Lecturer Dr.Mustafa Ghani
Histology
Digestive System
The oesophagus:It is a straight muscular tube that transports food from the pharynx to the stomach.
It is about 25cm long. Its wall consists of the usual 4 layers :1. The mucosa:a)
It
is
lined
by
a
protective,
non-keratinized
stratified
squamous
epithelium with mitotic figures in its basal layer, indicating a constant
shedding & renewal of the cells.
b) Lamina propria :- consists of loose C.T. , in the lamina propria of the region near
the stomach are groups of glands , the oesophageal cardiac glands that secrete mucus.
c) Muscularis mucosae:- it is thick & consists of longitudinal smooth muscle fibers.
In the lower part of the oesophagus, the muscularis mucosae may be formed of inner
circular & outer longitudinal muscles fibers.
2.
The
branched
submucosa:tubuloalveolar
contains
the
oesophageal
glands
which
are
glands.
mucous
in
These
nature,
are
their
ducts open on the epithelial surface.
3.
The muscularis externa:- consists of 2 layers, of which the outer
is mainly longitudinal & the inner mainly circular. In the upper part of
the oesophagus, the muscle fibers are striated, in the middle are mixed
(striated
& smooth)
& in
the
lower part
all
the muscle fibers
are
smooth. In the region of the cardiac sphincter the circular muscle is"
greatly
thickened.
Between
the
2
muscle
plexus.
1
layers
there
is
Auerbach's
4. The adventitial- consists of loose C.T. continuous with that of the surrounding
structures. There is a serous layer only on the abdominal part of the oesophagus.
Oesophagogastric junction:At this junction the stratified squamous epithelium of the oesophagus ends abruptly
to be replaced by the simple columnar epithelium of the stomach. The oesophageal
glands stop & the gastric glands appear in the lamina propria. Lymph nodules are
frequent around the junction. The oesophagogastric junction is an important site of
the pathologic abnormality. Lower oesophagus is an important site of common
diseases, particularly ulceration, stricture & cancer.
The epithelium of the oesophagus is protected from exposure to the gastric acid
by:l.
The
anatomical
arrangement
of
the
oesophagogastric
junction.
2. The cardiac sphincter which prevents reflux of the gastric contents
into the lower oesophagus.
Reflux of acid gastric secretions may occur into the lower esophagus causing
inflammation & pain. Under the constant irritating effect of reflux of acidic gastric
secretions, the epithelium in the lower esophagus changes to a gastric type.
Stratified
The
squameus
columnar
non-keratinized
epith
→
simple
ulceration
&
epithelium
prone
to→
columnar
inflammation
and predispose to the development of one type of oesophageal cancer.
Healing of such ulcers can lead to scarring of the lower end of the
oesophagus
&
thereby
narrowing
of
stricture) → difficulty of swallowing.
2
its
lumen
(i.e
oesophageal
Stomach:It is both endocrine & exocrine organ that digests food & secretes hormones. It is a
dilated segment of the digestive tract.
The main functions of the stomach:1. Continuation of digestion of carbohydrates initiated in the oral cavity (mouth) by
salivary amylase which starts the digestion of carbohydrates & this process will
continue in the stomach.
2. Secretion of acids (HCl) together with the muscular activity of the muscles of the
stomach will transform the digested food into a viscous mass known as chyme.
3. The stomach initiates the digestion of proteins by the enzyme pepsin so the
stomach secretes pepsinogen which is converted into pepsin which initiates the
digestion of proteins.
4. The stomach secretes or produces gastric lipase. Gastric lipase together with the
help of lingual lipase (Von Ebner's gland) start the digestion of triglycerides.
For descriptive purpose or anatomically the stomach can be divided into 4
divisions :1. Cardia
2. Fundus
3. Body
4. Pylorus or pyloric region
Because the fundus & body are identical in microscopic structure, only 3
histologic regions are recognized. In empty, contracted stomach, the mucosa &
submucosa form numerous longitudinal folds known as rugae. When the stomach is
filld with food, these folds flatten out.
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The wall of the stomach consists of the usual 4 layers of digestive tract :1. The mucosa:- consists of:
A) Surface epithelium ( simple columnar epithelium):- their cells secrete
mucin(mucus) & they are known as surface mucous cells. The- mucus forms a thick
layer that protects these cells from the effects of the strong acid secreted by the
stomach. The surface epithelium invaginates to various extents into the lamina
propria, forming gastric pits (lined by the same epithelium). Emptying into the gastric
pits are branched tubular glands (cardiac, fundic & pyloric) characteristic of each
region of the stomach. The surface mucous cells are thought to produce blood group
substances. Tight junctions present around surface & pits cells also form part of the
barrier to acid. Stress & other psychosomatic factor; ingested substances such as
aspirin or ethanol & some microorganisms (e.g. Helicobacter pylori) can disrupt this
epithelial layer & lead to ulceration.
B) The lamina propria:- is occupied by the gastric glands. These are branched
tubular glands, separated by little amount of C.T.. They open into the bases of the
gastric pits.
Types of gastric glands:1) The cardiac glands:- are present in the cardiac region. They are simple or
branched tubular glands, lined by mucus secreting columnar cells similar to the
mucous neck cells of the gastric gland proper. They secrete mucus & lysozyme.
2) Fundic glands (gastric gland proper):- are present in the lamina propria of the
fundus & body of the stomach. They are branched, tubular glands, 3 to 7 of which
open into the bottom of each gastric pit. The glands are composed of the following
cells:-
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a) Mucous neck cells:- are located between the parietal cells in the neck of the
gland. The cells are large irregular in shape, with clear cytoplasm & the nucleus is
flattened at the base of the cell. They secrete mucus which protects the stomach wall
from the action of the HC1 & proteolytic enzymes.
Peptic (chief or zymogenic)cells:- they predominate in the lower part of the gland &
have all the characteristics of the protein synthesizing & exporting cells. Their
basophilia is due to the abundant RER(rough endoplasmic reticulum). The granules in
their cytoplasm contain the inactive enzyme pepsinogen, the precursor of pepsin. In
human, chief cells also produce the enzymes, lipase & rennin. The E.M shows the
presence of small irregular microvilli on their free surfaces, a well developed Golgi
apparatus (complex) located in the supranuclear region, large amount of basally
located RER & many apical secretory granules.
c) Parietal(oxyntic) cells:- are present mainly in the upper half of the gland. They are
large rounded or pyramidal cells with one centrally placed spherical nucleus &
intensely eosinophilic cytoplasm. The most striking features of the active secreting
cell seen in the electron microscope are an
abundance of mitochondriaf
eosinophilic) & a deep circular invagination of the apical plasma membrane, forming
the intracellular secretory canaliculus lined with abundant microvilli. In the resting
cell, a number of tubulovesicular structures can be seen in the apical region of the cell.
At this stage, the cell has few microvilli. When stimulated to produce HC1,
tubulovesicles fuse with the cell membrane to
form the
canaliculus
& more
microvilli, suggesting that tubulovesicles are involved in secretion. The parietal cells
secrete the HC1 & gastric intrinsic factor (a glycoprotein that binds to vitamin B12 &
facilitate its absorption by the intestine). Lack of the intrinsic factor can lead to vit.
B12 deficiency →
pernicious anaemia. In cases of atrophic gastritis, both parietal
and peptic cells are much less numerous, and the gastric juice has little or no acid or
pepsin activity, and no intrinsic factor →
pernicious anaemia.
Parietal cells have abundant carbonic anhydrase, which is thought to play a vital role
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in generating H+ ions for the production of HCL
d)
Enteroendocrine
enterochromaffin
chromium
cells:-
cells,
stains.
the so-called APUD
were
owing
Most
cells
of
formerly
to
the
(amine
their
cells
called,
affinity
have
precursor
for
the
uptake
argentaffin
&
silver
&
characteristics
of
& decarboxylation),
which are wide spread in the body (found in the epithelium of GIT, respiratory tract,
in the pancreas & thyroid gland). Their cytoplasm contains either polypeptide
hormones or the biogenic amines epinephrine, norepinephrine, or
tryptamine
(serotonin).
These
cells
5-hydroxy
have characteristics of diffuse
neuroendocrine system(DNES).These cells can be identified & localized by
immunocytochemical methods or other cytochemical techniques for specific amines.
The enteroendocrine cells are small pyramidal cells found near the bases of gastric
glands. They are characterized by the presence of abundant dense secretory granules,
always located at the base of the cell between the nucleus'& basal lamina. This
suggests that they are endocrine cells that liberate their secretion into the blood vessels
in the lamina propria rather than into lumen of the gland. Some of these cells are
known as paracrine because they produce hormones that diffuse into the surrounding
extracellular fluid to regulate the function of neighboring cells without passing
through the vascular system.
Polypeptide-secreting cells of the digestive tract fall into 2 classes:1) The open type:- in which the apex of the cell presents microvilli & contacts the
lumen of the organ.
2) The closed type:- in which the cellular apex is covered by other epithelial cells.
In the fundus of the stomach, serotonin is one of the principal
secretory
carcinoids,
products.
are
Tumors
responsible
arising
for
from
the
clinical
overproduction of the serotonin.
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these
cells
symptoms
are
called
caused
by
d) Stem cells (undifferentiated cells):- are found in the neck region of all gastric
glands (cardiac, fundic & pyloric). These cells have a high rate of mitosis; some of
them move upward to replace the pit & surface mucous cells, which have a turnover
time of 4-7 days. Other daughter cells migrate more deeply into the glands &
differentiate into the different types of cells of the glands. These cells are replaced
much more slowly than are surface mucous cells.
3) Pyloric glands:- are present in the pyloric region of the stomach & are similar to
the cardiac glands. However, the pits are longer & the glands are shorter & coiled,
opposite to that of cardiac glands. The pyloric glands secrete mucus as well as
appreciable amounts
of the
enzyme
lysozyme.
These glands
also
have
enteroendocrine cells as follows:1. G cells release gastrin which stimulates the secretion of the acid
by the parietal cells.
2 D cells secrete somatostatin, which inhibits the release of some
other hormones, including gastrin.
C) The muscularis mucosae:- consists of smooth m. arranged as an inner circular &
an outer longitudinal layer, in some parts there is a third outer layer of circular fibres.
Strands of muscle extend from the inner layer into the lamina propria between the
glands, their contraction helps to empty the glands.
2. Submucosa:- is composed of dense C.T. containing blood & lymph vessels &
Meissner's plexus.
3. Muscularis externa:- consists of 3 illdefined layers of smooth m., an inner
oblique, a middle circular & an outer longitudinal. .At the pylorus, the middle layer
is greatly thickened to form the pyloric sphincter. Auerbach's plexus is found
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between the muscles.
e) Serosa:- derived from the visceral peritoneum consists of a thin layer of loose
C.T. covered by mesothelium (simple squamous epithelium).
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Regions of the stomach. The stomach is a muscular dilation of the digestive tract where mechanical
and chemical digestion occurs. The muscularis consists of three layers for thorough mixing of the
stomach contents as chyme: an outer longitudinal layer, a middle circular layer, and an inner oblique
layer. The stomach mucosa shows distinct histological differences in the cardia, the fundus/body,
and the pylorus. Cells that secrete HCl and pepsin are restricted mainly to the body and fundus
regions. Glands of the cardia and pylorus produce primarily mucus.
Wall of the stomach with rugae. Low magnification micrograph of the stomach wall at the fundus
shows the relative thickness of the four major layers: the mucosa (M), the submucosa (SM), the
muscularis externa (ME), and the serosa (S). Two rugae (folds) cut transversely and consisting of
mucosa and submucosa are included. The mucosa is packed with branched tubular glands
penetrating the full thickness of the lamina propria so that this sublayer cannot be distinguished at
this magnification. The muscularis mucosae (arrows), immediately beneath the basal ends of the
gastric glands, is shown. The submucosa is largely loose connective tissue, with blood vessels (V)
and lymphatics. X12. H&E.
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Esophagogastric junction. At the junction of the esophagus (E) and the cardiac region of the
stomach (C) there is an abrupt change in the mucosa from stratified squamous epithelium to simple
columnar epithelium invaginating as gastric pits (GP). The mucosa contains many mucus—secreting
esophageal cardiac glands (ECG), whose function is supplemented by mucous cardiac glands (CG)
opening into the superficial gastric pits. Strands of muscularis mucosae (arrow) separate the mucosa
and submucosa (SM). X60. H&E.
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Gastric pits and glands. (a): SEM of the stomach lining cleared of its mucus layer reveals closely
placed gastric pits (P) surrounded by polygonal apical ends of surface mucous cells. X600. (b):
Micrograph of the same lining shows that these surface mucous cells are part of a simple columnar
epithelium continuous with the lining of the pits (P). Each pit extends into the lamina propria and
then branches into several tubular glands. These glands branch further, coil slightly, and fill most of
the volume of the mucosa. Around the glands, which contain other cells besides columnar cells, a
small amount of connective tissue comprising the lamina propria is also seen. X200. H&E.
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Pyloric glands. The pyloric region of the stomach has deep gastric pits (P) leading to short, coiled
pyloric glands (G) in the lamina propria. Cardial glands are rather similar histologically and
functionally. Cells of these glands secrete mucus and lysozyme primarily, with a few G cells also
present. The glands and pits are surrounded by cells of the lamina propria (LP), connective tissue
also containing lymphatics and MALT. Immediately beneath the glands is the smooth muscle layer
of the muscularis mucosae. X140. H&E.
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Gastric glands. Throughout the fundus and body regions of the stomach the gastric pits lead to
glands with various cell types. (a): In the neck of the glands are mucous neck cells (MN), scattered
or present as clusters of irregular, low columnar cells with basophilic, granular cytoplasm and basal
nuclei. These cells produce mucus with a higher content of glycoproteins than that made by surface
mucous cells. Among the neck mucous cells are stem cells that give rise to all epithelial cells of the
glands. In the upper half of the glands are also numerous distinctive parietal cells (P), large rounded
cells often bulging from the tubules, with large central nuclei surrounded by intensely eosinophilic
cytoplasm with unusual ultrastructure. These cells produce HCl and the numerous mitochondria
required for this process cause the eosinophilia. Around these tubular glands are various cells and
microvasculature in connective tissue.
(b): Near the muscularis mucosae (MM), the bases of these glands contain fewer parietal cells, but
another cell type, chief cells (C), is abundant here. Chief cells are also known as peptic or zymogenic
cells. They are seen as clusters of cells with condensed, basal nuclei and basophilic cytoplasm. From
their apical ends chief cells secrete pepsinogen, the zymogen precursor for pepsin, a major protease.
Zymogen granules are often removed or stain poorly in routine preparations. Both X200. H&E.
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Parietal cells and chief cells. A plastic section of a gastric gland’s basal portion shows better detail
of parietal and chief cells than routine histological sections often allow. The large parietal cells’
characteristic intracellular canaliculi (arrowheads) can be seen, along with the numerous acidophilic
mitochondria. The smaller chief cells have cytoplasm containing numerous large red secretory
(zymogen) granules. X400. PT.
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Synthesis of HCl by parietal cells. Diagram showing the main steps in the synthesis of
hydrochloric acid. Active transport by ATPase is indicated by arrows and diffusion is indicated by
dotted arrows. Under the action of carbonic anhydrase, carbonic acid is produced from CO2.
Carbonic acid dissociates into a bicarbonate ion and a proton (H+), which is pumped into the
stomach lumen in exchange for K+. A high concentration of intracellular K+ is maintained by the
Na+, K+ ATPase, while HCO3– is exchanged for Cl− by an antiport. The tubulovesicles of the cell
apex are seen to be related to hydrochloric acid secretion, because their number decreases after
parietal cell stimulation as microvilli increase. Most of the bicarbonate ion returns to the blood and is
responsible for a measurable increase in blood pH during digestion, but some is taken up by surface
mucous cells and used to raise the pH of mucus.
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