-
1
Epithelium
There are two basic types of epithelial tissues: covering and
lining epithelia and glandular
epithelia
Diagrammatic illustration showing epithelial cells resting on
basement membrane
Lining epithelial cells form a continuous layer over all the
free surfaces of the body:
o The outer layer of the skin.
o The inner surface of the digestive and respiratory
cavities.
o The inner surface of the heart and blood vessels.
o The walls and the organs of the closed ventral body
cavities.
o The ducts of the exocrine glands.
Glandular epithelia make up most of the glands in the body.
General Features:
Maximum cell-to-cell contact.
Minimum extracellular material.
Cell junctions: Several types of junctional specializations
unite adjacent epithelial
cells (tight junctions, desmosomes and gap junctions).
Supported by basement membrane (basal lamella).
Avascular: There are no blood vessels within the epithelial
layer.
Derived from all embryonic germ layers, including endoderm,
mesoderm and
ectoderm
With the exception of endocrine glands,
a. All epithelia have one free surface, called the apical
surface, which is exposed
at the body surface or at the lumen (space) of the body cavity,
duct, tube or
vessel.
-
2
b. The basal surface of epithelium rests on a basement membrane:
a non-living
adhesive material secreted by the epithelium and the underlying
connective
tissue.
c. These cells are often characterized by frequent cell division
because they are
exposed to wear and tear and injury, necessitating
replacement.
Surface specializations or modifications (Cell polarity):
Polarity (structural and functional
asymmetry) is characteristic of most epithelial cells. It is
best seen in simple epithelia, where
each cell has 3 surfaces: an apical (Free) surface, lateral
surfaces, and a basal surface
attached to the basal lamina.
1- Apical (Surface or luminal) modifications: It is specialized
to carry out functions that
occur at these interfaces, including secretion, absorption, and
movement of luminal contents.
Cilia: are membrane-covered extensions of the entire apical
surface. They beat in
waves, often moving a surface coat of mucus and trapped
materials. Ciliated epithelia
include ciliated pseudostratified columnar (respiratory)
epithelium and the ciliated
simple columnar epithelium of the oviducts.
Flagella: are also concerned with movement. Spermatozoa, derived
from seminiferous
epithelia, are the only flagellated human cells.
Microvilli (brush border or striated border): are plasma
membrane-covered
extensions of the cell surface. Their cores are composed of
parallel actin
microfilaments; these are anchored in a dense mat of filaments
in the apical cytoplasm
called the terminal web. By interacting with cytoplasmic myosin,
the microfilaments
can contract, shortening the microvilli. The apical surface of
absorptive cells is usually
covered with microvilli, which greatly increase the apical
surface area when extended.
Microvillus-covered epithelia, said to exhibit a striated
border, or brush border,
include the absorptive simple columnar epithelium lining the
small intestines and the
absorptive simple cuboidal epithelium lining the proximal
tubules of the kidney.
Stereocilia: are not true cilia but very long microvilli. They
are found in the male
reproductive tract (epididymis. ductus deferens). where they
have an absorptive
function, and in the internal ear (hair cells of the maculae and
organ of Corti). where
they have a sensory function.
-
3
2- Lateral specializations (junctions and folding): cells attach
tightly to one another by
specialized intercellular junctions. Several types of junctions
can be seen, such as:-
o Zonula occludens (tight junctions, occluding junctions): are
located near the cell apex
and seal off the intercellular space, allowing the epithelium to
isolate certain body
compartments (they help keep intestinal bacteria and toxins out
of the bloodstream).
Their structure, best seen in freeze-fracture preparations,
results from the fusion of 2
trilaminar plasma membranes of adjacent cells to form a
pentalaminar structure; this
fusion may require specific "tight-junction proteins." In some
tissues, tight junctions
can be disrupted by removing calcium ions or treating with
protease.
o Zonula adherens (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. 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.
o 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. Desmosomes, distributed in patches along the
lateral membranes of
most epithelial cells. Transverse thin 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
o Junctional complex: combination of zonula occludens, zonula
adherens and
desmosomes.
o Communicating junctions (gap or nexus junctions): 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, 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 (
-
4
3- Basal specialization 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. A basal lamina underlies all true epithelial tissues. The
basal lamina is a sheet-like
structure, usually composed of type IV collagen, proteoglycan,
and laminin, a
glycoprotein that aids in binding cells to the basal lamina. The
basal lamina exhibits
electron-lucent and electron-dense layers termed the lamina
lucida (lamina rara) and
the lamina dense, respectively. 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 laminae accompanied by
reticular laminae are often
thick enough to be seen with the light microscope as PAS-
positive layers and are
sometimes termed basement membranes. 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. 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.
4- 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 RER is preferentially located in
the basal cytoplasm, the
nucleus in the basal to middle region just above the RER, and
the Golgi complex just
above the nucleus. Mature secretory vesicles collect in the
apical cytoplasm.
-
5
Cell polarity: the upper, lower and lateral surface modification
of epithelium.
Functions of epithelium
1. Protection: Epithelia protect underlying tissues against
physical damage, drying out,
chemical injury and infection.
2. Epithelia allow and regulate the passage of materials
(diffusion, absorption, filtration,
secretion, excretion) into and out of the tissues of the body
which they cover or line.
3. Sensory reception: Specialized epithelia form sensory parts
of organs such as the eye,
ear, mouth (taste buds), and nose (olfactory epithelium).
4. Most glands are derived from epithelial cells specialized for
producing secretions.
The classification of the surface epithelia is based on:
1. The number of cell layers
- Simple epithelium: Being composed of one layer of cells only,
they are very thin.
They are found in areas of minimum wear and tear. Their main
function is to allow
passage of substances between the lumen and the surrounding
tissues.
- Stratified epithelium: Being composed of several layers of
cells, they are very
thick. Their main function is to protect the tissues that they
cover. The shape of the
cells closest to the basement membrane is quite different from
that of the cells at the
-
6
top, near the lumen. The stratified epithelia are further
classified according to the
shape of the cells at the free surface.
2. The shape of the cells
o Flat: squamous epithelium
o Square: cuboidal epithelium
o Rectangular: columnar epithelium
o If the shape changes depending of the degree of stretching of
the tissue:
transitional epithelium.
Most often, it is very hard to distinguish the cell's boundary
by light microscope. To figure out
what type of cells you are dealing with, look at the shape and
position of the nuclei:
If the nuclei are flat and parallel to the free surface:
squamous epithelium,
If the nuclei are oval and parallel to the axis of the cell and
situated at its base:
columnar epithelium,
If the nuclei are round and situated in the middle of the cell:
cuboidal epithelium.
-
7
Types of epithelium and location
- Simple squamous: with flattened nuclei. Present in the alveoli
of lungs, Kidneys, Lining
of visceral organs and all blood vessels. Function: selective
diffusion, absorption or
secretion.
- Simple cuboidal: with central rounded nuclei. Present in
liver, pancreas, acini of glands,
lines small ducts and tubules. Function: excretory, secretory or
absorptive.
- Simple columnar: with basal oval nuclei. Present in the
absorptive surfaces (intestine);
s e c r e t o r y s u r f a c e s ( s t o m a c h ) ; l i n i n
g g a l l b l a d d e r ( a b s o r b s w a t e r ) .
- Simple columnar ciliated: Present in female reproductive tract
(fallopian tube, uterus).
- Pseudostratified columnar ciliated: nuclei disposed at
different levels; basal cells do
not extend to surface; Present in larger airways of respiratory
system (trachea, bronchi).
- Stratified squamous keratinizing: Upper cell layer composed of
squamous sells.
Present in surface of skin
- Stratified squamous non-keratinizing (mucous membrane):
resists abrasion;
moistened by glandular secretions. Present in oral cavity,
pharynx, esophagus, anal canal,
uterine cervix, and vagina.
- Stratified cuboidal: Upper cell layer composed of cuboidal
cells. Only 2 to 3 cell layers;
lining large excretory ducts of salivary gland.
- Stratified columnar: The surface cell layer is columnar in
shape, and it could be;
- Non-ciliated as in penile part of male urethra, large ducts of
glands, recto-anal
junction and fornicies of the conjunctiva.
- Ciliated as in fetal esophagus, nasal surface of the soft
palate, and the laryngeal
surface of the epiglottis.
-
8
- Transitional: urinary tract; accommodates stretching and
toxicity of urine; surface cells
larger, pale-staining, scalloped surface outline; luminal
surface appears thickened; may
be binucleate; large, round, prominent nucleoli.
Illustration showing types of the surface epithelium
-
9
Glandular epithelium (Parenchymous epithelium)
Generally formed by down growth of surface epithelium into
underlying connective tissue,
and separated from connective tissue by basal lamina.
Illustration showing development of glandular epithelium
Classification of glands:
1- According to presence or absence of ducts: exocrine glands
(have duct system) and
endocrine glands (ductless) secrete hormones glands.
2- According to Nature of secretion:
- Serous secretion: secret watery fluid rich in protein (parotid
glands)
- Mucous secretion: secret mucus; poor in protein (goblet
cells)
- Muco-serus secretion: as in mixed salivary glands
- Milky secretion: mammary gland
- Wax secretion: glands in external ear
- Fatty secretion: sebaceous glands
- Watery secretion: sweat glands
- Cellular secretion: ovary and testis.
-
10
3- According to mode of secretion:
- Apocrine glands: a small portion of the apical cytoplasm
discharged with
the secretory products. eg. Mammary glands and some sweat
glands.
- Holocrine glands: discharge who cell; sebaceous glands
(sebum).
- Merocrine glands: in which secretion occurs by exocytosis;
i.e. no cellular
changes as parotid glands.
4- According to Number of cells:
- Unicellular glands (goblet cells) secrete mucus.
- Multicellular glands that can be further classified according
to the shape of secretory
portion into:
Diagrammatic illustration of the types of glandular
epithelium
A- Simple tubular gland: large intestine.
B- Simple coiled tubular gland: sweat glands.
C- Simple acinar (alveolar) gland: (rounded secretory unit)
mucus-secreting glands of
penile urethra
D- Simple branched tubular gland: stomach.
-
11
E- Simple branched acinar gland: sebaceous gland
F- Simple branched tubulo-alveolar glands: glands of oral
cavity.
G- Compound tubular gland: liver, kidney
H- Compound acinar (alveolar) gland: mammary gland
I- Compound tubulo-acinar gland: pancreas
Striated ducts: striations due to mitochondria lined up along
folds of basal membrane;
transport Na and bicarbonate; cells high cuboidal to
columnar
Myoepithelial cells: Many exocrine glands have specialized type
of epithelial cells embedded
in basement membrane between basal lamina and the basal part of
secretory cells; may aid
secretion of acinus glands
Basement membranes (external lamina) is formed of 3 layers;
lamina lucid, lamina densa
and lamina fibroreticularis. The first two layers (are produced
by the epithelia)
Neuro-epithelium: special type of epithelium that has sensory
functions. It is found in;
- Tongue: taste buds - Nose: olfactory mucosa - Eye: retina -
Ear: organ of corti, crista ampullaris, macula sacculi, macula
utriculi.
VI. Major types of epithelial cells
A. Epithelial Cells Specialized for Transport:
1. Ion-transporting cells. Some epithelial cells are specialized
for trans-cellular transport;
ie. they can pump ions across their entire thickness, apex to
base. Sheets of such cells form
active barriers that control ion and water concentrations in
body compartments. Tight
junctions are often found between the cells and appear to
restrict back-flow.
Ion-transporting cells typically have highly infolded basal
plasma membranes that
interdigitate with numerous mitochondria. Commonly, the ion pump
is specific for sodium
tie, it is Na/K+-ATPase), and chloride ions and water follow the
sodium ion flow
passively. Important ion-transporting epithelia are found in the
kidney tubules, striated
ducts of the salivary glands, gallbladder, choroid plexus and
ciliary body of the eye.
-
12
2. Cells that transport by pinocytosis. Epithelial cells
specialized for pinocytosis have
tight junctions and abundant pinocytotic vesicles. The vesicles
transport substances across
the cell from the luminal surface to the basal surface or vice
versa. The best example is the
endothelial cells lining the blood vessels, where trans-cellular
transport is rapid (2-3
minutes).
B. Epithelial Cells Specialized for Absorption:
Specialized absorptive cells lining the digestive tract
(especially the small intestine) have
numerous microvilli on their apical surfaces to increase the
exposed area. Small nutrient
molecules diffuse into the microvilli, and contraction of the
microfilaments shortens the
microvilli, bringing the nutrients into the cytoplasm. Other
nutrients are pinocytosed between
microvilli. Absorptive cells with similar specializations occur
in the proximal tubules of the
kidney.
C. Epithelial Cells Specialized for Secretion:
1. Protein-secreting cells. Cells that synthesize proteins for
segregation and secretion have
abundant basophilic RER, a well-developed Golgi complex, and,
frequently, an
accumulation of secretory granules in the cell apex. Proteins
secreted by epithelial cells
include the digestive enzymes, produced by pancreatic acinar
cells and the chief cells of
the stomach: serum albumin, produced by liver hepatocytes; and
protein hormones (eg,
parathyroid hormone, produced by the chief cells of the
parathyroid gland).
2. Polypeptide-secreting cells. Secreted polypeptides have fewer
amino acids than the
secreted proteins. Polypeptide-secreting cells have a small
amount of RER, a supra-nuclear
Golgi complex, and an accumulation of secretory granules in
their bases.
3. Mucous cells occur as unicellular, sheet, or solid glands.
Histologic features include a
light staining, foamy appearance caused by numerous large
mucus-containing vesicles
concentrated near the cell apex. PAS-positive staining from an
abundance of
oligosaccharide residues, predominantly acidophilic staining
with H&E; a large
supranuclear Golgi complex and sparse RER in the base of the
cell.
4. Serous cells have characteristics of protein-secreting cells.
They are usually smaller,
darker staining, and more basophilic than mucus-secreting cells.
Serous cells include
pancreatic acinar cells and secretory cells of the parotid
salivary glands.
-
13
5. Steroid-secreting cells. Endocrine cells specialized to
secrete steroid hormones are
polygonal or rounded, with a central nucleus and pale-staining,
acidophilic cytoplasm that
often contains numerous lipid droplets. Their abundant SER
contains enzymes for
cholesterol synthesis and for converting steroid hormone
precursors (eg, progesterone) into
specific hormones leg, androgens, estrogens, and progesterone).
Their mitochondria
typically have tubular cristae and contain enzymes that convert
cholesterol to progesterone.
Steroid hormones include testosterone, produced by interstitial
cells of the testes: estrogen,
from follicle cells of the ovaries; progesterone, from granulosa
lutein cells of the corpus
luteum; and cortisone and aldosterone, from cells of the adrenal
cortex.