Lecture Notes
Veterinary Histology
VMED 7123
Dr. Charlotte L. Ownby
Fall Semester 2004
For Lecture Notes with Color Images use go to:
http://www.cvm.okstate.edu/instruction/mm_curr/histology/Hi
stologyReference/index.htm
Under Course Outline, double click on Organ System of Interest
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Digestive System I - lips, tongue, salivary glands, esophagus,
stomach, small and large intestines
The digestive system includes the gastrointestinal tract as well as associated organs like the
pancreas and liver. Digestive System I will cover the oral cavity (lips, tongue, major salivary
glands) and the gastrointestinal tract, i.e. esophagus, stomach, small and large intestines. The
digestive system consists throughout most of its length of a series of tubular organs lined with
specific types of epithelium to fulfill specific functions related to the digestion and absorption of
nutrients from a food source and the elimination of waste products.
Organ
Lips
Teeth
Tongue
Salivary Glands
Esophagus
Stomach
Small Intestine - duodenum
Small Intestine - jejunum &
ileum
FuFunctionnction
Ingestion and fragmentation of food
Fragmentation of food
Fragmentation and swallowing
Fragmentation and moistening of food; swallowing
Passage of food from oral cavity to the stomach
Completion of fragmentation and beginning of
digestion
Digestion; emulsificaton of fats by enzymes from the
pancreas and bile from the liver
Completion of digestion and absorption
Large Intestine- cecum
Absorption of water from liquid residue
Large Intestine - colon
Absorption of water from liquid residue
Large Intestine - rectum
Anus
Storage of feces prior to defecation
Route for defecation of feces outside the body
Oral Cavity
Organs that make up the oral cavity include the lips, teeth, tongue and major salivary glands.
These organs function to obtain and ingest food, fragment it into smaller particles, moisten and
swallow it. Teeth will not be covered in this course.
Lips
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The lips aid in obtaining food and placing it in the mouth so that the teeth and tongue can
manipulate it and begin fragmenting it. Lips are covered by a stratified squamous epithelium
that is usually keratinized on the outer surface and contains many hairs whereas the epithelium
on the inner surface is more moist and non-keratinized.
Tongue
The tongue is a highly muscular organ used to manipulate food in the mouth and for the sense of
taste. It is covered with stratified squamous epithelium that in the anterior part forms specialized
structures known as papillae that are involved in the manipulation of food as well as in the sense
of taste. The skeletal muscle of the tongue is unique in that it runs in three different directions
allowing for a wide range of movements needed to properly manipulate foodstuffs. The types,
numbers and distribution of papillae in the tongue vary greatly among species. In domestic
animals there are usually five different types of papillae.
1. Filiform papillae are highly keratinized, sharply pointed and aid in mechanically
breaking up food material. They are numerous in ruminants and cats where they are used in
lapping milk.
2. Fungiform papillae are smooth with a rounded surface. They help manipulate the food
but also have taste buds on their lateral surfaces.
3. Conical papillae are somewhat larger than fungiform papillae, are used in manipulating
and breaking down ingested food. They can be distinguished from fungiform papillae by
their larger size, tendency to project above other papillae and they do not have taste buds.
4. Foliate papillae, covered with non-keratinized stratified squamous epithelium, are leafshaped structures defined by an invagination of the mucous membrane on their sides. Many
taste buds on their lateral surfaces indicate their role in gustation. They are absent in
ruminants but well developed in the horse and dog.
5. Circumvallate papillae are the largest (up to 1/8 inch diameter) papillae. They are
surrounded by a deep indentation of the mucous membrane and are not numerous. They do
not rise above the surface of the tongue. Many taste buds are located on their sides. Serous
von Ebner's glands empty into the "moat" around these papillae and help keep it free of food
particles.
Salivary Glands
The salivary glands all empty their secretions into the buccal cavity. They vary as to their
distance from the buccal cavity, their size and the nature of their secretory products. They can
also be divided into major and minor glands. We will consider only the major salivary glands of
which there are three: parotid, sublingual and submandibular. These glands all have the
tubuloalveolar glandular structure and all are compound, i.e., composed of numerous secretory
endpieces connected by an elaborate system of branching ducts. In general saliva is a dilute,
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hypotonic solution containing various enzymes (esp. amylase and lysozyme) and other proteins
such as antibodies, glycoproteins as well as electrolytes. Saliva in the buccal cavity is the
combined secretion of the numerous salivary glands, both major and minor. The secretions of
salivary cells can be either of a serous type, i.e., watery and rich in enzymes and antibodies or
mucous, i.e., viscid containing more glycoproteins. Individual salivary glands may contain
mostly cells of the serous type, of the mucous type or a mixture of both types. The final
composition of saliva at any given time depends on the proportion contributed by specific
salivary glands and is determined in the major glands by the parasympathetic nervous system
resulting from physical, chemical and psychological stimuli.
Salivary Gland
Type of Secretory Cells
Parotid
Serous
Sublingual
Mucous
Submandibular
Mixed
Basic Plan of the Digestive Tube
From the esophagus to the anus, the digestive system is basically a tube very similar to other
tubular organs in the body. All such tubular organs are composed of several tissue layers
arranged around a lumen. In a "generic" tubular organ, these layers are as follows (from the
lumen to the ablumenal layer).
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Tunica mucosa: This layer is composed of epithelium, connective tissue and
muscle. These tissues can usually be found in distinct layers as follows:
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lamina epithelialis mucosae: consists only of epithelium
lamina propria mucosae: consists of either loose areolar or reticular
connective tissue
lamina muscularis mucosae: consists of smooth muscle
Tunica submucosa: consists of loose connective tissue, nerves, blood vessels,
and glands in some organs
Tunica muscularis: consists of at least two layers, an inner circular and an outer
longitudinal with parasympathetic ganglia located between the layers
Tunica adventitia or tunica serosa: consists of loose connective tissue
If the organ is surrounded by other tissues, this layer is called a tunica adventitia
and its connective tissue blends with that of the surrounding tissues.
If the organ is suspended in the body cavity, this layer is called a tunica serosa
and it is covered by a simple squamous epithelium that is called mesothelium.
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Esophagus
The esophagus connects the oral cavity with the stomach allowing and aiding in the movement
of food particles to the stomach. It is a muscular tube having the layers described above for the
typical tubular organ. In the esophagus the layers are specialized for the function of further
fragmenting food particles.
Layers of the Esophagus
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Tunica mucosa:
lamina epithelialis: consists of stratified squamous epithelium that can
be highly folded in an empty organ; may be highly keratinized in animals
that ingest hard, dry materials such as herbivores
o lamina propria: consists of loose connective tissue which often has
scattered lymph nodules esp. in pigs and humans
o lamina muscularis mucosae: consists of smooth muscle; distribution and
continuity is highly species variable as follows: (1) continuous in human
(2) separate muscle bundles that fuse in horses, ruminants and cats, (3)
absent in cervical part in dogs, (4) absent in pigs in cervical region but
complete near the stomach
o
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Tunica submucosa: consists of loose connective tissue that is very elastic allowing for
expansion when food is present; ties the overlying epithelium to the underlying muscle
layers; seromucous glands present in most species and numerous in the dog but absent in
horses and cats. Lymphoid nodules may be present in the pig esophagus.
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Tunica muscularis: consists of smooth and/or skeletal muscle; inner circular and outer
longitudinal layers usually begin as skeletal muscle at the cervical end (voluntary control
of swallowing) changing to smooth near the distal end close to the stomach; skeletal
muscle throughout in ruminants and the dog.
Tunica serosa/adventitia: consists of typical loose connective tissue that blends into the
connective tissue of surrounding tissues.
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Stomach
The stomach connects the esophagus to the intestines and in most species serves not only to
continue the breakdown of foodstuffs via the use of digestive enzymes and acid but it also as a
storage depot for food. Usually food remains in the stomach a few hours during which it is
converted into a liquid material called chyme.
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Stomachs are either simple or compound, i.e., consisting of one chamber or many
chambers. Simple stomachs are composed primarily of glands, that is the tunica
mucosa is filled with glands.
Ruminant stomachs are compound stomachs containing both non-glandular and
glandular regions. The non-glandular regions include the reticulum, rumen and
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the omasum. The glandular region is the abomasum which has its own regions
similar to those found in a simple stomach.
Regional variation in the glands of the tunica mucosa of the stomach
Not all regions of the stomach mucosa have the same histological structure. They vary as
follows:
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cardia: contains many mucus-secreting glands
fundus: mostly glands secreting acid-peptic gastric juices; some mucus-secreting glands
pylorus: contains two different types of mucus-secreting glands; also contains endocrine
cells secreting gastrin
Wall of the Glandular Stomach
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Tunica mucosa: in the empty stomach, this layer is thrown into deep longitudinal folds
called rugae that extend from the lamina muscularis mucosae to the lumen; in the full
stomach the rugae are much reduced in size as a result of distension of the tunica mucosa
to accomodate the presence of a large amount of food material
Tunica submucosa: typical loose connective tissue contains parasympathetic ganglia
located in submucosal plexuses also known as Meissner's plexuses
Tunica muscularis: typical smooth muscle consisting of at least two layers, an inner
circular layer and an outer longitudinal layer; parasympathetic ganglia located between
the two muscle layers in the myenteric or Auerbach's plexus
Tunica serosa: typical small amount of loose connective tissue with overlying simple
squamous epithelium or mesothelium
Layers of the Tunica Mucosa of the Stomach - Fundic Region
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Lamina epithelialis: consists of simple columnar epithelium that forms branched,
tubular glands; organized into gastric pits that open onto the lumen and gastric glands that
empty into the base of the gastric pits
Lamina propria: consists of loose areolar connective tissue that in the glandular
stomach is minimal between gastric glands and difficult to see in sections; highly
vascular containing many blood and lymphatic capillaries
Stratum compactum: consists of dense connective tissue containing thick collagen
fibers; located between the lamina propria and the lamina muscularis mucosae; prominent
in carnivores where it probably helps prevent the perforation of the wall of the stomach
by sharp objects such as bones that might be present in the lumen
Lamina muscularis mucosae: consists of several layers of smooth muscle oriented both
longitudinally and circularly; usually not very thick
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Four cell types in the gastric gland
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Surface mucous cells: line the gastric pit and secrete mucous and bicarbonate ions to
protect the epithelium from digestion by gastric juice (contains HCl and pepsin) present
in the stomach lumen
Neck mucous cells: found dispersed between the parietal cells; secret a mucus that is
thinner than that secreted by the surface mucous cells; mucus protects other glandular
cells from action of proteases and HCl
Parietal (oxyntic) cells: found throughout the gastric gland; round cells that contain
distinct eosinophilic (pink) cytoplasm and round, prominent nucleus; secrete HCl and
intrinsic factor, needed for absorption of vitamin B12.
Chief (peptic, zymogenic) cells: found mostly near the base of the gastric glands; very
basophilic (purple) containing basally positioned nucleus and prominent basophilic apical
cytoplasm filled with many ribosomes; secrete pepsinogen, which is converted to pepsin
in the acidic milieu of the stomach.
Neuroendocrine cells: difficult to distinguish by conventional light microscopy; several
types are present; some secrete serotonin, gastrin, glucagon, and somatostatin, among
other hormones
Stem cells: located primarily in the neck region; difficult to identify in routine H&E
sections; undergo mitosis to form more cells then differentiate into the other cell types
present in the gland
Parasympathetic Ganglia
Aggregations of parasympathetic ganglion cells are found throughout the digestive tube in two
locations. Some are located in the submucosa and are usually called Meissner's plexus; others
are located between the inner circular and outer longitundinal layers of smooth muscle in the
tunica muscularis. The latter ones are usually called myenteric or Auerbach's plexus.
Postganglionic fibers from Meissner's plexus innervate the lamina muscularis mucosae whereas
postganglionic fibers from the myenteric plexus innervate the smooth muscle of the tunica
muscularis. The two layers of smooth muscle in the tunica muscularis inherently contract in a
wave of peristalsis that helps move stomach contents toward the small intestine. However,
contractions of the smooth muscle are regulated by the autonomic nervous system as well as
other factors such as hormones released into the stomach. An increase in peristalsis results from
an increase in parasympathetic stimulation; a decrease in peristalsis results from an increase in
sympathetic stimulation.
Meissner's plexus and the myenteric plexus both consist of the cell bodies of parasympathetic
ganglion cells that are easily identified by their large size in comparison with other cells in the
area and also by the large, round nucleus that contains a prominent nucleolus. These cell bodies
are found in the midst of unmyelinated nerve fibers and near areas of myelinated axons.
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Compound stomach - found in ruminants and has four parts
Chamber
Rumen
(part of
forestomach)
Reticulum
(part of
forestomach)
Omasum
(part of
forestomach)
Abomasum
Histology
Function
non-glandular; keratinized stratified
squamous epithelium
mechanical and chemical
breakdown of food;
breakdown of food by
microbes; production of
volatile fatty acids;
absorption of volatile fatty
acids, lactic acid,
ammonia, inorganic ions
and water
non-glandular; keratinized stratified
squamous epithelium
"
non-glandular; keratinized stratified
squamous epithelium
"
glandular; simple columnar glandular
enzymatic digestion
epithelium
Rumen
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Tunica mucosa: characterized by the presence of long (1.0-1.5 cm long) conical
projections called papillae that extend in to the lumen
o Lamina epithelialis mucosae - keratinized stratified squamous.
o Lamina propria - typical; no glands
o Lamina muscularis mucosae- absent; NOTE: It is easy to confuse a thickened
layer of connective tissue that extends into the papilla with a lamina muscularis
mucosae but this tissue is connective tissue, not smooth muscle.
Tunica submucosa: merges with lamina
propria; no glands or lymphoid aggregates.
Tunica muscularis: typical
Tunica serosa: typical
Reticulum
Similar to rumen, except as noted below:
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Tunica mucosa: when viewed from the lumen of the reticulum, the mucosa looks like a
"honeycomb" or reticulum. The basis of this honeycomb is a series of connected vertical
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primary folds that give rise to secondary and tertiary papillae which project into the
lumen.
Lamina muscularis mucosa: a layer of smooth muscle extends from the tips of the
papillae down to the position of the lamina muscularis mucosa although in the reticulum
this layer is not quite typical. However, the smooth muscle in the reticulum is continuous
with the smooth muscle of the lamina muscularis mucosa in the esophagus.
Other tunics are typical
Omasum
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This part of the non-glandular region of the compound stomach is notable for the
complexity of the foldings of its tunica mucosa
These folds or laminae are covered with a highly keratinized stratified squamous
epithelium
Underlying this epithelium is the sparse loose connective tissue of the lamina propria.
The laminae muscularis mucosa extends into the primary laminae usually in two layers.
In between these two layers of the laminae muscularis mucosae there is a layer of smooth
muscle belonging to the tunica muscularis. These three layers of smooth muscle
intertwine as they extend toward the tip of the laminae and eventually fuse to form one
large mass of muscle at the tip.
Abomasum
The abomasum is the glandular part of the compound stomach and histologically it is essentially
the same as a simple stomach.
Small Intestine
The small intestine is a typical tubular organ in that it has all of the typical tunics and layers.
However, the tunica mucosa is especially modified to fulfill the function of absorption. Also, the
three regions of the small intestine, the duodenum, the jejunum, and the ileum, each have special
modifications to the wall to enable each region to better perform its particular function. In the
small intestine digestion occurs in the lumen as well as at the surface of the lining epithelial
cells. Pancreatic enzymes such as trypsin, chymotrypsin, elastase, carboxypeptidases, peptide
hydrolases, amylase and lipases are adsorbed onto the membrane surface of the epithelial cells
where they mix with the chyme present in the lumen catalyzing the breakdown of proteins,
carbohydrates and lipids. The smaller breakdown products are then absorbed by the lining
epithelial cells that are called enterocytes.
Layers of the Small Intestine
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Tunica mucosa: This layer protrudes out into the lumen as projections called villi and it
dips down to the underlying lamina muscularis mucosae forming pockets called crypts.
o Lamina epithelialis mucosae - simple columnar epithelium
- villus - a villus contains enterocytes (absorption), goblet cells (protective mucus)
in its upper region and neuroendocrine cells (local hormones)
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- crypt - a crypt (crypt of Lieberkühn) contains goblet cells, paneth cells
(defensive), neuroendocrine cells, stem cells, intraepithelial lymphocytes
(defensive)
Lamina propria - loose connective tissue rich in blood and lymphatic vessels
present in the core of the villi and between crypts
o Lamina muscularis mucosae- thin layer of smooth muscle located at the base of
the crypts
o
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Tunica submucosa: This layer blends with the lamina
propria and is typical. In the duodenum it has coiled branched glands known as
Brunner's glands, the ducts of which open into the base of the crypts.
Tunica muscularis: typical consisting of an inner circular layer and an outer longitudinal
layer
Tunica serosa: typical
Enteroendocrine cells: These cells secrete hormones such as secretin, somatostatin,
enteroglucagon and serotonin; one hormone per type of cell.
Paneth cells: These remarkable cells contain large granules that contain defensins
(antimicrobial peptides) as well as lysozymes and phospholipase A. These chemicals represent
the "first-line" of defense against microbes that enter through the digestive tract. Compared to
the other cells present in the epithelial lining, Paneth cells are long-lived, i.e., weeks versus a few
days for the other cells.
Specializations to enhance absorption ability
The small intestine has all of the "layers" of a typical tubular organ but the tunica mucosa is
highly specialized to perform the function of absorption. To fulfill this function it uses several
strategies to increase the surface area of the plasma membrane of the absorptive epithelial cells.
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individual cells have numerous projections of their apical plasma membranes called
microvilli
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the lamina epithelialis and lamina propria together form folds that project out into the
lumen called villi
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the tunica mucosa and tunica submucosa together form large transverse folds into the
lumen called plicae circulares
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the small intestine is extremely long (usually several meters)
Regional variations in the small intestine
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Duodenum
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presence of Brunner's glands in the submucosa
o serous in pig and horse
o mucous in ruminant and dog
o mixed in cat
presence of chyme in the small intestine induces cells of Brunner's glands to secrete
alkaline mucus that neutralizes gastric acid and pepsin and further promotes digestion
no plicae circulares
longest villi of all three
regions
highest number of goblet cells
Jejunum
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no glands in the submucosa
no lymphoid nodules
Ileum
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permanent aggregated lymphoid nodules in the submucosa
shortest villi; least number of goblet cells
Large Intestine
Unlike the small intestine, there are no plicae circulares or villi in the large intestine so the
surface of the tunica mucosa is more uniform and flatter than that of the small intestine.
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Tunica mucosa:
o
lamina epithelialis -simple columnar epithelium that forms straight tubular
glands lined with absorptive columnar cells (recovering water and salt) and
numerous goblet cells (producing mucus to facilitate passage of dry waste
material); stem cells and lymphocytes are also present
o
lamina propria- loose connective tissue that contains numerous blood and
lymphatic vessels, collagen, lymphocytes and plasma cells
o
lamina muscularis mucosae- present beneath the base of the crypts and
prominent; undergoes rhythmic contractions mixed in cat
Tunica submucosa: typical
Tunica muscularis: inner circular and outer longitudinal layers; outer longitudinal layer
is organized into three separate bands known as taenia coli; movement of more solid
waste to the rectum
Tunica serosa is typical.
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Commensal bacteria reside in the large intestine and play a role in the continued digestion of
food.
Digestive System II – Liver, gall bladder, pancreas
Other organs that are part of the digestive system include the liver, gall bladder and pancreas.
Liver
The liver is the largest gland in the body; it is multifunctional. To understand the function of the
liver it is necessary to understand the blood supply to the liver. It is supplied by the hepatic
artery in the typical manner but it is the only digestive organ drained by the inferior vena cava.
Other digestive organs such as the small intestine, parts of the large intestine, stomach and
pancreas are drained by the hepatic portal system which takes the blood directly to the liver.
Thus, the liver receives oxygen poor, nutrient rich blood from the hepatic portal system and
oxygen rich blood from the hepatic artery.
Functions of the liver.
Digestive and Metabolic Functions
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synthesis and secretion of bile
storage of glycogen and lipid reserves
maintaining normal blood glucose, amino acid and fatty acid concentrations
synthesis and release of cholesterol bound to transport proteins
inactivation of toxins
storage of iron reserves
storage of fat-soluble vitamins
Non-Digestive Functions
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synthesis of plasma proteins
synthesis of clotting factors
synthesis of the inactive angiotensinogen
phagocytosis of damaged red blood cells
storage of blood
breakdown of circulating hormones (insulin and epinephrine) and immunoglobulins
inactivation of lipid-soluble drugs
General organization of the liver
Structurally the liver is divided into lobules by loose connective tissue septae. These septae are
more prominent in some domestic animals than in others; the pig has the most prominent septae
and they are readily apparent grossly. For a long time the lobule as defined by these septae was
thought to be the basic functional unit of the liver but now it seems that another unit, i.e., the
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hepatic acinus, might better represent the functional unit of the liver. Both the hepatic lobule and
the hepatic acinus will be described but first the basic histology of the liver will be described.
At low magnification the liver looks relatively homogeneous and on first examination little
organization can be discerned. A closer look reveals the presence of "lobules" or groups of
hepatocytes arranged around a blood vessel, the central vein, and defined by loose connective
tissue in which the portal canals are found. This type of organization is most easily seen in the
pig liver.
Hepatocytes are one of the primary functional cells of the liver. They are located in flat
irregular plates that are arranged radially like the spokes of a wheel around a branch of the
hepatic vein, called the central vein or central venule since it really has the structure of a
venule.
Portal canal: Three structures are found gouped together in the loose connective tissue
surrounding the plates of hepatocytes. These include branches of the hepatic artery, the hepatic
portal vein (venule) and the intralobular bile ductule. This group of three structures has been
called a portal triad but now is called a portal canal.
Portal canal:
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branch of the hepatic artery
branch of the hepatic portal vein (venule)
section of an intralobular bile ductule
Hepatocytes are arranged in rows that radiate out from the central vein. These rows are one cell
wide and are surrounded by sinusoidal capillaries or sinusoids. This arrangement ensures that
each hepatocyte is in very close contact with blood flowing through the sinusoids, i.e. bathed in
blood.
The endothelial cells lining sinusoids are fenestrated and in most species lack a basal lamina.
Gaps are also present between the endothelial cells. Taken together these two properties make
the sinusoids extremely leaky and allow for the extremely close contact between the blood and
the surface of hepatocytes. Many materials in the blood, except for whole blood cells, can pass
between the spaces in the sinusoidal lining.
Although sinusoidal endothelial cells lie very close to hepatocytes, they do not actually make
contact. A narrow space is present between the surface of the hepatocyte and the surface of the
endothelial cell. This is called the space of Disse; it is filled with numerous microvilli from the
hepatocytes. As in other areas of the body, these structures serve to increase the surface area of
the cell membrane that comes in contact with the blood facilitating exchange of molecules
between hepatocytes and the blood.
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What is the basic functional unit of the liver?
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Hepatic lobule: The hepatic lobule is defined as having a central vein (CV) at its center
with its edges defined by portal canals (PC). This model only takes into consideration the
flow of blood in one direction, i.e., from the branch of the hepatic artery located in the
portal canal toward the central vein. Yet, in the liver, blood actually flows from branches
of the hepatic artery in several directions. Thus, the hepatic lobule does not define a
"functional unit" of the liver very well.
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Hepatic acinus: More recent terminology identifies the hepatic acinus as the "functional
unit" of the liver. This definition recognizes both the real pattern of blood flow in the
liver and the role of hepatocytes as secretory cells, secreting bile.
The hepatic acinus has three zones.
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Hepatocytes in Zone 1 are the first to receive blood and it is high in oxygen.
o
Hepatocytes in Zone 2 are the second cells to receive blood and it is lower in
oxygen.
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Hepatocytes in Zone 3 are the last to receive blood from a branch of the hepatic
artery and it is lowest in oxygen.
o
Thus, the cells with the highest metabolic potential are found in Zone 1 and those
with the least are found in Zone 3. Importantly, the cells in Zone 3 are the most
susceptible to ischemic conditions due to the already low level of oxygen that
reaches them through the blood.
Secretion of bile in the liver
Bile is produced and secreted by hepatocytes into a special "duct" called a bile canaliculus.
This "duct" is actually just a space formed between two hepatocytes that is separated from the
connective tissue space around the hepatocytes by the presence of tight junctions. The bile
canaliculi empty into branches of the bile ductules which eventually empty into the hepatic duct
that carries the bile out of the liver to the gall bladder for concentration and storage. In the duct
system, bile flows in the direction opposite to the flow of blood in the sinusoids.
Gall Bladder
The gall bladder receives bile from the liver. Bile is composed of bile salts that emulsify fats
forming water-soluble complexes with lipids (micelles) to facilitate the absorption of fat. Bile
salts in the small intestine also activate lipases in the intestine.
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Functions of the gall bladder
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storage of bile
concentration of bile
acidification of bile
send bile to the duodenum in response to cholecystokinin
secreted by enteroendocrine cells in small intestine;
horse does not have a gall bladder and bile is continuously received from the liver
Gall bladder structure
The gall bladder is a sac that is lined with a simple columnar epithelium and has a tunica
muscularis containing smooth muscle that is innervated by both the parasympathetic and
sympathetic branches of the autonomic nervous system.
Tunics (layers) of the Gall Bladder
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Tunica mucosa: When the gall bladder is empty, this layer is extremely folded. When
full, this layer is smoother but still has some short folds.
o lamina epithelialis: composed of simple columnar epithelial cells with numerous
microvilli on their luminal surfaces and connected by tight junctions near luminal
surfaces.
o lamina propria: composed of loose connective tissue rich in reticular and elastic
fibers to support the large shape changes that occur in the lamina
epithelialis; lamina propria may contain simple tubuloalveolar glands especially
in ruminants. May be mucous or serous depending on species.
o lamina muscularis mucosae: not present
Tunica submucosa: present and typical
Tunica muscularis: contains much smooth muscle, poorly organized
Tunica serosa: present and typical
Pancreas
The pancreas contains both exocrine and endocrine components that secrete digestive enzymes
and peptide hormones respectively. These two components are very different structurally and
functionally but are intermingled within the gland. However, the organization of the exocrine
part into acini make it fairly easy to recognize in histological sections as does the organization of
the endocrine part around areas of high vascularity.
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Organization of the pancreas
The bulk of the pancreas by volume consists of exocrine cells that secrete an alkaline solution of
digestive enzymes. This secretion moves through a duct system that eventually leads to the
pancreatic duct. Only about 5% of the volume of the pancreas consists of endocrine cells.
These cells secrete peptide hormones that play a role in controlling carbohydrate metabolism.
The endocrine cells are closely associated with large numbers of blood capillaries into which
they secrete the peptide hormones.
The exocrine pancreas . . .
The exocrine portion of the pancreas is a compound acinar gland. It has many small lobules, each
of which is surrounded by connective tissue septa through which run blood vessels, nerves,
lymphatics, and interlobular ducts.
Exocrine secretion by the pancreas is controlled by hormones and nerves.
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When the hormone, secretin, is released from neuroendocrine cells in the duodenum of
the small intestine, the pancreas secretion is watery and rich in bicarbonate. This "basic"
secretion helps to neutralize the acidic chyme as it comes into the small intestine.
•
When cholecystokinin-pancreozymin (CCK) is released by neuroendocrine cells in the
duodenum the pancreas secretes a product rich in enzymes that breakdown proteins,
carbohydrates, lipids and nucleic acids in the lumen of the small intestine.
•
When gastrin is secreted by pyloric neuroendocrine cells the pancreatic secretion is rich
in digestive enzymes. Two of the digestive enzymes secreted by the pancreas are trypsin
and chymotrypsin; they are secreted as non-active, pro- or zymogen forms and are
subsequently activated by enterokinase in the lumen of the duodenum to avoid digestion
of the pancreatic acinar cells.
A compound acinar gland
•
Acini: The secretory cells of the pancreas are arranged around a small lumen. The
pancreatic acinar cells produce the digestive enzymes in the typical pattern of protein
synthesis. However these cells are highly active in protein synthesis for export and this
high activity is reflected in their bizonal staining properties with the typical dyes used
for histology, i.e., hematoxylin and eosin. The basal region of these secretory cells
usually stains intensely with hematoxylin reflecting the presence of large amounts of
endoplasmic reticulum where the protein is being synthesized on ribosomes. These
proteins move from the rough endoplasmic reticulum to the Golgi apparatus where they
are glycosylated, then from the Golgi as secretory granules. In these granules the
enzymes are found in an inactive or zymogen form. They are activated after release in to
the duct system. The presence of numerous zymogen granules containing high
concentrations of protein is reflected in the intense eosin staining in the apical region of
16
the secretory cells. These granules are most abundant during fasting or between meals
and least abundant after a meal has been ingested.
•
Ducts: The secretory product of the acinar cells is carried out of the pancreas by a duct
system as in other exocrine glands.
o Intercalated duct. The first part of the duct system is called the
intercalated duct or intralobular duct. It is lined with low cuboidal
epithelial cells that secrete bicarbonate ion into the secretory product.
This duct actually extends into the acinar lumen, where its walls consist of
the pale staining centroacinar cells. Intercalated ducts empty into the
larger interlobular ducts.
o Interlobular ducts. These ducts are lined with a low columnar
epithelium that may contain goblet cells. Interlobular ducts empty into the
main pancreatic ducts that exit the pancreas.
The endocrine pancreas . . .
The cells of the endocrine portion of the pancreas are arranged either in round-to-oval shaped
areas rich in blood vessels known as the islets of Langerhans or they may be scattered
throughout the exocrine portions of the pancreas near the acini or ducts. There are several
different types of cells in the islet or other regions, each secreting a different peptide hormone. It
is not possible to distinguish among these cells with routine hematoxylin and eosin stain used for
histological preparations. Immunocytochemistry is necessary to identify which cells are
secreting a particular peptide. This is done by staining with an antibody made to the specific
peptide that is combined with a label that can be visualized at the light microscopic level such as
immunoperoxidase.
Examples of peptide hormones secreted by the endocrine pancreas:
•
•
•
•
•
•
insulin - increases uptake of glucose by most cells; reduces blood level
glucagon - decreases uptake of glucose; increases blood level
somatostatin - many effects of gastrointestinal function; may inhibit insulin and glucagon
secretion
vasoactive intestinal peptide
pancreatic polypeptide
motilin, serotonin, substance P
17
Hematopoiesis
Hematopoiesis is the process by which immature precursor cells develop into mature blood
cells. The currently accepted theory on how this process works is called the monophyletic theory
which simply means that a single type of stem cell gives rise to all the mature blood cells in the
body. This stem cell is called the pluripotential (pluripotent) stem cell.
SITES OF HEMATOPOIESIS
Age of animal
Site of hematopoiesis
Embryo
yolk sac then liver
3rd to 7th month
spleen
4th and 5th months
marrow cavity - esp. granulocytes and platelets
7th month
marrow cavity - erythrocytes
Birth
mostly bone marrow; spleen and liver when needed
Birth to maturity
number of active sites in bone marrow decreases but
retain ability for hematopoiesis
Adult
bone marrow of skull, ribs, sternum, vertebral column,
pelvis, proximal ends of femurs
STRUCTURE AND FUNCTION OF BONE MARROW
Bone marrow has a vascular compartment and an extravascular compartment. The vascular
compartment is supplied by a nutrient artery which branches into central longitudinal arteries
which send out radial branches that eventually open into sinuses. These sinuses converge into a
central vein that carries the blood out of the bone marrow into the general circulation.
Hematopoiesis takes place in the extravascular compartment. The extra vascular compartment
consists of a stroma of reticular connective tissue and a parenchyma of developing blood cells,
plasma cells, macrophages and fat cells. The high activity of the bone marrow is demonstrated
by its daily output of mature blood cells: 2.5 billion erythrocytes, 2.5 billion platelets, 50-100
billion granulocytes. The numbers of lymphocytes and monocytes is also very large.
Bone marrow is the site for other important activities in addition to hematopoiesis. These
include the removal of aged and defective erythrocytes and the differentiation of B lymphocytes.
It is also the site of numerous plasma cells.
18
THE PROCESS OF HEMATOPOIESIS
The monophyletic theory of hematopoiesis states that pluripotent stem cells multiply to produce
more pluripotent stem cells, thus ensuring the steady and lasting supply of stem cells. Some of
the pluripotent stem cells differentiate into precursor cells that are at least partially committed to
become one type of mature blood cell.
Pluripotent stem cells multiply slowly into one of five possible unipotential stem cells, which
then multiply rapidly into the precursor of the specific mature blood cell for which they are
destined.
Although the pluripotent stem cells and the unipotential stem cells cannot be distinguished from
one another histologically, the precursor cells can be distinguished with a trained and practiced
eye.
General Features: Understanding the general process of hematopoiesis will be extremely
helpful in distinguishing and identifying the different cells in a bone marrow smear or in an
intact bone marrow preparation. Basically an immature, precursor cell goes from a cell that is
making lots of protein to a cell that is making much less protein.
19
Since structure is (always) related to function, the structure of the precursor cell changes as it
goes from making more protein to making less protein. Thus, a cell that is making a lot of
protein will have a nucleus containing dispersed or active chromatin, i.e. that is being transcribed
actively. When this cell is making less protein, the chromatin is condensed or clumped because
it is not being transcribed. Likewise, a cell that is making a lot of protein will have many large
nucleoli, the site of ribosomal RNA synthesis and assembly; as protein secretion decreases there
are smaller and fewer nucleoli. Cells with high protein synthetic activity have more ribosomes in
their cytoplasm and consequently the cytoplasm stains more basophilic (hematoxylin staining of
the RNA in ribosomes). Cells with lower protein synthetic activity have fewer ribosomes, thus
less basophilic staining with hematoxylin leaving the cytoplasm appearing more acidophilic due
to the eosin staining of cytoplasmic proteins. In cells with high protein synthetic activity, the
Golgi apparatus is highly developed, occupies much of the cytoplasm thus pushing the nucleus
off to one side (acentric nucleus). Cells with low protein synthetic activity have a smaller Golgi
and the nucleus tends to be more centrally located.
The chart below summarized these features.
Cell Organelle
Making lots of Protein
Making less Protein
Nucleus
chromatin is dispersed
chromatin is clumped
Nucleoli
more
fewer
Cytoplasm
more ribosomes; basophilic
fewer ribosomes; acidophilic
Golgi apparatus
*large; nucleus off center
*smaller; nucleus more
centered
* except in RBC precursors where the nucleus becomes more off centered until it is extruded
from the cell at the last stage before maturity
ERYTHROPOIESIS
As the cells are maturing in the erythrocytic series, the cells are usually getting smaller, the
nucleus is becoming smaller and more condensed and is eventually lost, and the cytoplasm is
becoming pinker rather than blue.
The cells in the developing erythrocyte series are as follows:
•
Unipotent stem cell: cannot be distinguished from other unipotent stem cells by
histology
20
•
Proerythroblast: nucleus still rather large, taking up most of the cell; nucleus not
condensed; cytoplasm still very blue or basophilic
•
Basophilic erythroblast: very difficult to distinguish from the proerythroblast
•
Polychromatophilic erythroblast: nucleus is more condensed than that of the
proerythroblast; cytoplasm less blue, more grayish
•
Orthochromatophilic erythroblast: nucleus more condensed, smaller than that of
previous cells and looks pyknotic by comparison; cytoplasm beginning to take on a more
pinkish cast
•
Reticulocyte: no nucleus; cytoplasm still stains somewhat bluish due to presence of
remnants of polyribosomes
•
Erythrocyte: mature erythrocyte has no nucleus (in mammals); cytoplasm stains very
pink due to lack of ribosomes and presence of high amounts of protein, i.e., hemoglobin
GRANULOCYTE DEVELOPMENT
•
Unipotent stem cell: cannot be distinguished from other unipotent stem cells by
histology
•
Myeloblast: large cell with blue-staining cytoplasm; large nucleus; often has a clear area
near the nucleus - this is where the rather large Golgi is located
•
Promyelocyte: still a rather large cell with azurophilic (not specifically stained) granules
•
Myelocyte: overall cell still rather large; nucleus still round without indentation; granules
staining appropriately for the series, i.e., pink for eosinophilic, blue for basophilic, neutral
for neutrophilic
•
Metamyelocyte: cell is about the size of a mature granulocyte; nucleus with slight
indentation; granules present that stain appropriately for the series, i.e., pink for
eosinophilic, blue for basophilic, neutral for neutrophilic
•
Band cell: cell is about the size of a mature granulocyte; nucleus with definite
indentation - looks like a horseshoe; prominent granules that stain appropriately for the
series
•
Mature (segmented) granulocyte: cell is mature and looks like normal, mature
granulocytes in the blood with lobed nucleus and prominent granules that stain
appropriately for the series
21
MONOCYTES
Not responsible for knowing the sequence of development of monocytes.
PLATELETS
Platelets, also called thrombocytes, play an important role in hemostasis by:
•
•
•
plugging holes in blood vessels to prevent bleeding
promoting formation of clots to further prevent bleeding
helping to repair damaged blood vessels
Platelet granules contain (1) the secretory material that platelets produce to help repair damaged
blood vessels, (2) growth factor, and (3)many other proteins. Some of these are:
•
•
•
•
•
•
platelet factor 4 - regulates vascular permeability, calcium mobilization from bone,
chemotaxis of monocytes and neutrophils
beta thromboglobulin - function unknown; used to monitor activation of platelets in some
diseases
coagulation factors - fibrinogen, factor V, factor VIII
fibronectin, thrombospondin, platelet-derived growth factor - all may be involved in
repair of damaged blood vessels
serotonin (taken up from plasma and stored in granules)
lysosomal enzymes such as hydrolases
Platelets appear as round, oval or biconcave disks and have a diameter of about 1.5 to 3.5 µm.
They are somewhat difficult to see in blood smears because of their small size and because they
are often clumped together. Despite their small size, they contain all of the normal organelles and
are rich in granules that are difficult to resolve with the light microscope but can be easily seen
with the electron microscope.
Platelets are formed in the bone marrow from megakaryocytes (30-100 µm diameter), very large
cells with a polyploid, multilobed nucleus. Platelets are released from fragmenting
megakaryocytes in at least two ways:
•
extension of pseudopodia through the wall of the sinuses; pseudopodia contain "strings"
of platelets that are pinched off and released into the circulation
•
passage of mature megakaryocyte into circulation and fragmenation in the pulmonary
vascular bed
22
Immune System
This system consists of cells and tissues that have as their main function the protection of the
body from the invasion by microorganisms and disease-producing entities foreign to the animal.
To achieve this goal this system has components spread widely throughout the body with
concentrations in specific places. Components of the system may be single lymphocytes located
strategically in the epithelium of mucous membranes, aggregations of lymphocytes associated
with the mucosa of strategically placed organs, or entire organs highly organized and
strategically located in reference to lymph and blood flow patterns.
The components are:
•
•
•
•
•
•
•
Lymphocytes
T cells
B cells
Plasma cells
Bone marrow
Thymus
Lymph Node
Mucosa-associated lymphoid tissue (MALT)
Spleen
NOTE: The bone marrow and thymus are considered as primary immune/lymphoid components
because they contain the stem cells that will develop into T cells, B cells and natural killer cells
of the functioning immune and lymphatic systems.
Lymphocytes and Plasma Cells
There are basically two different types of lymphocytes, T lymphocytes (T cells) that are
involved in cell-mediated immunity and B lymphocytes (B cells) that are involved in humoral
immunity. Both types of cells originate from stem cells in bone marrow. In addition there are
many types of T lymphocytes depending on their specific role in the immune response. In
circulating blood, lymphocytes may be either small lymphocytes (6-9 µm) or large lymphocytes
(9-15 µm) with the latter representing only about 3% of circulating lymphocytes. Although it is
not possible by routine histological methods to differentiate the various types of small
lymphocytes found in blood, they are of several different types that are in the process of
migrating through the circulation to take up residence in an organ or they are "searching" for
foreign antigen. Large lymphocytes are mostly activated B lymphocytes.
Immature T lymphocytes move from the bone marrow into the thymus, take up residence and
become thymus-dependent or mature T lymphocytes. These mature T cells then pass through the
circulation to find homes in lymph nodes, mucosa-associated lymphoid tissue or the spleen.
There are several types of T lymphocytes, i.e., T helper cells, cytotoxic T cells and suppressor T
cells.
23
B lymphocytes originate and mature in the bone marrow then move through the circulation to
various sites throughout the body. Upon interaction with foreign antigen and usually with the
assistance of T helper cells, B lymphocytes become mature antibody secreting cells called
plasma cells. Clones of plasma cells making specific immunoglobulins are produced thus
providing the large numbers of plasma cells needed to mount a good antibody (humoral immune)
response. Plasma cells are rarely found in the circulation but reside mostly in connective tissue
(lamina propria) beneath epithelia, in the medullary cords of lymph nodes and in the white pulp
of the spleen.
These immune cells are strategically located in areas that come in close contact with foreign
substances. They represent one of the first lines of defense against invading microorganisms,
viruses and parasites. A good example is the small intestine. In these types of locations, they are
perfectly positioned to interact with invading foreign substances and they recognize these
substances as non-self or foreign. Upon such "recognition" lymphocytes are activated and
function to neutralize or destroy the invading foreign substance.
Plasma cells are derived from activated B lymphocytes that have left the blood stream and taken
up residence in connective tissue. They are easily identified in histological sections due to their
unique morphology which reflects their high protein synthetic activity. Usually the round to oval
nucleus is eccentrically located in the cell due to the presence of a large Golgi apparatus where
immunoblobulin synthesis is completed and the molecules packaged for secretion. The
predominant staining pattern of the cytoplasm is bluish to purple (basophilic) due to the large
amount of rough endoplasmic reticulum and associated ribosomes. Usually the cytoplasm is
packed with rough ER. In a very well stained, relatively thin seciton, the nucleus has the
appearance of being "spoked" or having a "clock face".
Thymus
Located posterior to the sternum in the anterior part of the mediastinum, the thymus is a bi-lobed
nodular organ that is very large in the first year or two of life reaching maximum size at puberty
then becoming smaller in a process called involution. During this degenerative process
connective tissue fibers and fat cells replace the previously functional tissue (parenchyma) of the
organ and even though only a few pieces of functional tissue remain, it is enough to continue to
supply the organism with sufficient mature lymphocytes. Immature T lymphocytes move from
the bone marrow into the thymus where they become immunocompetent T cells. These T cells
then leave the thymus, go into the circulation and eventually find their way to lymph nodes,
mucosa-associated lymphoid tissue or the spleen.
Functions
•
•
•
•
production of immunocompetent T lymphocytes
production of mature but naïve T cells for peripheral tissues and circulation
immunological self-tolerance
regulation of T cell maturation, proliferation and function via secretion of hormones
24
Histology of the Thymus
Each lobule has an outer, darker staining cortex and an inner, paler staining medulla. High
concentrations of T lymphocytes in the cortex are the basis for the intense basophilia of this
region and this is the site of precursor cell proliferation and maturation. Mature,
immunocompetent T cells then move from the cortex toward the medulla where they enter the
bloodstream to be taken out of the thymus.
The thymus has two tissue components: parenchyma and stroma. The parenchyma is composed
mostly of T lymphocytes in various stages of development into mature T cells whereas the
stroma is composed of special thymic epithelial cells.
The parenchyma and stroma have different appearances depending on whether you are looking at
the cortex or the medulla.
Cortex
In the cortex, the parenchyma consists mostly of the developing T lymphocytes. It is here that the
T cell receptor (TCR) genes are rearranged so that the mature T cells obtain their specific surface
markers. The stroma consists of sparse, delicate epithelial cells obscured by all of the
lymphocytes. These epithelial cells form the support structure for the developing T cells but also
play an important role in isolating the T cells from foreign anitgens during their development.
Medulla
In the medulla, the stroma consists of prominent epithelial cells that have large, pale-staining
nuclei and substantial amounts of eosinophilic (pink-staining) cytoplasm. There are fewer T
cells because most of them have entered the blood stream via vessels at the corticomedullary
junction. Antigen presenting cells (APC) are also found in the medulla where they are called
thymic interdigitating cells. These cells are thought to present self-antigens to the matured T
cells. T cells that recognize these self-antigens are removed by a process called apotosis. This
process helps to prevent autoimmune diseases.
One prominent and identifying feature of the medulla is the presence of Hassall's corpuscles
thought to represent degenerating epithelial cells. These impressive structures begin to form in
the fetus and increase in number and size as the animal ages.
25
Structural basis for function of the thymus
In performing its major functions of producing immunocompetent but naïve T cells and in
achieving immunological self-tolerance, the thymus has some special structural arrangements
unlike those found in other organs.
•
First, to keep the developing T lymphocytes "protected" so that they can develop their
surface receptors in a "climate" that is not influenced by antigens, the thymic epithelial
cells form a continuous layer along the inner surface of the capsule extending into the
thymus along the septa and along blood vessels. These cells actually provide a cellular
framework for a space that is kept separate from other spaces such as the bloodstream.
This separation is maintained by desmosomes between adjacent epithelial cells and their
close contact with endothelial cells of capillaries. The "barrier" that results is called the
blood-thymus barrier; it is similar in structure to the blood-brain barrier. It is within this
confined and protected space that the T lymphocytes develop into immunocompetent yet
naïve T cells. The integrity of the space within the epithelial cell framework is
extremely important because it prevents the premature stimulation of T cells by antigens.
Blood-thymus barrier - components
- tight junctions between endothelial cells
- basal lamina of endothelium
- small connective tissue space
- basal lamina of epithelial cell
- continuous sheet of epithelial cell
•
Second, to provide a mechanism by which the newly developed immunocompetent and
naïve T cells can be added back to the circulation, the blood supply of the thymus also
has some peculiarities. Most arteries enter the thymus through the capsule, course via
connective septae through the cortex down to the level of the corticomedullary junction
where they then actually enter the parenchyma of the organ. Capillaries from these
arterial branches return to the region of the cortex within the parenchyma. These
capillaries are special in that they are not permeable to macromolecules, thus preventing
any antigenic contact with developing T cells in the cortex. Postcapillary venules that
derive from these same capillaries are permeable to macromolecules and lymphocytes.
The new immunocompetent T cells move into these postcapillary venules to eventually
join the general circulation and move to the other tissues and organs that are part of the
immune system. Some capillaries from the arterial branches entering the thymus from
the capsule extend down directly into the medulla to supply the tissue with oxygen and
nutrients then reconvene as postcapillary venules that join the postcapillary venules
coming from capillaries in the cortex. Thus, blood draining the cortex and the medulla
combine in the postcapillary venules and exit the thymus through typical venous
pathways.
•
Third, to ensure that self-tolerance is acheived, the medulla of the thymus has antigen
presenting cells (APC) that are thought to present self-antigens to the matured T cells.
26
Any T cells that recognize these self-antigens are removed thus preventing development
of autoimmune diseases.
Lymph Nodes
After maturing in the thymus, T cells move through the circulation to other organs, including
lymph nodes. Lymph nodes are small lima-bean shaped organs that are spread throughout the
body but occur in groups in areas where lymphatic vessels come together to form larger vessels
such as in the groins, neck and axilla. Lymph nodes are also part of the lymphatic system that
includes the lymphatic vessels, lymphoid tissue and lymphoid organs. Lymphatic vessels drain
fluid (lymph) from peripheral tissues and bring it to the venous system. Lymph consists of
interstitial fluid that is similar to blood plasma but with a lower protein concentration,
lymphocytes and macrophages. Lymph nodes filter and purify the lymph before it flows into
the venous system.
Functions
•
•
•
•
filter debris and microorganisms via phagocytosis by fixed macrophages
facilitate the interaction between antigen presenting cells and circulating lymphocytes to
initiate an immune response
B lymphocytes: activation and proliferation; plasma cell formation and antibody
production in response to antigens
T lymphocytes: activation to become T helper and T cytotoxic cells
The location and structural organization of lymph nodes makes them perfect for the above
functions. They are positioned so that all lymphatic vessels draining back to the venous
circulation from the tissues pass through a lymph node. The afferent lymphatic vessels branch
outside the organ, penetrate the capsule and empty into the subcapsular sinus. From here the
lymph flows into and through cortical sinuses enabling the lymph to come in close contact with
cells in the cortex of the node. In the medulla there are also sinuses (medullary sinuses) that
enable the lymph to flow toward the hilum and enter efferent lymphatic vessels. Eventually the
filtered lymph enters the bloodstream through the thoracic duct or right lymphatic duct.
Lymph nodes are surrounded by a fibrous connective tissue capsule that enters the organ as
trabeculae which define a cortex and medulla. The capsule and trabeculae are the source of
reticulin fibers that are found throughout the node and form the main supporting network of the
organ. These fibers serve to keep the sinuses open and to support the massive number of
lymphocytes and macrophages. Beneath the capsule is a subcapsular sinus into which lymph
flows from the afferent lymphatic vessels.
27
Histological organization of Lymph Nodes
The Cortex
The cortex is composed of the cortical sinuses surrounded by dense accumulations of
lymphocytes. In the more superficial cortex the lymphocytes are arranged into spherical
follicles, lymphoid follicles. It is here that B lymphocytes are activated and undergo
proliferation.
Germinal Center (GC)
The open, pale-staining nature of the nuclei of these cells indicate that they are B lymphocytes
undergoing active proliferation. Other cells include:
•
•
follicular dendritic cells that present antigen to the B cells
tingible body macrophages that engulfed dead B cells that have died by apotosis
Resting B cells enter the lymph node parenchyma though the high endothelial venules and if they
encounter an antigen with which they can react, they then enter the cycle of blast transformation
to produce clones of plasma cells and B memory cells. This production of clones occurs in the
germinal centers of lymphoid follicles.
Paracortical zone
Deeper regions of the cortex contain primarily T lymphocytes that do not form into follicles. T
lymphocytes (helper and cytotoxic/suppresor) arrive through the circulation, enter the lymph
node parenchyma through the high endothelial venules and take up residence in the paracortical
zone. If activated, the T lymphocytes undergo active proliferation to produce expanded clones of
activated T lymphocytes.
T lymphocytes that arrive at the lymph node via the arterial blood stream gain access to the
parenchyma of the lymph node through the wall of the high endothelial venules located in the
paracortical zone. These blood vessels contain endothelial cells that are expressing specific
lymphocyte binding molecules called addressins. These surface molecules are available to bind
to lymphocytes that recognize them, the lymphoctyes bind to the surface of the endothelium,
then cross the vessel wall and enter the lymph node parenchyma.
Mantle zone (corona)
The germinal center is surrounded by a ring of darker-staining cells. The condensed nature of
their nuclei indicates that these are resting B cells. Also present in the mantle zone are T helper
cells, macrophages and dendritic cells.
28
In a T cell-dominated response, the paracortical zone of the lymph nodes may be greatly
enlarged. Interdigitating dendritic cells are the main antigen presenting cell in the paracortical
zone.
The Medulla
The medulla of a lymph node is composed of medullary cords interspersed between medullary
sinuses.
The medullary cords are composed of plasma cells producing antibodies, their precursors,
macrophages and T helper cells. The most prominent cell in the cord is the precursor to plasma
cells or immunoblasts that came from the germinal centers of the lymphoid follicles in the cortex
of the node.
In the medullary cords, the plasma cells undergo final maturation and secrete antibodies into the
lymph that is collected by efferent lymphatic vessels in the node and eventually carried to the
general circulation. Plasma cells may also get into the general circulation in this manner.
The medullary sinuses are composed primarily of reticular fibers (RF) providing the support
framework, reticular cells (fibroblast-like cells that secret the reticulin) (RC) and macrophages.
Overview of the Blood Flow Pattern in a Lymph Node
29
Mucosa-associated lymphoid tissue (MALT)
MALT is really connective tissue located beneath mucous membranes in which the lymphocyte
is the predominant cell type. Examples occur in the respiratory, gastrointestinal, urinary and
reproductive tracts. The exact extent of these aggregations of lymphocytes is not easily
discernible because they have no distinct capsule like that of lymph nodes. However, they are
like lymph nodes in that they often have a pale-staining germinal center containing actively
dividing lymphocytes like the germinal centers in lymph nodes. The larger aggregations contain
B and T cell zones and antigen processing cells; the smaller, more scattered MALT components
such as those in the intestines and respiratory tract are mostly T lymphocytes. Some B cells and
plasma cells are also present.
Distribution of MALT
In the digestive system:
•
•
•
•
in the wall of the pharynx - tonsils (palatine, lingual, pharyngeal)
in the wall of the small intestine - aggregate lymphoid nodules
in the wall of the colon-aggregate lymphoid nodules
in the walls of the appendix
In the reproductive system:
•
in the wall of the vagina
30
Types of MALT and Functional aspects
•
•
larger aggregates function much like lymph nodes
smaller, scattered MALT are mostly T lymphocytes but also have B cells and plasma
cells
-- mostly IgA in the intestines and respiratory tract to protect against pathogens that
may gain access to underlying tissues
-- IgG and IgM secreted into lamina propria to counteract pathogens that have gained
access to connective tissue
-- IgE secreted into lamina propria; mediates the release of histamine from mast cells
•
•
•
single lymphocytes found within the lamina epithelialis are mostly T lymphocytes
MALT is drained by efferent lymphatics but there are no afferent lymphatics
lymphocytes exposed in MALT regions go through regional lymph nodes then return to
the MALT region after activation
Spleen
Located between the stomach, left kidney and diaphragm, the spleen is the largest lymphoid
organ in the body, performing functions for the blood similar to those performed by the lymph
nodes for the lymph. It is a soft organ, conforming to the contours of the organs and structures
surrounding it. At the hilus on the visceral surface, the splenic artery brings blood into the
spleen, the splenic vein takes blood from the spleen to the hepatic portal system, and lymphatics
drain lymph from the spleen. In some domestic species such as the horse and dog, the spleen
functions as a reservoir from which blood can be mobilized when needed and in these species,
smooth muscle is a prominent feature of the capsule and trabeculae of the spleen.
Functions
•
•
•
•
removal of abnormal blood cells and particulate matter via phagocytosis
storage of iron from recycled red blood cells
initiation of the immune responses by B cells and T cells in response to antigens
circulating in the blood
hematopoiesis in fetus and sometimes in adult
Histology
The exterior surface of the spleen consists of a capsule containing collagen and elastic fibers; the
interior components are collectively called the "pulp". Upon gross examinagtion of a slice of the
spleen, the pulp has two very different appearances: red and white. The organ appears as a large
expanse of red pulp dotted with white pulp. Histologically,red pulp is "red" due to the presence
of large numbers of erythrocytes in blood vessels called sinuses and white pulp is "white" due to
lack of these sinuses and consequently fewer erythrocytes. The red pulp surrounds the white
pulp while the latter looks like lymphatic nodules. Closer inspection of the white pulp indicates
31
that there is a "central arteriole", sometimes called a central artery, close to the center of each
area of white pulp.
Red Pulp
The red pulp of the spleen is characterized by a parenchyma (PN) that consists of macrophages
of the sheathed capillaries as well as other macrophages and blood cells that have not yet entered
the venous sinuses. The rest of the red pulp is occupied by numerous venous sinuses (VS). The
walls of the sinuses are very open and can be easily traversed by blood cells. Their lining
consists of long endothelial cells (arrows) oriented along the longitudinal axis of the vessel.
Large spaces occur between adjacent endothelial cells and the underlying basement membrane is
discontinuous, thus blood cells can easily pass between the endothelial cells and gain access to
the bloodstream on the venous side. A continuous reticulin network forms the framework that
supports the macrophages and a few fibroblasts responsible for producing the reticulin fibers;
special stains are required to visualize the reticular network.
White Pulp
The white pulp of the spleen is characterized by a parenchyma that consists of two types of
lymphocytes, i.e., B cells and T cells located in two different areas of the spleen. B cells are
located in the lymphoid follicles scattered throughout the organ. In younger animals, a germinal
center can be seen as seen in lymph nodes. In fact, this type of white pulp functions much in the
manner that lymphoid follicles of lymph nodes function, i.e., initiation of immune responses by
B cells to foreign antigens in the blood. T cells are located around the central arteries and form a
kind of sheath. This site is called the periarteriolar lymphoid sheath.
Blood flow in the spleen
To properly understand the histology of the spleen, it is necessary to understand its blood supply.
The splenic artery enters the spleen at the hilus, then branches into numerous arterioles that run
through the parenchyma or pulp of the spleen. When these arterioles acquire a coating of T cells,
the arterioles are called central arteries and the surrounding lymphoid tissue is called the
PALS, i.e., the periarteriolar lymphoid sheath.
32
Smaller penicillary arteries branch off of the central arteries and end in sheathed capillaries that
are special capillaries which actually have no endothelial cells and end blindly. These unique
capillaries are surrounded by macrophages which serve to filter materials from the blood. After
the blood flows through these sheathed capillaries, it flows into a complicated system of sinuses
that drain into larger and larger sinuses which eventually drain into the splenic vein which joins
with the hepatic portal vein. Blood flows out of the sheathed capillaries into a space that is not
considered part of any "true" blood vessel, then the blood cells re-enter the bloodstream through
33
the walls of the sinuses. This particular arrangement of "blood flow" in the spleen is considered
to be an open circulation pattern.
Integument
The Integument - the skin and all of its derivatives
Components
•
•
skin (epidermis, dermis, hypodermis)
derivatives (sweat glands, sebaceous glands, mammary glands, hair, nails, claws,
hooves, horns, antlers, combs, wattles, and feathers)
Functions
•
•
•
•
protection - from drying out, from invasion by microorganisms, from UV light and from
insults (mechanical, chemical or thermal)
sensation - for touch, pressure, pain and temperature
thermoregulation - decreases heat loss in cold temperatures; increases heat loss in hot
temperatures
metabolic functions - energy stored in fat deposits; synthesis of vitamin D
Structure of the Skin
Three distinct layers can be seen in the skin
•
•
•
Epidermis - consists of keratinizing stratified squamous epithelium
Dermis - consists of fibroelastic connective tissue
Hypodermis - consists mostly of white adipose tissue (sometimes referred to as the
subcutis)
The thickness of these layers varies depending on the specific location in/on the body and in a
given location on the amount of exposure to wear and tear. For example, in the buccal cavity,
the epidermis consists of a moist stratified squamous epithelium which is relatively think and not
highly keratinized whereas the epidermis of skin on the ball of the foot is thick and highly
keratinized. The skin covering the dorsum of the hand has a rather thin hypodermis whereas the
skin over the buttocks has a very thick hypodermis containing numerous fat cells.
34
Epidermis
Layers of the Epidermis
- in order from outermost (surface) to innermost (deepest)
•
Stratum corneum - consists of the remains of keratinocytes; mostly composed of the
protein, keratin
•
Stratum lucidum - present only in very thick skin; pale-staining layer of cells between
the stratum corneum and stratum granulosum in which the dying keratinocytes contain a
lot of keratin but are not completely replaced by it
•
Stratum granulosum - consists of keratinocytes containing large numbers of granules
that contribute to the process of keratinization
•
Stratum spinosum- consists of large, polyhedral keratinocytes that are actively
synthesizing keratin which is inserted as tonofibrils into the area of the plasma membrane
beneath desmosomes that connect adjacent cells together. These "connections" or
desmosomes between cells in this layer help hold them together and result in the "spiny"
appearance of the cells that gives this layer its name.
•
Stratum basale - consists of keratinocytes undergoing mitosis to produce the constant
supply of keratinocytes needed for replacement of the dead and dying cells in the more
superficial layers of the epidermis
Dermis
Two zones of the dermis:
•
•
papillary zone - consists of loose areolar connective tissue containing collagen and fine
elastic fibers; connects the epidermis to the thicker and denser reticular zone of the
dermis
reticular zone - contains dense, irregular and coarse collagen fibers and thick elastic
fibers interspersed with fibroblasts and blood vessels and nerves
Glands in the skin.
Several different types of glands are located in the dermis of the skin serving a variety of
functions.
•
Sebaceous glands
•
Apocrine sweat glands
35
•
Merocrine (= eccrine) sweat glands
Sebaceous Glands: The epithelium of this gland is an outgrowth of the external root sheath of
the hair follicle and the gland empties its oily product directly into the follicle itself. The glands
are of a branched acinar type and produce a lipid product called sebum that serves to reduce the
entry of microorganisms into the body through the skin, to lubricate the hair and preventing it
from drying out. The secretory cells die and become part of the product; a holocrine mode of
secretion. These glands are not found in hooves, foot pads, claws or horns.
Apocrine Sweat Glands: These glands are coiled, tubular glands with a large lumen and a duct
connecting it to an adjacent hair follicle. These glands secrete a viscid, milky product and are
analogous to odiferous glands of many mammals. Once thought to use the apocrine mode of
secretion, it is now known that their mode of secretion is more like that of the merocrine sweat
glands. These glands are the primary sweat gland of domestic animals and are especially
prominent in the horse.
Merocrine Sweat Glands: These glands are unbranched tubular in form and appear as a mass
of tubules in cross section. They are plentiful in the upper regions of the fatty hypodermis. They
secrete a watery product that is hypotonic to the plasma. It is the evaporation of this secretion on
the surface of the skin that aids in thermoregulation. These are sometimes called eccrine sweat
glands.
Hair
General structure of hair and associated structures:
•
•
•
•
•
Hair shaft: the part of the hair above the surface of the skin
Hair root: the part of the hair below the surface
Bulb: an enlarged, hollow portion at the base of the root
Hair papilla: projection of dermis into center of the bulb
Follicle: the indentation in the skin within which the root lies
Hair has three layers:
•
•
•
Cuticle: the outermost layer. Single layer of flattened, keratinized cells. Overlap like
shingles, with free edge distally.
Cortex: the thickest, intermediate layer. Consists of several layers of keratinized cells
containing hard keratin. If hair is colored, these cells contain pigment. Cells held together
by desmosomes.
Medulla: central core; loosely packed cuboidal cells.
The structure and organization of the cuticle and medulla cells are species-specific.
36
Hair follicles
The hair follicle is the structure that anchors the hair in the dermis and produces the hair itself. It
is composed of five layers of epithelial cells arranged concentrically. The inner three layers
form the hair shaft through a process of keratinzation while the outer two layers form the hair
sheath.
1.
2.
3.
4.
5.
cells in the innermost layer form the medulla of the hair or core of the hair shaft
cells in the next layer form the cortex that makes up most of the hair
cells in the third layer form the cuticle on the surface of the hair
cells in the fourth layer make up the internal root sheath
cells in the fifth or outermost layer form a layer called the external root sheath that does
not take part in hair formation
The external root sheath is separated from the surrounding connective tissue by a thick
basement membrane known as the glassy membrane.
Types of follicles
Hair follicles can be classified in two ways: based on their size, i.e., diameter and based on their
organization.
Based on size (diameter):
•
•
Primary hair follicle: large having sweat gland, sebaceous gland and arrector pili
muscle; ex. overcoat or guard hairs in dogs
Secondary hair follicle: smaller, lacking sweat glands and arrector pili muscle; ex.
underhair
Based on Organization:
•
•
Simple follicle: a single hair from one follicle
Compound follicle: cluster of several follicles with several hairs emerging from one
opening onto surface of skin
The Hoof
The equine foot includes the hoof, dermis, first, second and third phalanges and associated
structures. The hoof itself is the cornified layer of the epidermis, lacking the stratum granulosum
and stratum lucidum. It is important to understand the histology of the hoof because a disease
involving the epithelium of the foot, called laminitis, is the most devastating clinical disease of
the foot.
37
The peculiar histology of the hoof is formed from special relationships between the dermis (or
corium) and the overlying epidermis. In some places, the dermal papillae and epidermal pegs are
confluent forming apparent layers, i.e., they are laminar or consist of lamellae; in other places
they are more typical. It is this lamellar interaction between the epidermis and dermis that gives
the hoof its strength.
The wall of the hoof is that part of the hoof which is visible when the foot is on the ground, and
it can be divided into three layers. From outside to inside, they are the stratum externum
(tectorium), the stratum medium, and the stratum internum (lamellatum).
The Wall of the Hoof
•
•
•
The stratum externum or tectorium is an extension of the perioplic epidermis and is
composed of cornified eipithelial cells which appear as a soft, white, shiny material. This
tissue attaches the hoof to the epidermis of the skin of the foot.
The stratum medium or coronary epidermis is composed of prominent tubular and
intertubular horn, and this layer comprises the bulk of the wall of the hoof.
The stratum internum or stratum lamellatum is the epidermis in the laminar region.
Stratum Lamellum
This layer is made of nontubular horn which fuses with the stratum medium and helps hold the
wall to the foot. In this region, the dermal papillae and epidermal pegs form elongated ridges
oriented perpendicular to the ground. These ridges are formed from primary and secondary
laminae - the secondary laminae being oriented at close to a right angle to the primary laminae.
There are about 600 primary laminae and about 100-200 secondary laminae for each primary
lamina. This system of interdigitating primary and secondary laminae provides the tight bond
between the wall of the hoof and the underlying dermis. Thus, damage to the laminae leads to
disruption of this interdigitating system which results in separation of the hoof wall from the
dermis and phalanx beneath it.
Laminitis (acute laminar degeneration) is an inflammation of the laminae within the hoof. Many
pathophysiologic mechanisms are thought to cause laminitis, among them vasoconstriction
within the digit, perivascular edema, arteriovenous shunting of blood at the level of the coronary
band, venoconstriction and microthombosis. These lead to less than normal perfusion of blood to
the digit resulting in ischemia, edema and eventually necrosis of the laminae.
38
Male Reproductive System
The primary function of the male reproductive system is reproduction, which includes the
production of spermatozoa, the transportation of spermatozoa from the testes out of the male
body, the secretion by glands, and the placement of spermatozoa in the female reproductive
tract. Spermatozoa are produced in the testes then transported from the testes by a series of
ducts which become gradually larger and connect with the urethra of the penis. Various
accessory glands in the male system secrete materials which together with the spermatozoa
constitutes the semen. A secondary function of the male reproductive system is the production of
the male hormones which are responsible for the secondary sex characteristics of the male
animal.
The male reproductive system is composed of several distinct organs. These include the testes,
epididymis, deferent ducts, accessory glands, and the penis. The testis (plural, testes) is both an
exocrine organ (compound, coiled, tubular gland) producing cells, i.e., spermatozoa, and an
endocrine organ, secreting hormones, i.e., testosterone. Accessory glands (not all are present in
all species) include the ampullary glands, vesicular glands, prostate gland, bulbourethral gland
and urethral glands.
The testes are paired organs, and each one is enclosed in a fibrous white capsule of dense
connective tissue (tunica albuginea) containing blood vessels (the stratum vasculare). A layer of
peritoneum is tightly adhered to the tunica albuginea of each testis. The stallion has obvious
smooth muscle fibers in the capsule. The connective tissue of the capsule continues into the testis
on the posterior aspect as the mediastinum testis.
The dense connective tissue of the tunica albuginea is continuous with the loose areolar
connective tissue of the septuli testis (septa) which extend through the parenchyma of the testis
and divide it into lobules. Each lobule is composed of several seminiferous tubules (tubuli
contorti) and the surrounding connective tissue. Spermatogenesis (formation of spermatozoa)
occurs in the epithelial lining of the seminiferous tubules. The interstitium is composed of
loose connective tissue containing fibroblasts and Leydig cells (interstitial cells). Spermatozoa
produced in the seminiferous epithelium move through the lumen of the tubules to the tubuli
recti (straight tubes) which extend to a network of spaces in the mediastinum, the rete testis
(except in the stallion). Efferent ductules (ductuli efferentes) carry the spermatozoa from the
rete testis, then converge to form the ductus epididymis, a convoluted duct. The ductus
epididymis straightens and becomes the ductus deferens. In domestic mammals, testes are not
in a major body cavity, but are enclosed in the scrotum. Each testis is suspended at the end of a
tissue called the spermatic cord which contains the ductus deferens, the blood vessels, and the
nerves supplying the testis.
Testis. Each testis is composed of an exocrine part (seminiferous tubules) and an endocrine part
(interstitial or Leydig cells). The testis is divided into lobules by septa consisting of loose
areolar connective tissue. Several seminiferous tubules are found in each lobule, and interstitial
cells are found in the connective tissue septa surrounding the seminiferous tubules. The
seminiferous tubules are the exocrine portion of the testis producing and "excreting"
39
spermatozoa. These tubules are lined by a stratified epithelium that consists of the developing
spermatozoa and supporting cells (Sertoli cells).
Seminiferous tubules. The stratified epithelium of the seminiferous tubules is composed of
different stages of developing sperm cells. Spermatogonia are stem cells located near the
basement membrane of the tubule which proliferate by mitosis. Some of the progeny cells
differentiate into sperm and move away from the basement membrane toward the lumen of the
tubule. These differentiating cells first undergo meiosis then undergo a morphological change to
become spermatozoa. Some of the progeny cells undergo mitosis again to produce more
progeny cells providing a continuous source of stem cells for the production of spermatozoa.
Interstitium. The interstitial tissue of the testis consists of loose areolar connective tissue
containing numerous reticular fibers which serves to support the seminiferous tubules. The
interstitial cells (Leydig cells), located in this connective tissue, are responsible for the synthesis
and secretion of the steroid hormone testosterone.
Spermatogenesis: the process by which stem cells develop into mature spermatozoa. There are
three phases: (1) Spermatocytogenesis (Mitosis), (2) Meiosis, and (3) Spermiogenesis.
1. Spermatocytogenesis (also called Mitosis): Stem cells (Type A spermatogonia; singular =
spermatogonium) divide mitotically to replace themselves and to produce cells that begin
differentiation (Type B spermatogonia). Spermatogonia have spherical or oval nuclei, and rest
on the basement membrane. (You are not responsible for distinguishing between Type A and
Type B spermatogonia in lab.)
2. Meiosis: Cells in prophase of the first meiotic division are primary spermatocytes. They are
characterized by highly condensed chromosomes giving the nucleus a coarse chromatin pattern
and an intermediate position in the seminiferous epithelium. This is a long stage, so many
primary spermatocytes can be seen. Primary spermatocytes go through the first meiotic division
and become secondary spermatocytes. The cells quickly proceed through this stage and
complete the second meiotic division. Because this stage is short there are few secondary
spermatocytes to be seen in sections. You are not responsible for identifying secondary
spermatocytes in lab. Meiosis is the process by which the diploid number of chromosomes
present in spermatogonia (the stem cells) is reduced to the haploid number present in mature
spermatozoa.
The products of the second meiotic division are called spermatids. They are spherical cells with
interphase nuclei, positioned high in the epithelium. Since spermatids go through a
metamorphosis into spermatozoa, they occur in early through late stages. You are not
responsible for distinguishing the different stages of spermatids, but you are required to identify
a spermatid.
All of these progeny cells remain attached to each other by cytoplasmic bridges. The bridges
remain until sperm are fully differentiated.
3. Spermiogenesis:
This is the metamorphosis of spherical spermatids into elongated spermatozoa. No further
mitosis or meiosis occurs. During spermiogenesis, the acrosome forms, the flagellar apparatus
40
forms, and most excess cytoplasm (the residual body) is separated and left in the Sertoli cell.
Spermatozoa are released into the lumen of the seminiferous tubule. A small amount of excess
cytoplasm (the cytoplasmic droplet) is shed later in the epididymis.
Sertoli Cell & Developing Sperm Cells: an interaction
The Interaction At all stages of differentiation, the spermatogenic cells are in close contact
with Sertoli cells which are thought to provide structural and metabolic support to the developing
sperm cells. A single Sertoli cell extends from the basement membrane to the lumen of the
seminiferous tubule although its cytoplasm is difficult to distinguish at the light microscopic
level. They are characterized by the presence of a vesicular, oval, basally positioned nucleus
which contains a prominent nucleolus. The nuclear envelope often contains a definite fold. The
significance of the very close association of the two types of cells is unknown. Sertoli cells are
endocrine cells - they secrete the polypeptide hormone, inhibin. Inhibin acts at the level of the
pituitary to reduce the secretion of follicle stimulating hormone.
Blood-testis barrier. Large molecules cannot pass from the blood into the lumen of a
seminiferous tubule due to the presence of tight junctions between adjacent Sertoli cells. The
spermatogonia are in the basal compartment (deep to the level of the tight junctions) and the
more mature forms such as primary and secondary spermatocytes and spermatids are in the
adluminal compartment. The function of the blood-testis barrier (red highlight in diagram above)
may be to prevent an auto-immune reaction. Mature sperm (and their antigens) arise long after
immune tolerance is established; therefore, a male animal is capable of making antibodies
against his own sperm. Injection of sperm antigens causes inflammation of the testis
(autoimmune orchitis) and reduced fertility. Thus, the blood-testis barrier may reduce the
likelihood that sperm proteins will induce an immune response.
The Duct System
After production in the testes, spermatozoa pass through a series of ducts in their journey out of
the male system.
Tubuli recti, Rete Testis, Efferent ductules
The genital ducts are tubular organs in which the lamina epithelialis varies from the stratified
epithelium in the testes to a transitional epithelium in the urethra. In the terminal part of the
seminiferous tubules, the epithelium contains only Sertoli cells which gradually blends with the
squamous, cuboidal or columnar epithelium (species variation) of the tubuli recti and the rete
testis. These epithelial cells may actually represent a continuation of the Sertoli cells which line
the seminiferous tubules.
Epididymis
The ductus epididymis is lined with a pseudostratified stereociliated columnar epithelium.
Stereocilia are actually nonmotile, long microvilli which serve to increase the absorptive and/or
secretory surface of this epithelium. With its associated connective tissue and muscle, the ductus
epididymis coils to form the head, body and tail of the epididymis which then continues into the
41
ductus deferens. Spermatozoa are stored within the epididymis while they undergo maturation to
become mature sperm.
Ductus Deferens
The ductus epididymis continues as the ductus deferens which is also lined by a pseudostratified
columnar epithelium. However, the lamina propria submucosa of the ductus deferens is areolar
loose connective tissue, and the tunica muscularis is very thick and contains two layers of
smooth muscle. The tunica serosa is present and typical.
Urethra
The male urethra consists of two portions; the pelvic urethra and the penile urethra. Both
portions are lined with transitional epithelium, both contain erectile tissue, and both contain
(species variable) branched tubular mucous glands, the urethral glands. In the pelvic urethra, the
three layers of smooth muscle in the tunica muscularis near the bladder are replaced (or joined in
some species) by the striated urethral muscle. The tunica adventitia is present and typical.
The tunica muscularis is smooth muscle, and cavernous (corpus cavernosum urethra) tissue is
present in the connective tissue beneath the epithelium. In the penile urethra, the corpus
cavernosum penis is also present.
Accessory Glands
Ampullary, vesicular, prostate, bulbourethral, and urethral glands
The products of these glands serve to nourish and activate the spermatozoa, to clear the urethral
tract prior to ejaculation, serve as the vehicle of transport of the spermatozoa in the female tract,
and to plug the female tract after placement of spermatozoa to help ensure fertilization.
Although the glands are ususally described as being branched tubular or branched
tubuloalveolar, they vary in their organization and in their distribution in different species.
Ampullary Glands
This branched tubular gland lined by simple columnar epithelium is an enlargement of the ductus
deferens in its terminal portion. This is a typical tubular gland in ruminants, horses and dogs;
absent in the cat and poorly developed in boars. The function of the white serous secretion is not
known.
Vesicular Glands
The secretory endpieces of this gland are lined with simple columnar epithelium; the main ducts
are lined with stratified columnar epithelium. These glands do not occur in carnivores, but are
present in some form in horses, ruminants and swine. Seminal fuid, the product of this gland,
serves as a vehicle for the transport of spermatozoa.
Prostate Gland
42
Grossly the prostate gland can be divided into two parts: the body and the disseminate part. Low
cuboidal to low columnar epithelium provides the lining for this compound, tubuloalveolar gland
which consists primarily of serous secretory end pieces. The secretion of this gland is more
serous in dogs and more mucous in bulls. It serves to promote the movement of spermatozoa
and to form a vaginal plug. Additionally, in bulls, the secretion contains high amounts of
fructose and citric acid. Concretions may be present in the secretory end pieces as well as parts
of the duct system.
Bulbourethral Glands
The lining of these paired, compound, tubuloalveloar glands is simple columnar epithelium. A
capsule of dense connective tissue contains some smooth muscle as well as skeletal muscle of
the bulbocavernous and urethral muscles. All domestic species have these glands except the dog,
and their mucous secretion serves to clear the urethra of urine and to lubricate it and the vagina.
The product may also serve as an energy source for the spermatozoa.
Urethral Glands
In some species, branched tubular mucous glands are found along the length of the urethra,
especially dorsal to the lumen of the urethra. The exact function of their product is not clear.
Penis: the copulatory organ
The penis provides an outlet for both urine and the copulatory ejaculate (spermatozoa and
semen). The histology and gross anatomy of the penis varies dramatically from species to species
and from region to region within the same species. In general, the body of the penis consists of
the urethra, erectile tissue (corpora cavernosa penis and corpora cavernosum urethra), smooth
and skeletal muscle, touch and pressure receptors (Pacinian corpuscles) and a dense connective
tissue capsule (tunica albuginea).
Erectile tissue and the erectile mechanism. The erectile tissue is composed of dense irregular
connective tissue which contains numerous elastic fibers and sinuses. Under stimulation, the
primary blood supply of the penis is directed through helicine arteries which open into the
venous sinuses. During erection, these vessels and the sinuses become engorged with blood,
and the thin-walled veins beneath the tunica albuginea are effectively closed, further increasing
the rigidity of the organ. Because the capsule around the erectile tissue of the corpus cavernosum
urethra is not as thick as that around the corpus cavernosum penis, the urethra is not occluded
during erection. After ejaculation, the helicine arteries contract and regain their normal tone
resulting in a relaxing of the pressure around the veins which leads to the restoration of normal
bloo
d flow to the region.
43
Female Reproductive System
The primary function of the female reproductive system is reproduction, which includes
the production of ova
the transportation of ova from the ovary to the site of fertilization
transportation of spermatozoa from the point of deposition in the female tract to the site
of fertilization
nourishment of the developing embryo and fetus
parturition and nourishment of the infant
Included in the reproductive function of this system is the production of the female hormones
which are responsible for the secondary sex characteristics of the female animal as well as the
development of follicles in the ovary, ovulation, preparation of the uterus for implantation of an
embryo, maintenance of pregnancy, parturition and preparation of the mammary glands for milk
production.
Structure
The female reproductive system is composed of several distinct organs. These include the paired
ovaries, paired uterine tubes, uterus (uterine horns), cervix, vagina, and the mammary glands.
The ovaries are both an exocrine organ producing cells, i.e., ova, and an endocrine organ,
secreting hormones, i.e., estrogen and progesterone. Note: in domestic animals the oviducts are
usually called uterine tubes and the uterus is called uterine horns due to the structure of these
organs.
Ova are produced in the ovaries then transported from the ovaries to the site of fertilization in the
upper part of the uterine tube. Sperm are transported from the site of deposition near the vagina
and uterus to the site of fertilization. If fertilization occurs, the uterus serves to nourish the
developing embryo and fetus until the time of parturition. The vagina receives the male
copulatory organ, the penis, during copulation and is the birth canal for the infant during
parturition. Mammary glands serve to nourish the infant.
Although all mammals have the same basic organs, their individual structure and association
with each other varies according to species. The structure of the uterine tubes and uterus are
especially variable.
Function
The ovary, or female gonad, is:
1. an exocrine gland, producing ova
2. an endocrine gland, secreting
44
a. the female hormones estrogen and progesterone androgens, typically considered male
hormones
The surface of the ovary is covered with surface epithelium, a simple epithelium which changes
from squamous to cuboidal with age. Immediately beneath this surface epithelium there is a
dense connective tissue sheath, the tunica albuginea ovarii.
In most species, the ovaries are composed of an outer cortex and inner medulla (except in the
mare where the cortical region is interior to the medulla). The cortex is composed of ovarian
follicles (developing oocytes with their associated follicular cells), interstitial gland cells and
stromal elements. Ovarian follicles are in different stages of development (least mature to most
mature): primordial, primary, secondary, secondary-vesicular and mature.
The cortex usually also contains the remains of degenerated follicles called atretic follicles
which may arise at any stage of follicular development.
Interstitial gland cells are also present in the cortex. Although the function of these cells is not
known for sure, they are thought to secrete estrogen since they have the structure of steroidsecreting cells.
The atretic follicles and the interstitial gland cells, though not shown on this diagram, will be
discussed later.
The medulla is composed of loose areolar connective tissue containing numerous elastic and
reticular fibers, large blood vessels, nerves and lymphatics.
The hilus is the region through which blood vessels, lymphatics and nerves enter and leave the
ovary. It is contiguous with and histologically similar to the medulla.
Oogenesis: the production of female gametes, the ova
1. Early in embryogenesis, primordial germ cells migrate from the yolk sac endoderm to the
genital ridge (developing ovary) where they take up residence and are called oogonia.
2. These diploid oogonia undergo several mitotic divisions prior to or shortly after parturition,
thus providing the developing ovary with a large supply of future ova (eggs).
3. When oogonia begin the first meiotic division, they are called primary oocytes.
4. Primary oocytes are arrested in prophase of Meiosis I until the female reaches sexual maturity.
They grow in size during this arrested phase, but do not divide. A human female is born with
about 2 million primary oocytes in her ovaries, but by the time of puberty only about 400,000 are
left due to atresia (degeneration).
When the female reaches sexual maturity and under the influence of follicle stimulating hormone
(FSH), a small number of primary oocytes are stimulated to continue through Meiosis I.
45
6. During this process the number of chromosomes is reduced from the diploid number (2N) to
the haploid number (1N).
7. This division is uneven in that although the chromosomes are divided equally, most of the
cytoplasm stays with the oocyte. The smaller polar body contains half the chromosomes but
only a small amount of cytoplasm and will eventually degenerate.
8. After a primary oocyte completes the first meiotic division, it is called a secondary oocyte
(1N). In most species Meiosis I is completed just before ovulation (release of the ovum from the
ovary). However, in horses and dogs Meiosis I is completed after ovulation.
9. If a secondary oocyte is not penetrated by a sperm, it will degenerate.
10. If fertilization and pregnancy do not occur, a new cycle will begin in which FSH from the
pituitary gland will stimulate a few more primary oocytes to continue through Meiosis I. 11. The
process is the same as previously described and a secondary oocyte is formed.
12. However, some of the time a sperm will penetrate the zona pellucida and the secondary
oocyte is stimulated to continue through Meiosis II, forming a second polar body and a mature
ovum (1N). Again, the polar body contains half of the chromosome material, but little
cytoplasm, and it eventually degenerates.
13. After a sperm enters the cytoplasm of the ovum, two pronuclei form, containing genetic
material from the ovum or the sperm.
14. Fertilization is complete when the two pronuclei fuse and restore the diploid chromosome
number.
15. If fertilization is completed, the zygote undergoes several mitotic changes to become an
embryo; otherwise it degenerates.
There are three stages in the development of follicles:
1. pre-ovulation
2. ovulation
3. post-ovulation.
The pars distalis of the pituitary gland (hypophysis) controls the process of follicular
development and maturation by the secretion of:
1. gonadotropic hormones FSH (follicle stimulating hormone)
2. LH (luteinizing hormone)
3. prolactin (in some species).
The hormonal interactions will be explained in the discussion of the ovarian cycle.
Pre-ovulation: Development of Follicles in the Ovary.
46
The primordial (quiescent) follicle consists of a primary oocyte and a single layer of flattened
follicular cells. As the follicle develops, alterations occur in the primary oocyte and the
surrounding follicular cells. The primary oocyte produces yolk granules and the follicular cells
change from flattened to cuboidal or columnar.
The primary follicle consists of a primary oocyte with a single layer of cuboidal/columnar
follicular cells. As development proceeds, the number of follicular cells increases by mitosis
forming several layers around the primary oocyte. As these cells enlarge they release steroid
hormones called estrogens of which estradiol is the dominant one prior to ovulation. During
each cycle, a few primary follicles will continue to develop into secondary follicles.
The secondary follicle consists of several layers of cuboidal/columnar follicular cells, now
collectively called the membrana granulosa which begin to secrete follicular fluid. A thick,
amorphous layer, the zona pellucida, forms between the primary oocyte and the membrana
granulosa. Previously undifferentiated stromal cells now develop into two distinct layers around
the developing follicle: the theca interna and the theca externa . Cells in the theca interna are
large, rounded and epithelial-like; cells in the theca externa are smaller, fibroblasts. Both layers
of theca cells are separated from the membrana granulosa cells of the follicle by a basement
membrane. As the follicular fluid secreted by the membrana graulosa cells accumulates, small
pockets of fluid between granulosa cells begin to appear. Usually in human females only one
secondary follicle will continue to develop.
The secondary-vesicular follicle is characterized by the presence of pockets of follicular fluid
within the membrana granulosa. As the follicle continues to develop, the separate pockets fuse to
form one large pocket of fluid called the follicular antrum.
During this development of the follicular antrum, the oocyte is still a primary oocyte, arrested in
prophase of Meiosis I. It is still surrounded by granulosa cells which are contiguous with the
membrana granulosa present around the periphery of the growing follicle.
Two regions of cells can be identified in the layer of granulosa cells surrounding the oocyte:
1. the corona radiata contains granulosa cells which remain attached to the oocyte after
ovulation and are in close contact with the oocyte through cytoplasmic processes which
pass through the zona pellucida and contact microvilli of the oocyte;
2. the cumulus oophorus contains granulosa cells which surround the oocyte and are
continuous with the displaced cells of the membrana granulosa but remains in the ovary
after ovulation.
The other granulosa cells form a layer around the periphery of the follicle and are separated from
the theca interna cells by a distinct basement membrane.
The mature follicle, sometimes called the pre-ovulatory follicle, has all of the components of the
secondary-vesicular follicle but is much larger and contains one single large antrum of follicular
fluid. These follicles are very large and usually extend from the deepest parts of the cortex and
protrude from the surface of the ovary. In some species just before ovulation, the primary oocyte
in the mature follicle completes meiosis I producing a secondary oocyte and a polar body.
47
Pre-ovulation: Development of Theca Cells in the Ovary.
As the oocyte and follicular (granulosa) cells are growing and developing in the ovary, the
stromal cells differentiate and develop into the theca interna and theca externa cells. As a
follicle goes from a primary to a secondary follicle, the stromal cells immediately surrounding
the follicle differentiate into the theca folliculi. The cells closest to the follicle become the
theca interna cells, round, foamy cells that secrete androgens, including testosterone. These
two “male” hormones are converted by the granulosa cells to estrogens. The stomal cells farther
away from the developing follicle become the theca externa cells, fibroblast-like cells arranged
around the follicle outside the theca interna cells.
Post-ovulation
1. corpus hemorrhagicum After ovulation, hemorrhage into the remains of the follicle usually
occurs resulting in a structure called a corpus hemorrhagicum. This transitory structure develops
into a corpus luteum.
2. corpus luteum (yellow body) In most species LH from the pituitary gland initiates this
luteinization and stimulates the granulosa cells to secrete progesterone. The granulosa cells
undergo hyperplasia (proliferation), hypertrophy (enlargement) and are transformed into
granulosa lutein cells. In several species, including the human, the accumulation of a yellow
lipid pigment (lutein) and other lipids marks the transition to granulosa lutein cells. The cells of
the theca interna are also transformed into lipid-forming cells called theca lutein cells. The
resulting structure is highly vascular. If fertilization occurs, the corpus luteum persists and
secretes progesterone.
3. orpus albicans If fertilization does not occur, the corpus luteum degenerates and is replaced
by connective tissue forming a corpus albicans.
The Ovarian Cycle
Primordial follicles in the cortex of the ovary are stimulated by FSH (follicle stimulating
hormone) secreted by the pars distalis of the pituitary gland. FSH stimulates the development of
one or more primordial follicles in the ovary to begin the development into a mature (Graafian)
follicle ready for ovulation. A surge of LH from the pars distalis initiates ovulation and induces
luteinization of the granulosa and theca cells of the ruptured follicle. The oocyte with its
surrounding corona radiata and cumulus cells moves into the uterine tubes while the ruptured
follicle left behind becomes a corpus hemorrhagicum and under the influence of LH develops
into a corpus luteum. If fertilization and implantation occur, the corpus luteum persists as the
corpus luteum of pregnancy, but if fertilization does not occur, the corpus luteum degenerates
into a corpus albicans which remains as a scar in the ovary.
The Uterine Tubes
Function The uterine tubes (also called Fallopian tubes or oviducts):
1. transport the ovum from the ovary to the site of fertilization
48
2. help transport spermatozoa, the haploid male gametes, from the site of deposition to the
site of fertilization
3. provide an appropriate environment for fertilization
4. transport the fertilized ovum (embryo) to the uterine horns where implantation and
further development may occur.
Structure The uterine tubes can be divided into three major parts:
1. the infundibulum
2. the ampulla
3. the isthmus
The infundibulum is the region most proximal to the ovary. It is funnel-shaped and has fingerlike projections called fimbriae that extend into the pelvic cavity and make close contact with the
ovaries. The tunica mucosa occupies most of the thickness of the wall of the organ.
The ampulla is the middle, one-third region in which fertilization usually occurs. Histologically
it is very similar to the infundibulum having a very thick tunica mucosa and relatively thick
tunica muscularis.
The thick-walled isthmus is the lower one-third region most proximal to the uterine horns. The
smooth muscle in the wall of the isthmus helps propel (by peristalsis) the fertilized ovum toward
the uterine horns and body of the uterus where implantation occurs. The tunica muscularis is the
thickest part of the wall and the tunica submucosa is very thin as in the infundibulum and
ampulla.
About the time of ovulation, the infundibulum, closest to the ovary, moves to cover the site of
rupture of the mature (Graafian) follicle.
The ovum moves down the infundibulum of the uterine tube toward the ampulla, assisted by
peristaltic contractions of the smooth muscle in the wall of the tube as well as fluid moved by
ciliated epithelial cells in the mucosa of the tube. The ampulla is usually the site of
fertilization.
After fertilization, the embryo moves down through the isthmus which connects the uterine tube
with the uterine horns or uterus. The thick muscular wall of the isthmus of the uterine tube helps
propel the embryo into the uterus where it can be nourished during further development.
Regional Variations:
The uterine tubes are paired tubular organs with the typical organization of a tubular organ, i.e.,
four tunics consisting of:
1.
2.
3.
4.
tunica mucosa
tunica submucosa
tunica muscularis
tunica serosa.
49
The thickness and specific characteristics of these tunics varies with the region of the uterine
tube. The tunica mucosa of the infundibulum helps capture the ovum from the surface of the
ovary, bathes it in a supportive fluid and helps move it toward the uterus. The tunica mucosa of
the ampulla provides the proper environment for fertilization. Consequently, in the infundibulum
and ampulla the tunica mucosa is thick and highly developed.
It is the tunica muscularis of the isthmus that provides the strong contractions that at the right
time propel the ovum or embryo into the uterine horns.
As a result, in the isthmus the tunica mucosa is reduced in thickness and the tunica muscularis
is much thicker
Histology:
The lamina epithelialis of the tunica mucosa of the uterine tubes is an intermittently ciliated
columnar epithelium that contains two types of cells: a ciliated cell and a non-ciliated, secretory
cell. In the cow and sow the lamina epithelialis may be pseudostratified intermittently ciliated
columnar. The secretory product of the non-ciliated, secretory cells is moved toward the uterine
horns by the movement of the cilia on the ciliated cells. This secretion probably also protects
and nourishes the ovum.
The lamina propria consists of a typical loose areolar connective tissue without glands, and it
blends with the underlying, thin tunica submucosa. There is no lamina muscularis mucosae in
the entire female reproductive tract. The tunica muscularis is sparse in the infundibulum and the
ampulla but thick in the isthmus consisting of an inner circular and an outer longitudinal layer of
smooth muscle. The tunica serosa is typical containing many blood vessels in a distinct vascular
layer.
Cyclic Changes in the Epithelium : Under the influence of estrogen, the ciliated epithelial cells
increase in height and in the number of cilia. Under the influence of progesterone, these cells
decrease in height and in the number of cilia. These cells are at their tallest with the most
numerous cilia at the time of ovulation. Their main function is to assist in the movement of the
ovum toward the site of fertilization and the embryo toward the uterus. This action is secondary
to the peristaltic movement of the isthmus region.
Clinical: The uterine tubes are the site of tubal ectopic pregnancies. They can also be the site of
bacterial infection which can lead to Pelvic Inflammatory Disease, a major cause of infertility in
women.
The Uterus
Functions
1. serves to receive the sperm in mares
2. transports sperm from site of deposition to uterine tubes for fertilization
3. provides suitable environment for
a. implantation of the embryo
b. nourishment of the embryo & fetus during pregnancy
50
4. provides mechanical protection of the fetus
5. expels the mature fetus at the end of pregnancy
In the fundus and body of the uterus, the wall is divided into the
1. endometrium = tunica mucosa and tunica submucosa
2. myometrium = tunica muscularis
3. perimetrium= tunica serosa
These are terms specific to the uterus that apply to the typical tunics of a tubular organ.
The endometrium comprises the tunica mucosa and the tunica submucosa of the uterus. In the
tunica mucosa the lamina epithelialis is usually simple columnar except in the sow and ruminants
where it may be pseudostratified columnar. The lamina propria consists of loose connective
tissue full of neutrophils and lymphocytes. It blends with the underlying tunica submucosa since
there is no lamina muscularis mucosae in the entire female reproductive tract. Uterine glands are
simple or branched tubular glands located in the lamina propria-tunica submucosa. Some regions
of the endometrium in ruminants are void of glands and are highly vascular. It is in these
regions, called caruncles, that contacts between the uterus and the extraembryonic membranes
are made.
The myometrium is the tunica muscularis of the uterus. It is composed of a thick inner circular
layer and a thinner outer longitudinal layer of smooth muscle. The region in between the two
layers of smooth muscle contains large blood vessels.
The perimetrium is the tunica serosa of the uterus. It has the typical composition of loose
connective tissue, but contains a large number of lymphatic vessels.
The stratum vasculare is a layer of large blood vessels located between the inner and outer
layers of smooth muscle of the myometrium.
In the sow the stratum vasculare is indistinct and in the cow it may be located in the outer half of
the circular muscle layer.
Glands
Uterine glands are simple or branched tubular glands, and may be coiled distally. Distal portions
of the glands are in the lamina propria/tunica submucosa. The glands secrete mucus, glycogen,
proteins, and lipids. The remainder of the endometrial tissue is loose connective tissue.
The Uterus: Changes in the Uterus during the Estrous Cycle
Domestic animals are either monestrous or polyestrous. Monestrous animals such as bitches
have one estrous cycle per year; each cycle is followed by a long anestrous period. Polyestrous
animals are either continuously cycling without an anestrous period such as cows and sows or
seasonally cycling with an anestrous period such as mares, ewes, goats and queens. The uterine
wall, especially the endometrium, undergoes greater changes in the monestrous animal than in
the polyestrous animal but changes can be clearly seen in polyestrous animals.
51
During the estrous cycle the uterus undergoes changes controlled by hormones secreted by the
ovary. The changes are most pronounced in the glands in the tunica submucosa of the
endometrium and in the smooth muscle and stratum vasculare located in the tunica muscularis.
The Chart and Diagrams below allow you to see the differences in the uterine wall between
Proestrus, early in the estrous cycle, and Diestrus, later in the estrous cycle. First read through
the text and view the associated images, then review the material by comparing the appearance
of the uterus in proestrus versus diestrus by using the colored bars below the text.
Proestrus
1.
2.
3.
4.
5.
endometrial glands at lowest level of size & secretion
estrogen rises from granulosa cells in the developing follicles of the ovary
uterine epithelial cells hypertrophy
uterine glands proliferate
vascular supply increases
Estrus
1. after ovulation progesterone rises from
· granulosa & theca lutein cells in the corpus hemorrhagicum
· granulosa & theca lutein cells in the corpus luteum
2. uterine epithelial cells continue to hypertrophy
3. uterine glands continue to proliferate
Metestrus
1. progesterone levels remain high
2. uterine glands undergo hyperplasia & coiling
3. uterine glandular secretory activity high
Diestrus
1. endometrial glands at maximum size, coiling & secretion
2. normal state (no fertilization)
· secretory activity is arrested
· lining cells and glands begin to involute
· vascularity of the entire wall decreases
fertilized state
· secretory activity is maintained
(in order to nourish embryo/fetus during pregnancy)
3. the wall of the uterus returns to a state that resembles that of the Proestrus uterus
52
Vagina
Histology:
The epithelium of the vagina is stratified squamous, usually nonglandular. It increases in
thickness during proestrus and estrus. In some species (especially rodent and carnivores), the
epithelium keratinizes during estrus.
The tunica mucosa and submucosa are highly folded.
Lymphocytes, and lymphatic nodules can be found in the connective tissue.
The cranial portion of the vagina has a tunica serosa; the larger caudal portion has a tunica
adventitia.
Tunica mucosa: lamina epithelialis of stratified squamous epithelium which is nonglandular;
highly folded; lamina propria is loose connective which blends with the denser connective tissue
of the tunica submucosa since a lamina muscularis mucosae is not present; the tunica mucosa is
thin prior to the onset of puberty and in old age; thickens under the influence of estrogens during
the reproductive years; superficial cells accumulate glycogen which is maximum at the time of
ovulation. The acid pH (app. 3.0) in the vagina is due to the breakdown of this glycogen by
commensal lactobacilli which produce lactic acid. Thus, only acid-loving bacteria and fungi can
exist in this low pH environment, thus detering bacterial pathogens and fungi such as Candida
albicans.
Tunica submucosa: highly folded
Tunica muscularis: composed of two-three layers of smooth muscle
Tunica serosa: present cranially then turns into a tunica adventitia caudally
53
Endocrine System
Endocrine organs are organs whose cells secrete their products, i.e., hormones, into the
bloodstream whereas exocrine organs such as sweat glands, salivary glands and
sebaceous glands secrete their products into a duct system. Hormones travel via the
blood circulation and when they reach their "target organ" they exert their specific
effect. Some organs are primarily exocrine while some are primarily endocrine and
some contain elements of both. The ovary and testes are both exocrine organs,
"secreting" ova and spermatozoa, respectively yet they are endocrine organs as well
secreting hormones such as estrogen, progesterone and testosterone. In the pancreas,
part of the organ is an exocrine gland (the acini) secreting digestive enzymes and part
is endocrine , i.e., the islets of Langerhans which secrete various hormones such as
insulin and glucagon. In the digestive system, some cells are endocrine cells, i.e., the
enteroendocrine cells.
Other endocrine organs include the thyroid, parathyroid, adrenal, and pituitary gland.
The Thyroid Gland
Gross Anatomy
The thyroid gland is located dorsolateral to the trachea, close to the larynx. It has two
lobes that are connected by a narrow isthmus.
II. Histology
The thyroid gland is composed of follicles and interfollicular connective tissue. The
capsule, classified as loose areolar connective tissue, surrounds the mass of thyroid
54
follicles and sends smaller pieces of connective tissue into the gland to surround the
individual thyroid follicles.
Near the thyroid gland and embedded in the same connective tissue capsule is the
parathyroid gland.
Sometimes patches of lymphocytes can be observed in the thyroid/parathyroid glands.
Thyroid follicles consist of a layer of simple epithelium surrounding a gel-like pinkish
material called colloid.
The principal cell is the most numerous cell present in the simple epithelial layer and
is responsible for secreting the thyroid hormones as well as thyroglobulin, a
glycoprotein.
Thyroid hormones are stored extracellularly as part of the thyroglobulin which is the
main component of the colloid.
The size of follicles and the height of principal cells varies even within one section of the
gland. Squamous principal cells indicate a relatively inactive gland whereas cuboidal to
columnar cells indicate more activity in removing the hormone from the stored form.
In addition to principal cells there is another type of functional cell in the thyroid gland.
This is the parafollicular cell which may be found as single cells in the epithelial
lining of the follicle or in groups in the connective tissue between follicles. They usually
appear as large, clear cells since they do not stain well with hematoxylin and eosin.
They are sometimes called parafollicular cells based on their location and clear cells
(C cells) based on their appearance of their cytoplasm. Parafollicular cells secrete
calcitonin, a hormone that lowers the level of calcium in the blood.
III. Function
•
•
secretes the thyroid hormones tri-iodothyronine (T3 ) and tetra-iodothyronine (T4
or thyroxin) that help to regulate the metabolic rate
also secretes calcitonin that helps control blood calcium concentration
IV. Mechanism of Secretion of T3 and T4 (thyroxin).
•
•
Under the influence of increased TSH from the pituitary gland, principal cells
concentrate iodine by active transport. At the same time they synthesize
thyroglobulin and secrete it into the lumen of the thyroid follicle
The iodination reaction, catalyzed by the enzyme peroxidase, is carried out on
the large thyroglobulin molecule at the luminal surface of the principal cell.
Various combinations of iodinated and non-iodinated tyrosine are possible. If the
two molecules of tyrosine are both fully iodinated, the hormone resulting upon
55
•
cleavage is T4 but if one of the tyrosines has only one iodine, then the hormone
that results is T3. In the circulation T4 is converted to T3 which appears to be
the active form of the hormone.
Under the influence of rising TSH levels, the principal cells take up colloid by
pinocytosis, the vesicles fuse with lysosomes which hydrolyze throglobulin
releasing T3 and T4 (thyroxin) which diffuse into the blood and lymph
V. Parafollicular Cells
•
•
Secrete calcitonin which inhibits osteoclasts from resorbing bone resulting in
decrease in calcium in the blood
Controlled by the level of calcium in the blood
PARATHYROID GLAND
I. Gross Anatomy
The parathyroid gland is difficult to see at the gross level. It is very close to and usually
embedded within the capsule of the thryroid gland.
II. Histology
There are three types of cells in the parathyroid gland: adipocytes, chief cells and
oxyphil cells. A reticular connective tissue framework surrounds and supports these
cells.
The main secretory cell is the chief cell. These cells secrete parathyroid hormone.
Unfortunately these cells have no distinguishing features.
Another cell type present is the oxyphil cell in the human, ox and horse. These are
large cells that contain numerous mitochondria. Their function is unknown.
III. Function
The parathryoid gland secretes parathyroid hormone which is essential for
regulating the levels of calcium and phosphate in the blood. Parathyroid hormone acts
on the following target organs.
•
•
•
Bone: increases blood calcium by inhibiting osteoblast deposition of calcium
and stimulating osteoclast removal of calcium.
Kidney: increases blood calcium by increasing calcium ion reabsorption by
kidney tubular cells; inhibits reabsorption of phosphate ion from the glomerular
filtrate
Small intestine: increases the absorption of calcium from the small intestine
IV. Summary of hormonal control of blood calcium levels through action on
bone.
56
Calcitonin As the level of calcium in the blood rises, the amount of calcitonin
secreted by the C cells of the thyroid increases. Calcitonin stimulates osteoblasts to
form bone taking calcium out of the circulation. At the same time, calcitonin inhibits
the mobilization of bone (and calcium) by osteoclasts. The end result is a decrease in
the level of calcium in the blood thus helping to maintain proper blood calcium levels.
Parathyroid Hormone A decrease in the normal levels of calcium in the blood causes
the chief cells of the parathyroid gland to secrete more parathyroid hormone which
stimulates osteoclasts to mobilize bone resulting in an increase in the level of calcium in
the blood. Parathyroid hormone also increases Ca ion reabsorption in the kidney and
decreases the reabsorption of phosphate ions.
Note: Whereas calcitonin is important in regulating the level of calcium in the
blood, parathyroid hormone is essential!!
ADRENAL GLAND
I. Gross Anatomy The adrenal glands are located at the cranial end of the kidneys.
They are flat organs embedded in fat. Each gland has an outer cortex that appears
yellow in fresh tissue and an inner medulla that appears gray in fresh tissue.
II. Histology of the Adrenal Gland and Adrenal Cortex
The adrenal gland is surrounded on the surface by a connective tissue capsule. This
capsule has projections into the cortex and through the cortex down into the medulla in
some species.
In most species 4 cortical zones can be identified. From the zone nearest the capsule
these are:
•
•
•
Zona glomerulosa
o
In ruminants this zone consists of cells in clusters.
o
In carnivores, horses and pigs this zone consists of columnar cells
in arches and is sometimes called a zona arcuata.
Zona intermedia: This zone is relatively thin and contains mostly
undifferentiated cells.
Zona fasciculata This is the largest zone containing large, round, foamyappearing cells arranged in cords that radiate from the zona glomerulosa down
toward the medulla.
o
Sinusoidal capillaries are located between cords of cells that are
one cell thick.
57
o
•
Cells in the zona fasiculata have a foamy appearance due to the
presence of many lipid droplets prior to processing for microscopy.
These lipid droplets represent the precursors for steroid hormones.
Zona reticularis
o
Cells of the zona reticularis are arranged in a network of cords no
longer arranged in parallel as in the zona fasiculata.
III. Function of the Adrenal Cortex
•
Zona glomerulosa cells secrete mineralocorticoids (principal one is aldosterone).
These steroid hormones act on the kidney to:
o
o
•
Increase Na+ reabsorption
Increase K+ secretion
Zona fasciculata and reticularis cells secrete glucocorticoids (e.g., cortisol,
cortisone, corticosterone) which control glucose metabolism.
IV. Histology of the Adrenal Medulla
•
•
•
•
Cells in the medulla are arranged in groups or cords, clustered around capillaries
and venules.
The cells have secretory granules which contain either epinephrine or
norepinephrine.
When fixed in potassium bichromate, the medullary cells become brown.
Therefore, they are called chromaffin cells. The color is the result of a reaction
between chromate and epinephrine or norepinephrine.
Chromaffin cells are derived from neural crest cells. They are innervated by
preganglionic sympathetic fibers. They release hormone by exocytosis when
stimulated by those fibers.
V. Function of the Adrenal Medulla
•
Secretes catecholamines (epinephrine and norepineprine).
VI. Vasculature of the adrenal gland.
58
Blood is supplied to the adrenal gland via the suprarenal arteries. These arteries
penetrate the capsule and form a plexus just beneath it. From this plexus, blood is
further supplied via two different routes.
Cortical arteries of the subcapsular plexus branch into sinusoids in the cortex which
intimately surround cells in the zona fasciculata and zona reticularis. These sinusoids
drain into venules that empty into the central vein of the medulla. This blood supply
provides a mechanism by which cells in the cortex can influence cells in the medulla.
The second route of blood supply is more direct to the medulla. In this case, cortical
arteries run along trabecular branches of the connective tissue capsule directly into the
medulla without forming capillaries or sinusoids in the cortex. These arteries then
branch into capillaries in the medulla supplying the chromaffin cells with oxygenated
blood. These capillaries join the same small medullary venules as the sinusoids of the
cortex and all the blood flows into the central vein of the medulla. Because of this
peculiar blood supply to the adrenal gland, the cortex has no veins.
PITUITARY GLAND (HYPOPHYSIS)
The pituitary gland or hypophysis is located at the base of the brain. Its function
reflects its development from two different types of ectoderm, i.e., oral ectoderm which
forms Rathke's pouch and neural ectoderm from the base of the diencephalon which
forms the infundibulum. As developments proceeds, the ectoderm of the infundibulum
grows downward and wraps around Rathke's pouch. In the adult the infundibulum
forms the infundibular stalk and the pars nervosa; the ectoderm of Rathke's pouch
forms the pars distalis and the pars intermedia. Some of the oral ectoderm remains
associated with the infundibular stalk to form in the adult the pars tuberalis. A vestige
of Rathke's pouch often remains visible in the adults of many species and forms a cleft
that serves to distinguish the anterior lobe of the pituitary from the posterior lobe
(useful in humans but not in domestic species).
The gland remains "connected" to the brain via both vascular and nervous routes (see
below under Function). These relationships with the hypothalamus of the brain provide
a basis for understanding the different functions of the pituitary gland.
Vascular Connection: The vascular connection provides the mechanism by which
factors secreted by neuroendocrine cells in the hypothalamus reach and affect secretory
cells in the pars distalis. These secretory cells respond to the "releasing factors"
brought to them through the portal blood vessels.
Neural Connection: The neural connection provides the mechanism by which
hormones secreted by neuroendocrine cells located in specific nuclei in the brain are
actually released into the bloodstream at the level of the pars nervosa of the pituitary
gland.
II. Histology
59
The pituitary gland can be divided into various regions based on structure and
function.
One way to describe the pituitary gland is by the type of tissue present in different
regions. If this system is used then regions are as follows:
Adenohypophysis - based on grouping of all regions composed of glandular tissue
This includes the...
•
•
•
pars distalis
pars intermedia
pars tuberalis
Neurohypophysis - based on grouping of all regions composed of neural or
neurosecretory tissue
This includes the . . .
•
•
•
median eminence (not shown)
infundibular stalk
pars nervosa ( infundibular process)
Another way to describe the pituitary gland is using the terms anterior lobe and
posterior lobe (in domestic species, ventral lobe and dorsal lobe) as follows . . .
Anterior or Ventral Lobe
This includes the . . .
•
•
pars distalis
pars intermedia
Posterior or Dorsal Lobe
This includes the . . .
•
pars nervosa
The Adenohypophysis
Pars distalis: This region of the pituitary gland is organized as cords or clusters of cells
supported by a reticular connective tissue. With routine staining two types of cells can
be observed: (1) chromophiles which stain readily and are either red (acidophiles), blue
or purple (basophiles) depending on the type of secretory material present, and (2)
chromophobes which do not take up the stain and thus appear unstained or rather
clear. Chromophobes may be chromophiles that have lost their secretory granules or
chromophiles that have not accumulated large numbers of secretory granules. Use of
specific antibodies against the protein secretory products has allowed the identification
of the different cells. The cells of the par distalis are:
60
•
•
•
•
•
Somatotrophs secrete growth hormone.
Mammotrophs secrete prolactin.
Corticotrophs secrete ACTH.
Thyrotrophs secrete TSH.
Gonadotrophs secrete FSH and LH.
Pars intermedia: With routine histological staining, the cells in the pars intermedia stain
blue-purple and thus are basophilic.
•
Cells secrete ACTH, MSH, endorphins and lipotrophins.
Pars tuberalis: This region is an extension of the glandular pituitary gland and its cells
resemble those of the pars intermedia and pars distalis. The specific function of the
cells in the pars tuberalis, however, is not clear.
The Neurohypophysis
Pars nervosa: This region consists of unmyelinated nerve axons (cell bodies are in the
hypothalamus) and supportive cells called pituicytes.
•
•
Secretes ADH (antidiuretic hormone) which is synthesized by neurons in the
supraoptic nucleus of the hypothalamus.
Secretes vasopressin which is synthesized by neurons in the paraventricular
nucleus of the hypothalamus.
III. Function - Control of Secretion
Adenohypophysis.
The cells in the adenohypophysis secrete two classes of hormones: (1) direct acting and
(2) trophic. Direct acting hormones include growth hormone (GH) and prolactin from
the pars distalis, and melanocyte stimulating hormone (MSH) from the pars intermedia.
Trophic hormones include adrenocorticotrophic hormone (ACTH), thyroid stimulating
hormone (TSH), follicle stimulating hormone (FSH) and luteinizing hormone (LH).
Secretion of these hormones is controlled by specific releasing hormones in the
hypothalamus. Most of the releasing hormones are stimulatory in their action except
for the one for prolactin which is inhibitory and the one for growth hormone which has
both inhibitory and stimulatory releasing hormones. Releasing hormones are produced
in the median eminence of the hypothalamus and reach the adenohypophysis via a
portal system of veins known as the pituitary portal system.
Neurohypophysis.
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The cells in the neurohypophysis secrete only direct acting hormones : (1) antidiuretic
hormone (ADH) also known as vasopressin secreted by neurons in the supraoptic
nucleus in the hypothalamus and (2) oxytocin secreted by neurons in the
paraventricular nucleus in the hypothalamus. After synthesis in the hypothalamus,
these hormones move down the axons of the hypothalamohypophyseal tract through
the infundibular stalk and terminate near blood vessels in the pars nervosa.
Accumulations of these hormones bound to specific glycoproteins can be observed
along the axons of the hypothalamophypophyseal tract and in the pars nervosa. These
"accumulations" often called Herring bodies represent a storage form of the hormone.
Release of these hormone stores is determined by impulses in the axons of the
hypothalamophypophyseal tract originating in the hypothalamus. Such a mechanism of
secretion controlled by nerve impulses is called "neurosecretion".
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