College of Applied Medical Sciences- Department of Medical lab Histology and Histological Techniques course MLAB241 Level 5 Gp.495
UNIT 1: Epithelium Study Guide
Epithelial tissue comprises one of the four basic tissue types. The others
are connective tissue (support cells, immune cells, blood cells), muscle
tissue(contractile cells), and nervous tissue. Organs represent various
combinations of these four basic tissue types, which thus comprise the entire
body. Each tissue type retains its fundamental character wherever it occurs.
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OVERVIEW.
CHARACTERISTICS of epithelial tissue.
Contrast with connective tissue.
TERMINOLOGY for describing epithelial tissue.
COMMON TYPES of epithelial tissue.
Glands.
REPLACEMENT of epithelial tissue.
Epithelial tissue pathology.
Examples.
OVERVIEW
The boundary between you and your environment is marked by a
continuous surface, or epithelium, of contiguous cells.
The shape of this boundary is complex, continuing unbroken from
skin through various orifices and including the many invaginations into the internal
organs of the respiratory, urinary, digestive, and gastrointestinal tracts.
Several of the body's organs are primarily epithelial tissue, with each cell
communicating with the surface via a duct or tube. Examples include lung, kidney,
and liver.
Throughout these organs (with very few exceptions) the surface remains
uninterrupted by any gap between adjacent cells. All exchange of materials and
information (nutrients, gases, wastes, sensation, heat) between the body and the
environment must take place across this boundary.
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Dr. ebtsam mohamad
College of Applied Medical Sciences- Department of Medical lab Histology and Histological Techniques course MLAB241 Level 5 Gp.495
Epithelial tissue thus serves both as a protective barrier for the body and as an
active interface with the environment. The structural and functional integrity of
this epithelium is vital for normal health.
Epithelial tissue is one of the four basic tissue types.
Several features characterize epithelial tissue and distinguish it from connective
tissue, muscle tissue, and nervous tissue.
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Epithelial tissue covers surfaces with an uninterrupted layer of cells.
Epithelial cells are attached to one another.
Intercellular spaces in epithelium are small.
Epithelial cells are polarized.
Epithelial cells are separated from the underlying tissue by a basement
membrane.
CHARACTERISTICS OF EPITHELIAL TISSUES
Nearly all epithelial tissues share some common features:
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Epithelial tissue comprises an uninterrupted
layer of cells. Epithelium covers nearly all
external and internal body surfaces.
Even when an epithelial surface seems to be
penetrated by a hole (such as a the pore of a
gland) that hole is really just an invagination of
the epithelium (i.e., the glandular duct is itself
lined by epithelium, and the secretory portion of
the gland is also epithelial tissue). [Example]
Epithelial cells are attached to one
another. Special devices (intercellular
junctions, tonofilaments) provide for structural
integrity of the epithelium. There are several
types of cell junctions.
o
Adhering junctions (e.g.,
desmosomes) provide mechanical
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Dr. ebtsam mohamad
College of Applied Medical Sciences- Department of Medical lab Histology and Histological Techniques course MLAB241 Level 5 Gp.495
o
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attachment. Keratinocytes are joined to
one another by many adhering junctions.
Tight (occluding) junctions block
diffusion; they typically form a seal or
gasket around the apical end of cells
comprising simple epithelia (i.e.,
epithelia comprising only a single layer of
cells). This junction helps assure
adequate separation between different
fluid compartments (i.e., between the
contents of the intestine and the
interstitial fluid of the body).
Gap junctions provide direct
intercytoplasmic communication
between joined cells. That is, ions or
small molecules can pass through gap
junctions directly from the cytoplasm of
one cell into the cytoplasm of an
adjacent cell, without passing into
intercellular space.
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Intercellular spaces in epithelium are
small. Within the epithelium, the narrow
spaces between cells are sealed off from the
external environment by cell junctions near the
free surface of the cells. Compared to ordinary
connective tissue, epithelial tissue has very
little interstitial fluid.
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Epithelial tissue is polarized. An epithelium
has a free surface, the apical surface, exposed
to the outside, and an attached surface,
the basal surface, resting on the underlying
connective tissue.
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In a simple (single-layered) epithelium,
each cell is polarized. The base of each
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Dr. ebtsam mohamad
College of Applied Medical Sciences- Department of Medical lab Histology and Histological Techniques course MLAB241 Level 5 Gp.495
o
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cell is attached to an
underlying basement membrane while
the apical end faces free space. Lateral
surfaces are attached to neighboring
epithelial cells.
Polarization of epithelial cell structure is
particularly evident in secretory and
absorptive cells where the arrangement
of intracellular organelles reflects the
directional transport of material.
Less visible but equally important is the
localization of membrane proteins, which
confer special properties to the
membrane (i.e., for cell junctions or for
active or passive transport of ions and
molecules in and out of the cell), to
apical, lateral or basal surfaces of the
epithelial cell.
Epithelial cells are separated from the
underlying tissue by a basement
membrane. The basement membrane is a thin
sheet of collagen and glycoproteins produced
in part by the epithelial cells themselves and in
part by underlying connective tissue cells
(specifically, fibroblasts).
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The basement membrane serves to
regulate cell behavior and can limit the
spread of some neoplasms.
In certain places, the basement
membrane assumes major
significance. (For example, in glomeruli
of the kidney it serves as a filter for blood
plasma on its way to becoming urine.) In
other places, the absence of a basement
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Dr. ebtsam mohamad
College of Applied Medical Sciences- Department of Medical lab Histology and Histological Techniques course MLAB241 Level 5 Gp.495
o
membrane is notable and functionally
significant. (For example, the absence of
a basement membrane in
the liver permits blood plasma to contact
hepatocytes directly.)
Basement membranes are at best
inconspicuous in H&E stained
preparations, but they can be
demonstrated with PAS stain or
certain silver stains.
These common features confer a characteristic appearance upon most epithelial
tissues, no matter how various their cells may be. In nearly all epithelia, cells are
attached to one another and thus evenly spaced, in distinct contrast with connective
tissue cells which are scattered and not attached.
CONTRAST WITH CONNECTIVE TISSUE
The characteristics of epithelial cells (above) contrast with those of connective
tissue cells, which are not attached to one another and so tend to be randomly
distributed, often with conspicuous spaces containing extracellular matrix in
between neighboring cells.
Many support cells and immune cells are typically unpolarized
and quiescent, with little cytoplasm and with nuclei containing
condensed chromatin.
In contrast, because many epithelial cells are
actively manufacturing, secreting, absorbing or dividing, epithelial
cells usually have conspicuous cytoplasm and large, relatively
euchromatic nuclei (with finely granular chromatin) and with
prominent nucleoli.
Each of the following examples illustrates the distinct difference between epithelial
tissue and connective tissue.
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Dr. ebtsam mohamad
College of Applied Medical Sciences- Department of Medical lab Histology and Histological Techniques course MLAB241 Level 5 Gp.495
Skin
Cornea
Trachea
DESCRIPTIVE TERMINOLOGY
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"Simple" signifies a single layer of epithelial cells.
"Stratified" signifies two or more layers of epithelial cells.
o By convention, a stratified epithelium is described according to the
shape of cells on its free surface. Regardless of whether the surface
cells are squamous, cuboidal, or columnar, the underlying cells are
usually cuboidal.
o Pseudostratified signifies that two or more rows of nuclei give the
(false) appearance of a stratified epithelium. But because each cell
rests on the basement membrane, this is "really" a single layer of
cells.
"Squamous" (from squama, scale) describes cells that are flattened.
o Squamous epithelial cells are typically inconspicuous in cross section,
appearing as thin lines with a bulge at the nucleus.
o A simple squamous epithelium is so thin as to be barely visible by
light microscopy.
o A stratified squamous epithelium is quite thick, with squamous cells
on the surface overlying deeper layers of taller cells.
"Cuboidal" describes cells that are rather box-shaped, not flattened.
o Cuboidal epithelial cells typically appear square or rectangular in
cross section, with a round, centrally placed nucleus.
o A cuboidal epithelium is typical of glandular ducts.
"Columnar" signifies that the cells at the free surface are considerably taller
than wide.
o By convention, cells are called columnar if their height is at least
twice their width, and cuboidal if not so tall.
o A columnar epithelium is often associated with secretion or
absorption.
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Dr. ebtsam mohamad
College of Applied Medical Sciences- Department of Medical lab Histology and Histological Techniques course MLAB241 Level 5 Gp.495
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"Glandular" describes epithelial tissue found in glands and specialized for
exocrine or endocrine secretion.
"Transitional" describes a specialized type of epithelium which lines urinary
passages.
TYPES OF EPITHELIAL TISSUE
The most common types of epithelial tissue are regularly associated with particular
functions and locations.
Stratified Squamous
Epithelium
Cuboidal Epithelium
Endothelium
Simple Columnar
Epithelium
Simple Squamous Epithelium
Mesothelium
Pseudostratified
Epithelium
Transitional Epithelium
Glandular Epithelium
Stratified squamous epithelium consists of flattened (squamous) cells on the
surface overlying multiple layers of cells that usually are more cuboidal toward the
base of the epithelium.
Stratified squamous epithelium is usually protective. The multiple layers are too
thick for efficient transport of materials (neither secretory or absorptive). The
innermost layer continually produces cells (via mitosis) to replace those lost from
the outer surface.
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Outer layers may be keratinized (dead and hardened) on dry surfaces such
as skin. Keratinization provides extra protection against abrasion and
dessication.
Alternatively, outer layers may be nonkeratinized on wet, mucous surfaces,
such as the cornea of the eye or the lining of the upper GI tract (e.g., cheek
and esophagus).
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Dr. ebtsam mohamad
College of Applied Medical Sciences- Department of Medical lab Histology and Histological Techniques course MLAB241 Level 5 Gp.495
Skin
Cornea
Esophagus
Simple columnar epithelium consists of a single layer of tall (columnar cells).
Simple columnar epithelium is usually involved in active secretion and/or
absorption of material across the single cell layer, or (if ciliated) in movement
along the surface. Simple columnar epithelium lines the digestive tract and the
female reproductive tract (as well as numerous other surfaces).
Stomach
Small
intestine
Uterus
Fallopian
tube
© Blue Histology
How many cell layers
appear in a section
depends on the angle
between the section
plane and the surface
of the epithelium. A
single surface is
usually not lined by
several types of
epithelia, so the
number of epithelial cell layers will normally be the smallest
number of layers visible anywhere along the surface lined by
the epithelium.
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Dr. ebtsam mohamad
College of Applied Medical Sciences- Department of Medical lab Histology and Histological Techniques course MLAB241 Level 5 Gp.495
A pseudostratified columnar epithelium appears stratified, typically with nuclei
located in at least two more-or-less distinct levels. But in fact every cell rests on
the basement membrane, so the epithelium is technically "simple", in spite of
appearances.
Trachea
Epididymis
A pseudostratified columnar epithelium is characteristic of the respiratory tract and
of the ducts in the male reproductive system.
Cuboidal epithelium consists of boxy (cuboidal) cells on the surface. If stratified,
the deeper layers are usually also cuboidal.
Cuboidal epithelium is commonly encountered in glandular ducts. Cuboidal
epithelial cells may be active (pumping material into or out of the lumen) or
passive, depending on location and cellular specialization. Small ducts typically
have a simple cuboidal epithelium. Larger ducts may have a stratifiedcuboidal
epithelium.
duct
kidney
kidney
Simple squamous epithelium consists of a single, very thin layer flattened
(squamous) cells.
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Dr. ebtsam mohamad
College of Applied Medical Sciences- Department of Medical lab Histology and Histological Techniques course MLAB241 Level 5 Gp.495
Simple squamous epithelium may be located at sites of rapid diffusion, such as
the lining of lung alveoli, the lining of blood vessels (calledendothelium), and at
sites where very little activity is occuring, such as Bowman's capsule in the kidney
and the lining of major body cavities (calledmesothelium).
endothelium
lung
kidney
mesothelium
Transitional epithelium, also called urothelium, is a stratified epithelium lining
the distensible walls of the urinary tract. The name "transitional" derives from this
tissue's ability to change its shape from cuboidal to squamous when stretched.
renal pelvis
Endothelium and mesothelium are special names given to the lining of certain
internal surfaces.
The entire circulatory system (heart, arteries, veins, capillaries, sinusoids and
lymphatics) is lined by a simple squamous epithelium that is
calledendothelium. (The inner lining of the cornea is also called "endothelium".)
The major body cavities (peritoneal, pleural, pericardial) are lined by a
mesodermally derived simple squamous epithelium that is called mesothelium.
endothelium mesothelium
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Dr. ebtsam mohamad
College of Applied Medical Sciences- Department of Medical lab Histology and Histological Techniques course MLAB241 Level 5 Gp.495
Glandular epithelium is specialized for secretion Glandular epithelium forms
more-or-less complex invaginations of epithelial tissue that lie deep to the surface
epithelium lining obvious external and internal body
surfaces.
Glandular epithelium may be differentiated into secretory
units (which can be specialized for various products)
and ducts(which typically appear less specialized, although
ducts may function actively to reabsorb water and thus
concentrate the secretory product).
Although large exocrine glands like the liver and pancreas may appear solid, each
secretory cell has an apical surface exposed to a lumen. This lumenal space, in
turn, leads through ducts to the outside of the body.
REPLACEMENT OF EPITHELIAL TISSUE
Certain epithelia, notably epidermis and intestinal epithelium, are continually
recycled, with with new cells being created by mitotic activity while old cells are
sloughed off (from the surface of the epidermis or the tips of intestinal villi). Many
additional epithelial cells (not just those of skin and intestine) have the ability to
respond to the stimulus of injury with mitotic activity and cell migration, to
regenerate tissue following damage. This confers on most epithelia an "automatic"
ability to deal effectively with injury by replacing lost tissue with new growth from
undamaged edges.
EPITHELIAL TISSUE PATHOLOGY
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Dr. ebtsam mohamad
College of Applied Medical Sciences- Department of Medical lab Histology and Histological Techniques course MLAB241 Level 5 Gp.495
The surface location of many epithelial tissues exposes them to a variety of insults,
ranging from mechanical damage (cuts, scrapes) and active penetration
(mosquitoes, parasites, hypodermics) to bacterial and fungal attack and poisoning
by toxic chemicals.
In simple clean wounds of the skin, one of the earliest healing accomplishments
may be proliferation and spread of epithelial keratinocytes, re-establishing
epidermal continuity in as little as 24 hours. Deep (third degree) burns are so
serious largely because they destroy the many hair follicles and sweat glands that
invaginate deep into the dermis and serve as efficient sources of epithelial regrowth
after more superficial injury.
The importance of epithelial cell regeneration is dramatically illustrated by
recovery from cholera. A toxin from the cholera vibrio kills the intestinal
epithelium. Resulting loss of bodily fluid from the uncovered mucosa leads
to copious diarrhea, massive dehydration, and death within a few days. However,
if patients can be kept hydrated for those few days, epithelial replacement by stem
cell division will restore normal function.
When an epithelial cells' ability to divide is stimulated inappropriately, it can result
in the formation of a tumor. Cells in epithelial tumors often retain their basic
epithelial character, remaining attached to one another and differentiating to form
layered structures. As long as the neoplastic cells respect the basement membrane,
the tumor will remain localized. But once cells break through this boundary they
can enter circulation and metastasize.
The name carcinoma is applied to any cancer (malignant neoplasm) of epithelial
origin; adenocarcinoma names a cancer of glandular origin. (Cancers of
mesenchymal origin are called sarcomas.)
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