TISSUES. EPITHELIAL TISSUES. LINING EPITHELIA.
Using lectures (on the web-page of histology department), lecture presentations
textbooks, additional literature and other resources, students should prepare such
theoretical questions:
1.
List the principal functions of epithelial tissues.
2.
From which embryonic germ layer(s) epithelial tissues are derived? Give
examples of epithelia that derived from each embryonic germ layer.
3.
Structural and functional characteristics of epithelial tissues that
distinguish them from other tissue types.
4.
Classification of the epithelial tissues.
5.
Describe the basal lamina in terms of its location, composition, staining
properties. Compare basal lamina and basement membrane.
6.
4 types of epithelial cells junctions.
7.
Types of simple epithelia. Give examples of body’s sites where each
type can be found.
8.
Types of stratified epithelia. Give examples of body’s sites where each
type can be found.
Human’s body consists of billions of cells. A tissue is a collection of cells and
noncellular structures, which have the similar origin, structure and functions. Organs
are groups of tissues. Most organs are complex groups of different tissue types. An
organism is composed of organs that are grouped together and functionally
integrated.
Cells are bound together with varying amounts of intercellular substance to
form tissues and organs. This applies also to the blood, the plasma of which
constitutes the intercellular substance. Even in the earliest stages, when the fertilized
egg divides repeatedly, the resulting cells stick together and pass through a series of
complex developmental processes which take place in an orderly sequence. There is a
phase when the early embryo consists only of simple epithelial layers. Later, the three
primitive germ layers give rise to 4 primary tissues:
1.
Epithelium – the cells are generally applied closely one to another with
little cementing substance. The sheets of cells may develop into the covering of the
outer and inner surfaces of the body and the glands and other structures derived from
them.
2.
Connective tissues – the cells are generally separated one from another
to a greater or less degree by a rather rich amount of intercellular substance. Blood
cells are derived from these cells and the blood cell-forming tissues, the connective
tissue in its varieties, cartilage and bone. These derivates differ not only in their cell
populations, but also in the nature and amount of the intercellular substance.
3.
Muscle tissue – the cells are of several varieties, which are associated
with movement of the skeleton and contractility in many organs, including those of
the vascular system.
4.
Nerve tissue - the cells are concerned primarily with rapid conduction of
impulses in the integration of numerous functions. The brain, spinal cord, autonomic
ganglia, peripheral nerves, and portions of sensory organs are composed of nerve
tissue.
The epithelia are a diverse group of tissues, which, with rare exceptions, line
all body surfaces, cavities and tubes. Epithelia thus function as interfaces between
biological compartments. Epithelia interfaces are involved in a wide range of
activities such as absorption, secretion and protection and all these major functions
may be exhibited at a single epithelial surface. For example, the epithelial lining of
the small intestine is primarily involved in absorption of the products of digestion,
but the epithelium also protects itself from noxious intestinal contents by the
secretion of a surface coating of mucus.
Epithelial tissues have morphofunctional features, which may be changed in
some pathologic states, for example inflammation, dystrophy, dysplasia and
metaplasia.
Epithelial tissues usually occur as structurally minor but functionally
important components of complex organs. Glands, derived from the invagination and
ingrowths of lining epithelia into underlying connective tissue, are composed mainly
of epithelial cells in adults and are considered a type of epithelial tissue.
Epithelial tissues are very diverse in form and function. They range from one
to several cell layers in thickness, forming sheets, solid organs, or glands. Their
functions range from protection to secretion and absorption.
Epithelial cells line surfaces. Epithelia line and protect virtually all-free
surfaces in the human body except joint cavities and the anterior surface of the iris,
which is a naked connective tissue domain.
a. The outer surface of the body is covered by the epithelial epidermis of skin.
The hair follicles and glands in skin also have epithelial components.
b. Epithelium lines the digestive system and its diverticula, such as the
respiratory system, liver, pancreas, and gallbladder.
c. The cardiovascular system is lined by an epithelium called endothelium.
d. Body cavities derived from the intraembryonic coelom (the pericardial
cavity, thoracic cavity, and peritoneal cavity) are lined by an epithelium called
mesothelium.
e. The urogenital system is lined by a layer of epithelial cells as well.
Ectoderm, mesoderm, and endoderm can all give rises to epithelia.
Epithelial cells are polarized. The epithelial apical surface typically faces a
free surface of the body or the lumen of an organ or blood vessel and may be covered
by microvilli or cilia. The basal surface rests on an extra cellular layer of fibrils and
glycoproteins called the basement membrane, or basal lamina, which is the boundary
between the epithelium and the underlying connective tissue.
Epithelial cells have tight lateral adhesions. An epithelium is one or more
layers of cells that are tightly joined together. The adhesions hold the epithelial cells
together into a coherent barrier tissue. The apical junctions between cells in many
epithelia have a sealing and adhesive structure called the junctional complex, which
isolates the internal milieu of the organism and tightly joins epithelial cells together.
Epithelial tissues have very little intercellular substance. The cells are usually
densely packed, closely apposed, and joined by specialized junctions.
Epithelia are continuously renewed and replaced. The epithelial cells closest
to the basal lamina undergo continuous mitosis, and their progeny replace the surface
cells.
When faced with a chronic change of environmental conditions, epithelia are
capable of metaplasia, i.e., they may change from one type to another.
Epithelia are avascular. In most organs, the connective tissue beneath or
around the epithelium contains blood vessels and lymphatics, which nourish the
epithelium by diffusion.
Classification of the epithelial tissues. Epithelia are classified and named
according to the number of their cell layers and the shape of the cells in the surface
layer.
1. Number of cell layers
a. Simple epithelia have one cell layer. All cells rest on the basement
membrane and reach the apical surface.
b. Stratified epithelia have 2 or more cell layers, consequently not all cells
rest on the basement membrane or reach the apical surface.
c. Pseudostratified epithelia have all their cells resting on the basal lamina,
but not all the cells extend to the surface. The nuclei lie at different depths, giving the
appearance of multiple cell layers.
Diagrams of simple epithelial tissue.
A: Simple squamous epithelium.
B: Simple cuboidal epithelium.
C: Simple ciliated columnar epithelium. All are separated from the subjacent
connective tissue by a basement membrane.
In C, note the terminal bars that correspond in light microscopy to the zonula
occludens and the zonula adherens of the junctional complex.
Diagrams of stratified and pseudostratified epithelial tissue.
A: Stratified squamous epithelium.
B: Transitional epithelium.
C: Ciliated pseudostratified epithelium. The goblet cells secrete mucus, which
forms a continuous mucous layer over the ciliary layer.
2. Shape of the surface cells
a. Squamous cells are flat and plate-like.
b. Cuboidal cells are polygonal and about as tall as they are wide.
c. Columnar cells are polygonal and taller than they are wide.
Type of Epithelium
Localizations
Lining of peritoneal and pleural cavities;
Simple squamous
endothelial lining of all normal blood vessels
Simple cuboidal
Proximal convoluted tubules; sweat glands;
epithelium of small and large intestines
Pseudostratified
columnar
ciliated
epithelium
Nasopharynx; trachea; bronchi
with
goblet cells
Pseudostratified columnar
Epididymis; ductus deferens
with stereocilia
Stratified squamous
Vagina; parts of the oral cavity; pharynx;
esophagus; anal canal
Stratified
squamous
Epidermis; hard palate; gingiva
keratinized
Stratified cuboidal
Sweat gland ducts
Stratified columnar
Male urethra
Transitional
Renal pelvis; ureters; urinary bladder
Specific Epithelial Types:
1. Simple squamous epithelium is a single layer of flat, plate-like cells that
usually functions as a semipermeable barrier between compartments.
Simple squamous epithelium coats, or partially coats, the stomach, liver,
gallbladder, and other visceral organs. It also is found in respiratory system alveoli
and in the thin limbs of the loop of Henle in the kidney.
Mesothelium is the simple squamous epithelium that lines serous cavities
(peritoneal, pleural, and pericardial cavities) and coats many of the organs in these
cavities.
Simple squamous epithelium covering the peritoneum (mesothelium).
Some blood capillaries are indicated by arrows. PT stain. Medium magnification
Simple squamous epithelium covering the peritoneum (mesothelium).Surface
view. Silver impregnation.
Endothelium is the simple squamous epithelium that lines the lumen of the
cardiovascular system. Cells of this type have three zones: central perinuclear, zone
of organelles and peripheral thin part through which exchanges occur.
Electron micrograph of simple squamous epithelium (endothelium) x44,600.
Section of a vein. All blood vessels are lined with a simple squamous
epithelium called endothelium (arrowheads). Smooth muscle cells in the vein
wall are indicated by arrows. Pararosaniline–toluidine blue (PT) stain. Medium
magnification
2. Simple cuboidal epithelium is a single layer of cells that are equal in height
and width. It typically exists in areas where ion transport occurs (e.g., in kidney
tubules, sweat glands and some glandular ducts). It also covers the choroid plexus—
the four clusters of capillaries in the walls of the ventricles of the brain that help
produce cerebrospinal fluid (CSF).
1. Simple cuboidal epithelium is abundant in the kidney, in the proximal and
distal tubules and parts of the collecting-duct system.
Simple cuboidal epithelium from kidney collecting tubules. Cells of these
tubules are responsive to the antidiuretic hormone and control the resorption of
water from the glomerular filtrate, thus affecting urine density and helping
retain the water content of the body. PT stain. Low magnification
2. Many glands are composed of simple cuboidal epithelial cells assembled
into round acini. In the configuration, the cuboidal cells are distorted into a rough
pyramidal shape.
3. Follicular epithelial cells in the thyroid gland can assume a cuboidal shape,
depending on their function.
4. Often, cuboidal epithelial cells have many apical microvilli and
mitochondria, which facilitate ion pumping and fluid transport.
3. Simple columnar epithelium is a single layer of roughly cylindrical cells
whose apical (free) surfaces may be covered with cilia or microvilli. It functions in
secretion, absorption, and propulsion of mucus (if ciliated) and often as a protective
barrier. It lines the stomach, intestines, rectum, uterus, and oviducts. It is found in
some glandular ducts and the gallbladder, and it lines papillary collecting ducts in the
urinary system.
Simple columnar epithelium that covers the inner cavity of the uterus. Note
that the epithelium rests on the loose connective tissue of the lamina propria.
The epithelium and the lamina propria constitute the mucosa. H&E stain.
Medium magnification
5. Pseudostratified columnar epithelium is a layer of cells, in which all of the cells
rest on the basement membrane but only some extend to the apical surface of the
epithelium. Cells reaching the surface are often ciliated. This epithelium forms a
protective barrier and, when ciliated, moves surface mucus and trapped debris.
Ciliated pseudostratified columnar epithelium, or respiratory epithelium, lines the
larger diameter respiratory passageways. Pseudostratified columnar epithelium
often covered with nonmotile stereocilia, also lines some parts of the male
reproductive tract, where its apical surfaces are.
Pseudostratified ciliated columnar epithelium with goblet cells of trachea.
H&E stain. Low magnification
6.
Stratified squamous epithelium contains many layers of cells,
including an apical layer of flat cells. These epithelia are classified according to the
characteristics of the apical layer.
a. The keratinized (cornified) type is a multilayered sheet of cells. The
superficial cells are squamous, dead, and filled with the scleroprotein keratin; they
lack nuclei. Deeper layers have polygonal cells in progressive stages of
keratinization. The deepest layer has cuboidal to columnar cells and lies on the basal
lamina. Keratinized stratified squamous epithelium is found mainly in the skin and
forms a highly specialized barrier against friction, abrasion, infection, and water loss.
The digestive system contains some stratified squamous epithelium. Stratified
squamous epithelium on the tongue and lining the esophagus resists the abrasion of
mastication, swallowing, and the passage of food from the oral cavity to the stomach.
Stratified squamous epithelium in the anal canal resists the abrasion of passing semisolid feces.
In the female reproductive system, stratified squamous epithelium is abundant
in the vagina and covering the cervix, where it resists abrasion during sexual
intercourse.
Stratified squamous keratinized epithelium of epidermis of the skin.
H&E stain. Low magnification
b. The nonkeratinized (noncornified) type is similar in structure but is thinner
and lacks heavily keratinized cells. Its surface cells are flattened but nucleated.
Nonkeratinized stratified squamous epithelium, also called mucous membrane, and
forms a protective barrier that is less resistant to water loss than the keratinized type.
It lines wet cavities subject to abrasion (e.g., mouth, esophagus, vagina, and anal
canal, vocal folds).
Stratified squamous nonkeratinized (moist) epithelium of the esophagus. PT
stain. Medium magnification.
Stratified squamous nonkeratinized epithelium of cornea. H&E stain. Low
magnification
6. Stratified cuboidal epithelium usually has 2-3 layers of cuboidal cells. This
relatively rare epithelium lines the ducts of epidermal glands (sweat) and salivary
glands.
Stratified cuboidal epithelium that lined the ducts of salivary glands.
H&E stain. Low magnification
7. Stratified columnar epithelium is similar to stratified cuboidal, but its
superficial cells are columnar and may be ciliated. Also rare, it lines the larger ducts
of some large glands, forms the conjunctiva, and occurs in small, isolated patches in
some mucous membranes. It sometimes covers the respiratory surface of the
epiglottis.
Stratified columnar epithelium. H&E stain. Low magnification
8. Transitional epithelium is a stratified epithelium found exclusively in the
urinary passages of the urinary system. It contains many layers of polyhedral cells
and an outer apical layer of round pillow-shaped cells. It is found in the minor
calyces, major calyces, renal pelvis, ureters, urinary bladder, and proximal urethra.
Transitional epithelium in the bladder undergoes a reversible morphologic change
during bladder distension and evacuation. When the bladder is empty, the surface
cells appear domelike, giving the epithelium a "cobblestone" appearance. When the
bladder is full, the surface cells stretch and flatten.
Stratified transitional epithelium of the urinary bladder. PT stain.
Medium magnification
Stratified transitional epithelium of the urethra. The basement membrane
between the epithelium and the underlying loose connective tissue is indicated by
arrows. PSH stain. Medium magnification.
Some epithelia are highly specialized and, therefore, defy simple classification
or description.
For example,
1. Chorionic villi in the placenta consist of a syncytial epithelial layer
(syncytiotrophoblast)
lying
on
a
proliferative
cuboidal
epithelial
layer
(cytotrophoblast), which, in turn rests on a basement membrane.
2. Seminiferous epithelium in the male is stratified, highly proliferative, and,
after puberty, sheds live haploid gametes from its apical surface.
3. Follicular epithelial cells in the ovary are surrounded by a basement
membrane and make intimate contact with the developing oocyte.
4. Some epithelia, such as those in the thyroid follicular epithelium, are in a
constant state of flux. They may be squamous, cuboidal, or columnar, depending on
the amount of thyroglobulin contained in the follicle lumen.
5. Many sensory epithelia (e.g., organ of Corti in the cochlear duct; retina) are
highly complex epithelia.
POLARITY & SPECIALIZATIONS OF EPITHELIAL CELLS
Polarity (structural and functional asymmetry) is characteristic of most
epithelial cells. It is best seen in simple epithelia, where each cell has 3 types of
surfaces: an apical (free) surface, a variable number of lateral surfaces that abut
neighboring cells, and a basal surface attached to the basal lamina.
Specializations of the Apical Surface. The cell's apical surface is on the
organ's external or internal (lumen) surface. It is specialized to carry out functions
that occur at these interfaces, including secretion, absorption, and movement of
luminal contents.
An unusual, but functionally crucial, feature of most epithelial apices is that
they are not adhesive. For example, when you clap your hands they do not adhere,
no matter how hard you clap or how long you hold your hands together.
Similarly, the apices of the mesothelium lining the thoracic cavity, pericardial
cavity, and peritoneal cavity do not adhere to one another, and organs in these
cavities move past one another without impediment or pain. However, if infection or
surgery destroys the mesothelium, the organs will stick together, resulting in painful
adhesions.
The apex of luminal epithelial cells also exhibits this nonadhesive property,
even in the smallest tubules in the body. Consequently, a lumen is maintained in all
tubules, including the ductus deferens and capillaries, because walls cannot collapse
and adhere to one another.
In many instances, especially in the small intestines, the apical portion of the
cell is coated with a thick, glycoconjugate-rich layer external to the outer leaflet of
the plasma membrane called the glycocalyx, which makes apices nonadhesive.
Apical protrusions. The epithelial apex often contains protrusions from the
cell surface. The protrusions may be scattered, as in mesothelium and endothelium,
and may take the form of microvilli.
a.
Microvilli are cylindrical, cell-surface projections, 80 nm wide and 1-2
(mm long, which increase the cell surface area for absorbing materials from the
lumen. Microvilli can be sparse (as in mesothelium), numerous (as in the
syncytiotrophoblast), or very dense (as in the brush border of kidney tubules and
intestinal epithelium). Microvilli are motile. They have many actin-containing
microfilaments and large amounts of myosin.
Electron micrograph of the apical region of an intestinal epithelial cell.
Note the terminal web composed of a horizontal network that contains mainly
actin microfilaments. The vertical microfilaments that constitute the core of the
microvilli are clearly seen. An extracellular cell coat (glycocalyx) is bound to the
plasmalemma of the microvilli. x45,000.
Electron micrograph of a section from the apical region of a cell from the
intestinal lining showing cross-sectioned microvilli. In their interiors, note the
microfilaments in a cross section. The surrounding unit membrane can be
clearly discerned and is covered by a layer of glycocalyx, or cell coat. x100,000.
b.
Stereocilia are long microvilli present in the male reproductive tract and
in the membranous labyrinth of the inner ear. They are similar to microvilli except
that they are longer and constricted at the point where they join the cell apex. Little is
known about their movements in vivo. Sensory stereocilia contain many actincontaining microfilaments and large amounts of myosin. In epididimus and ductus
deferens they have an absorptive function, and in the internal ear (hair cells of the
maculae and organ of Corti), they have a sensory function.
c.
Cilia. Many epithelia apices are ciliated in some locations :
(1) In the trachea, certain columnar epithelial cells have many apical cilia that
move mucus along the apical surface of the trachea lumen (airway).
(2) Epithelial cells in the uterine tubes have apical cilia that help move the
fertilized ova into the reproductive tract.
(3) The apical portions of many sensory epithelial cells have modified cilia or
cilia-like structures involved in the energy transduction function of the cell. However,
these cilia are not motile like the cilia in the respiratory and female reproductive
tracts.
Electron micrograph of the apical portion of a ciliated epithelial cell. Cilia
are seen in longitudinal section. At the left, arrowheads point to the central and
peripheral microtubules of the axoneme. The arrowhead at right indicates the
plasma membrane surrounding the cilium. Each cilium has a basal body (B)
from which it grows. Microvilli (MV) are shown. X59,000. Inset: Cilia in cross
section. The 9 + 2 array of microtubules in each cilium is evident. X80,000.
d.
Flagella are also concerned with movement. Spermatozoa, derived from
seminiferous epithelia, are the only human flagellated cells.
Specializations of the Lateral Surfaces:
Epithelial cells are usually tightly attached to one another by specialized
intercellular junctions composed of opposing membranes and some intra– and
extracellular material. Junctions occur in 3 major forms: Zonulae are band-like and
completely encircle the cell; maculae are disk-like and attach 2 cells at a single spot;
gap junctions are similar in shape to maculae but differ in composition and function.
A junctional complex (formerly called a terminal bar) is any combination of
intercellular junctions close to the cell apex that looks dark in the light microscope.
1. Zonula occludens. Zonulae occludentes (tight junctions, occluding
junctions) are located near the cell apex and seal off the intercellular space, allowing
the epithelium to isolate certain body compartments (e.g., they help keep intestinal
bacteria and toxins out of the bloodstream). Their structure, best seen in freezefracture preparations, results from the fusion of 2 trilaminar membranes to form a
pentalaminar structure (as seen in transmission electron microscopy [TEM]); this
fusion may require specific "tight-junction proteins." In some tissues, tight junctions
can be disrupted by removing calcium ions or treating with protease.
2. Zonula adherens. Zonulae adherentes (sometimes called belt desmosomes)
are usually just basal to the tight junctions. The membranes of the adhering cells are
typically 20-90 nm apart at a zonula adherens; the gap may be wider there than in
nonjunctional areas. An electron-dense plaque containing myosin, tropomyosin, alpha
actinin, and vinculin is found on the cytoplasmic surface of each of the membranes
participating in the junction. Actin-containing microfilaments arising from each cell's
terminal web insert into the plaques and appear to stabilize the junction.
3. Macula adherens. A macula adherens, or desmosome, consists of 2 dense,
granular attachment plaques composed of several proteins and borne on the
cytoplasmic surfaces of the opposing cell membranes. Transverse thin electron
microscopic (EM) sections show dense arrays of tonofilaments (cytokeratin
intermediate filaments) that insert into the plaques or make hairpin turns and return to
the cytoplasm. The gap between the attached membranes is often over 30 nm.
Sometimes fibrillar or granular material (probably glycoprotein) is seen as a dense
central line in the intercellular space. Desmosomes, distributed in patches along the
lateral membranes of most epithelial cells, form particularly stable attachments but do
not hamper the flow of substances between the cells.
4. Gap junction. A gap junction (nexus) is a disk- or patch-shaped structure,
best appreciated by viewing both freeze-fracture and transverse thin EM sections.
The intercellular gap is 2 nm, and the membrane on each side contains a circular
patch of connexons. Each connexon is a protein hexamer with a central 1.5-nm
hydrophilic pore. The connexons in one membrane link with those in the other to
form continuous pores that bridge the intercellular gap, allowing passage of ions and
small molecules (< 800 daltons). As sites of electronic coupling (reduced resistance
to ion flow), gap junctions are important in intercellular communication and
coordination; they are found in most tissues.
Specializations of the Basal Surface. The basal surface contacts the basal
lamina. Because it is the surface closest to the underlying blood supply, it often
contains receptors for blood-borne factors such as hormones.
1.
Basal lamina underlies all true epithelial tissues.
Structure. The basal lamina is a sheet-like structure, usually composed of type
IV collagen, proteoglycan, and laminin, a glycoprotein that aids in adhesion of the
basal lamina to the cells. The basal lamina exhibits electron-lucent and electron-dense
regions termed the lamina lucida (lamina rara) and the lamina densa, respectively.
The lamina densa is composed of a 20- to 100-nm-thick fibrillar network; the amount
of lamina lucida is variable. Basal lamina components are contributed by the
epithelial cells, the underlying connective tissue cells, and (in some locations)
muscle, adipose, and Schwann cells. In some sites, a layer of type III collagen fibers
(reticular fibers), produced by the connective tissue cells and termed the reticular
lamina, underlies the basal lamina. Basal lamina accompanied by reticular lamina are
often thick enough to be seen with the light microscope as PAS-positive layers and
are sometimes termed basement membranes.
Functions. The basal lamina forms a sieve like barrier between the epithelium
and connective tissue. It aids in tissue organization and cell adhesion and (through
trans-membrane linkages with cytoskeletal components) helps maintain cell shape. It
has a role in maintaining specific cell functions, probably through its effect on shape.
Muscle basal laminae are critical in establishing neuromuscular junctions.
2.
Hemidesmosomes are located on the inner surface of basal plasma
membranes in contact with the basal lamina. Essentially half desmosomes, they help
to attach epithelial cells to the basal lamina. The best examples are found in the basal
layers of stratified squamous epithelium.
3.
Sodium-potassium ATPase is a plasma membrane-bound enzyme
localized preferentially in the basal and basolateral regions of epithelial cells. It
transports sodium out of and potassium into the cell.
Intracellular Polarity. The nucleus and organelles are often found in
characteristic regions of epithelial cells, a feature particularly important to glandular
cells. For example, in protein-secreting cells, the rough endoplasmic reticulum is
preferentially located in the basal cytoplasm, the nucleus in the basal to middle region
just above the rough endoplasmic reticulum, and the Golgi complex just above the
nucleus. Mature secretory vesicles collect in the apical cytoplasm.
Stem cells of epithelial tissue. Like all mitotic cells, stem cells divide into two
cells. One daughter cell remains as a stem cell while the other differentiates and
moves toward the apex. Proliferative stem cells are undifferentiated and usually rest
on the basement membrane. The cell cycle varies tremendously among stem cell
populations in different epithelia, ranging from days to years. For example, the
epidermis turns over every 27 days, while seminiferous epithelium turns over every
72 days.
MEDICAL APPLICATION
Both benign and malignant tumors can arise from most types of epithelial cells.
A carcinoma (Gr. karkinos, cancer, + oma, tumor) is a malignant tumor of epithelial
cell origin. Malignant tumors derived from glandular epithelial tissue are usually
called adenocarcinomas (Gr. adenos, gland, + karkinos); these are by far the most
common tumors in adults. In children up to age 10 years, most tumors develop (in
decreasing order) from hematopoietic organs, nerve tissues, connective tissues, and
epithelial tissues. This proportion gradually changes, and after age 45 years, more
than 90% of all tumors are of epithelial origin.
Carcinomas composed of differentiated cells reflect cell-specific morphologic
features and behaviors (eg, the production of keratins, mucins, and hormones).
Undifferentiated carcinomas are often difficult to diagnose by morphologic analysis
alone. Since these carcinomas usually contain keratins, the detection of keratins by
immunocytochemistry often helps to determine the diagnosis and treatment of these
tumors.
Epithelia are normally capable of rapid repair and replacement of apoptotic or
damaged cells. In some large glands, most notably the liver, mitotic activity is
normally rare but is actively renewed following major damage to the organ. When a
portion of liver tissue is removed surgically or lost by the acute effects of toxic
substances, cells of undamaged regions quickly begin active proliferation and normal
functional mass of liver tissue is soon regenerated.
Some epithelial cells are prone to abnormal growth called neoplasia that may
lead to cancers. Neoplastic growth is reversible and does not always result in cancer.
Under certain abnormal conditions, one type of epithelial tissue may undergo
transformation into another type in another reversible process called metaplasia,
which is illustrated by the following examples.
In heavy cigarette smokers, the ciliated pseudo-stratified epithelium lining the
bronchi can be transformed into stratified squamous epithelium.
In individuals with chronic vitamin A deficiency, epithelial tissues of the type
found in the bronchi and urinary bladder are gradually replaced by stratified
squamous epithelium.
Metaplasia is not restricted to epithelial tissue; it may also occur in connective
tissue.
The second type of classification for covering and lining epithelium [the
arrangement of cells within their layer(s)] has three types: simple, stratified, and
pseudo-stratified. Which of them contain(s) only one layer?
Simple and pseudo-stratified. Both simple and pseudo-stratified are one-layered.
Simple epithelium is a single layer which functions for diffusion, secretion and
absorption. An example is blood vessels which are simple squamous epithelia.
Pseudo-stratified epithelia look like they have multiple layers, because the cells are
not all at the same level, but they do actually only contain one layer. Cells that don't
extend up to the apical surface either normally have cilia, or they secrete mucus.
Examples are the epithelia in the upper respiratory tract and part of the male urethra,
both of which are pseudo-stratified columnar epithelia. Stratified epithelia have
simply two or more layers, which helps to protect the layers underneath that would
otherwise be prone to abrasion (wear-and-tear).
ATTENTION IFOM!!!
Histology is the study of normal tissues. Groups of cells make up tissues, tissues
form organs, organs form organ systems, and systems make up the organism.
Each organ consists of 4 types of tissue: epithelial, connective, nervous, and
muscular
Only certain aspects of epithelia will be reviewed here; other aspects of
histology are reviewed elsewhere in this book
Epithelial cells are often polarized: the structure, composition, and function of
the apical surface membrane differ from those of the basolateral surfaces. The
polarity is established by the presence of tight junctions that separate these 2
regions. Internal organelles are situated symmetrically in the cell. Membrane
polarity and tight junctions are essential for the transport functions of epithelia.
Many simple epithelia transport substances from one side to the other (kidney
epithelia transport salts and sugars; intestinal epitelia transport nutrients, antibodies, etc.). There are 2 basic mechanisms used for these transports:
Transcellular pathway through which larger molecules and a combination of
diffusion and pumping in the case of ions that pass through the cell
Paracellular pathway that permits movement between cells
Tight junctions regulate the paracellular pathway, because they prevent
backflow of transported material and keep basolateral and apical membrane
components separate.
Epithelial polarity is essential to the proper functioning of epithelial cells; when
polarity is disrupted, disease can develop. For example, epithelia lining the
trachea, bronchi, intestine, and pancreatic ducts transport chloride from
basolateral surface to lumen via pumps in the basolateral surface and channels
in the apical surface.
The transport provides a driving force for Na by producing electrical
polarization of the epithelium. Thus NaCl moves across, and water follows. In
cystic fibrosis the apical Cl channels do not open. Failure of water transport
results in thickening of the mucous layer covering the epithelia.
Transformed cells may lose their polarized organization, and this change can
be easily detected by using antibodies against proteins specific for either the
apical or basolateral surfaces. Loss of polarity in the distribution of membrane
proteins may eventually become useful as an early index of neoplasticity.
Epithelia are always lined on the basal side by connective tissue containing
blood vessels. Since epithelia are avascular, interstitial tissue fluids provide
epithelia with oxygen and nutrients.
Epithelia modify the 2 compartments that they separate. The epithelial cells
may either secrete into or absorb from each compartment, and may selectively
transport solutes from one side of the barrier to the other.
Epithelia renew themselves
continuously, some very rapidly (skin and
intestinal linings), some at a slower rate. This means that the tissue contains
stem cells that continuously proliferate. The daughter cells resulting from each
cell division either remain in the pool of dividing cells or differentiate.
Simple:
Simple cuboidal epithelium (e.g., renal tubules, salivary gland acini)
Simple columnar epithelium (e.g., small intestine)
Simple squamous epithelium (e.g., endothelium, mesothelium, epithelium lining
the inside of the renal glomerular capsule)
Pseudostratified columnar epithelium (e.g., trachea, epididymis)
Stratified:
Stratified squamous epithelium:
nonkeratinized (e.g., esophagus)
and keratinizing (e.g., skin)
Transitional epithelium (urothelium) (e.g., ureter and bladder)
Stratified cuboidal epithelium (e.g., salivary gland ducts)
The most common way to stain tissues for viewing in the light microscope is to
utilize hematoxylin-and-eosin (H&E) staining.
Hematoxylin is a blue dye, which stains basophilic substrates that are the
acidic cellular components such as DNA and RNA. Hematoxylin stains nuclei
blue, and may tint the cytoplasm of cells with extensive mRNA in their
cytoplasm.
Eosin is a pink-to-orange dye, which stains acidophilic substrates such as basic
components of most proteins. Eosin stains the cytoplasm of most cells and many
extracellular proteins, such as collagen, pink.
Microfilaments
Microfilaments are actin proteins. They are composed of globular monomers
of G-actin that polymerize to form helical filaments of F-actin. Actin
polymerization is ATP dependent. The F-actin filaments are 7-nm-diameter
filaments that are constantly ongoing assembly and disassembly. F-actin has a
distinct polarity. The barbed end (the plus end) is the site of polymerization and
the pointed end is the site of depolymerization. Tread milling is the balance in
the activity at the 2 ends.
In conjunction with myosin, actin microfilaments provide contractile and
motile forces of cells including the formation of a contractile ring that provides a
basis for cytokinesis during mitosis and meiosis. Actin filaments are linked to
cell membranes at tight junctions and at the zonula adherens, and form the core
of microvilli.
Clinical Correlate
A first step in the invasion of malignant cells through an epithelium results
from a loss of
expression of
cadherins
that
weakens
the
epithelium.
Changes in intermediate filaments are evident in neurons in Alzheimer’s disease
and in cirrhotic liver diseases
Intermediate Filaments
Intermediate filaments are 10-nm-diameter filaments that are usually stable
once formed. These filaments provide structural stability to cells. There are 4
types:
Type I: keratins (keratins are found in all epithelial cells)
Type II: intermediate filaments comprising a diverse group
Desmin is found in skeletal, cardiac, and GI tract smooth muscle cells.
Vimentin is found in most fibroblasts, fibrocytes, endothelial cells,
an
d vascular smooth muscle.
Glial fibrillary acidic protein is found in astrocytes and some Schwann cells.
Peripherin is found in peripheral nerve axons.
Type III: intermediate filaments forming neurofilaments in neurons
Type IV: 3 types of lamins forming a meshwork rather than individual
filaments inside the nuclear envelope of all cells
Clinical Correlate
Colchicine prevents microtubule polymerization and is used to prevent
neutrophil migration in gout. Vinblastine and vincristine are used in cancer
therapy because they inhibit the formation of the mitotic spindle
Microtubules consist of 25-nm-diameter hollow tubes. Like actin, microtubules
undergo continuous assembly and disassembly. They provide “tracks” for
intracellular transport of vesicles and molecules. Such transport exists in all cells
but is particularly important in axons.
Transport requires specific ATPase motor molecules; dynein drives retrograde
transport
and
kinesin
drives
anterograde
transport.
Microtubules are found in true cilia and flagella, and utilize dynein to convey
motility to these structures. Microtubules form the mitotic spindle during
mitosis and meiosis.
Cell adhesion molecules are surface molecules that allow cells to adhere to one
another or to components of the extracellular matrix. The expression of
adhesion molecules on the surface of a given cell may change with time, altering
its interaction with adjacent cells or the extracellular matrix.
Cadherin and selectin are adhesion molecules that are calcium ion-dependent.
The extracellular portion binds to a cadherin dimer on another cell (trans
binding). The cytoplasmic portions of cadherins are linked to cytoplasmic actin
filaments by the catenin complex of proteins.
Integrins are adhesion molecules that are calcium-independent. They are
transmembrane surface molecules with extracellular domains that bind to fibronectin and laminin, which are components of extracellular basement
membrane. The cytoplasmic portions of integrins bind to actin filaments.
Integrins form a portion of hemidesmosomes but are also important in
interactions between leukocytes and endothelial cells.
A cell must physically interact via cell surface molecules with its external
environment, whether it be the extracellular matrix or basement membrane.
The basement membrane is a sheet-like structure underlying virtually all
epithelia, which consists of basal lamina (made of type IV collagen,
glycoproteins [e.g., laminin], and proteoglycans [e.g., heparin sulfate]), and
reticular lamina (composed of reticular fibers).
Cell junctions anchor cells to each other, seal boundaries between cells, and
form channels for direct transport and communication between cells.
The 3 types of junctional complex include anchoring, tight, and gap junctions.
•
Cell Junctions
Tight junctions (zonula occludens) function as barriers to diffusion and
determine cell polarity. They form a series of punctate contacts of adjacent
epithelial cells near the apical end or luminal surface of epithelial cells. The
major components of tight junctions are occludens (ZO-1,2,3) and claudin
proteins. These proteins span between the adjacent cell membranes and their
cytoplasmic parts bind to actin microfilaments.
Zonula adherens forms a belt around the entire apicolateral circumference of
the cell, immediately below the tight junction of epithelium. Cadherins span
between the cell membranes. Like the tight junctions immediately above them,
the cytoplasmic parts of cadherins are associated with actin filaments.
Desmosomes (macula adherens) function as anchoring junctions. Desmosomes
provide a structural and mechanical link between cells. Cadherins span between
the cell membranes of desmosomes and internally desmosomes are anchored to
intermediate filaments in large bundles called tonofilaments.
Hemidesmosomes adhere epithelial cells to the basement membrane.
The basement membrane is a structure that consists of the basal membrane of
a cell and 2 underlying extracellular components, the basal lamina and the
reticular lamina. The basal lamina is a thin felt-like extracellular layer
composed of predominantly of type IV collagen associated with laminin,
proteoglycans, and fibronectin that are secreted by epithelial cells. Fibronectin
binds to integrins on the cell membrane, and fibronectin and laminin in turn
bind to collagen in the basal lamina. Internally, like a desmosome, the
hemidesmosomes are linked to intermediate filaments. Below the basal lamina is
the reticular lamina, composed of reticular fibers.
Through the binding of extracellular components of hemidesmosomes to inte
grins, and thus to fibronectin and laminin, the cell is attached to the basement
membrane and therefore to the extracellular matrix components outside the
basement membrane. These interactions between the cell cytoplasm and the
extracellular matrix have implications for permeability, cell motility during
embryogenesis, and cell invasion by malignant neoplasms.
Gap
junctions
(communicating
junctions)
function
in
cell-to-cell
communication between the cytoplasm of adjacent cells by providing a
passageway for ions such as calcium and small molecules such as cyclic
adenosine monophosphate (cAMP). The transcellular channels that make up a
gap junction consist of connexons, which are hollow channels spanning the
plasma membrane. Each connexon consists of 6 connexin molecules. Unlike
other intercellular junctions, gap junctions are not associated with any
cytoskeletal filament
Kartagener syndrome is characterized by immotile spermatozoa and
infertility. It is due to an absence of dynein that is required for flagellar motility.
It is usually associated with chronic respiratory infections because of similar
defects in cilia of respiratory epithelium
1. Cilia contain 9 peripheral pairs of microtubules and 2 central microtubules.
The microtubules convey motility to cilia through the ATPase dynein. Cilia bend
and beat on the cell surface of pseudostratified ciliated columnar respiratory
epithelial cells to propel overlying mucous. They also form the core of the
flagella, the motile tail of sperm cell. (Kaplan Medical. USMLE Step 1
Lecture Notes 2018: Pathology, Pharmacology, Physiology, Biochemistry and
Medical Genetics, Immunology and Microbiology, Anatomy, and Behavioral
Science / Social Sciences / Kaplan Medical., 2018. – 2567 р.)
Student’s Practical Activities
Task No 1. Students must know and illustrate such histologic specimens.
Specimen 1. Simple squamous epithelium (mesothelium of the peritoneum).
Silver impregnation method.
In this preparation, the mesothelial lining of the peritoneal cavity has been stripped
from the underlying connective tissue. This lining is supported by a thin basement
membrane. The intercellular substance has been stained with silver salts thereby
outlining the closely interdigitating cell boundaries. Epithelium is composed of
flattened, irregularly-shaped cells. The nuclei have been stained with the dye, neutral
red.
Illustrate and indicate:
1.Cell boundaries.
2.Cytoplasm.
3.Nuclei.
Specimen 2. Simple cuboidal epithelium of the kidney.
Stained with hematoxylin and eosin.
This specimen of the cells that line a small collecting tubule in the kidney
shows simple cuboidal epithelium in section. Although the boundaries between
individual cells are indistinct, the nuclear shape provides an approximate indication
of the cell size and shape. Epithelium is supported by the underlying basement
membrane.
Illustrate and indicate:
1.Basement membrane.
2.Cuboidal epithelial cells.
3.Lumen of the tubule.
Specimen 3. Stratified squamous nonkeratinized epithelium of the human cornea.
Stained with hematoxylin and eosin.
The epithelium of the cornea demonstrated in specimen is a typical example of
stratified squamous epithelium. Note the highly cellular basal layer and the
transformation through the large polygonal cells of the intermediate layers to the
superficial squamous cells. Intercellular substance between cells is absence. The
junction between epithelium and underlying connective tissue is regular, and, as in all
epithelia, blood vessels do not extend beyong the basement membrane.
Illustrate and indicate:
1. Basement membrane.
2. Stratum basale.
3. Stratum spinosum.
4. Squamous cells.
Specimen 4. Stratified squamous keratinized epithelium (fingertip skin).
Stained with hamatoxylin and eosin.
Stratified squamous keratinized epithelium (epidermis) lies on the basement
membrane and is supported by loose connective tissue with blood vessels. The
junctions between epithelium and loose connective tissue are irregular and form
papillae. Note the five morphological layers of this epithelium: 1. Stratum basale –
the cells are cuboidal and form a single layer separated from the loose connective
tissue by a thin basement membrane; 2. Stratum spinosum – consists of 3-4 layers of
prickle cells; 3. Stratum granulosum consists of layers of cells, cytoplasm contains
dense basophilic granules; 4. Stratum lucidum appears as a homogenous layer
between stratum granulosum and stratum corneum; 5. Stratum corneum consists of
layers of the flattened, fused cells devoid of organelles.
Illustrate and indicate:
1. Connective tissue.
2. Basement membrane.
3. Epidermis:
a) stratum basale;
b) stratum spinosum;
c) stratum granulosum;
d) stratum lucidum;
e) stratum corneum.
References:
a) main
1. Practical classes materials:
2. Lecture presentations:
3. Stevens A. Human Histology / A. Stevens, J. Lowe. – [second edition]. –Mosby,
2000.
4. Wheter’s Functional Histology : A Text and Colour Atlas / [Young B., Lowe J.,
Stevens A., Heath J.]. – Elsevier Limited, 2006.
5. Inderbir Singh Textbook of Human Histology with colour atlas / Inderbir Singh. –
[fourth edition]. – Jaypee Brothers Medical Publishers (P) LTD, 2002.
6. Ross M. Histology : A Text and Atlas / M. Ross W.Pawlina. – [sixth edition]. –
Lippincott Williams and Wilkins, 2011.
7. Kaplan Medical. USMLE Step 1 Lecture Notes 2018: Pathology, Pharmacology,
Physiology, Biochemistry and Medical Genetics, Immunology and Microbiology,
Anatomy, and Behavioral Science/Social Sciences / Kaplan Medical., 2018. – 2567 р.
8. T.W. Sadler. Langman's Medical Embryology. Fourteenth edition. Wolters Kluwer;
2019. 456 р.
9. Wojciech Pawlina. Michael H. Ross. Histology: A Text and Atlas: With Correlated
Cell and Molecular Biology 8th Edition. Wolters Kluwer; 2020. 928 p.
b) additional
1. Eroschenko V.P. Atlas of Histology with functional correlations / Eroschenko V.P.
[tenth edition]. – Lippincott Williams and Wilkins, 2008
2. Volkov K. S. Ultrastructure of cells and tissues / K. S. Volkov, N. V. Pasechko. –
Ternopil : Ukrmedknyha, 1997. – P. 95.
Internet sources:
1. https://moodle.tdmu.edu.ua/course/view.php?id=1688
2. https://moodle.tdmu.edu.ua/course/view.php?id=233
3. http://aminbiol.com.ua
4. www.cytology.net
5. www.cytology-education.com
6. www.cytology.com.ua
7. http://www.histologyguide.com/
8. http://217.196.164.19/data/books/Volkov(atlas).pdf
9. http://en.wikipedia.org/wiki/Histology
10. http://www.meddean.luc.edu/LUMEN/MedEd/Histo/frames/histo_frames.htm
11. http://www.udel.edu/biology/Wags/histopage/histopage.htm
Created by Larysa Tupol