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 epithelial
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 epithelial boundary.
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.
- Epithelial tissue covers surfaces with an uninterrupted
layer of cells. [More]
- Epithelial cells are attached to one another. [More]
- Intercellular spaces in epithelium are small. [More]
- Epithelial cells are polarized. [More]
- Epithelial cell nuclei are typically fairly large and euchromatic. [More]
- Epithelial cells are separated from the underlying tissue
by a basement membrane. [More]
CHARACTERISTICS OF EPITHELIAL TISSUES
Nearly all epithelial tissues share some common features:
- 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]
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- 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.
- Adhering junctions provide mechanical
attachment. Their shape may be that of bands (zonulae adherens) or spots
(maculae adherens, also called desmosomes).
Keratinocytes (epidermal cells)
are joined to one another by many desmosomes.
- 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 junctional type 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.
- In typical columnar epithelia, these junctions usually occur
in a junctional complex near the apical end of the cell.
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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.
- In a simple (single-layered) epithelium, each
cell is polarized.
The base of the cell is attached to an
underlying basement membrane while
the apical end faces free space.
- Lateral surfaces are attached to neighboring epithelial cells.
In light microscopy, the junctional complex is sometimes visible as a terminal bar.
A cytoskeletal support of intracellular fibers is also sometimes visible as a terminal web.
- 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.
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- Epithelial cell nuclei are often similar in size and shape. Because typically epithelial cells are metabolically active,
their nuclei tend to be fairly large and euchromatic (in contrast to those of most connective tissue cell nuclei).
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- 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).
- 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 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.
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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.
TOP OF PAGE
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 (i.e., heterochromatic).
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. Click
on a thumbnail image for an annotated enlargement.
DESCRIPTIVE TERMINOLOGY
- "Simple" signifies
a single layer of epithelial cells.
- "Stratified" signifies two or more layers
of epithelial cells.
- 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.
- 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.
- Squamous epithelial cells are typically inconspicuous
in cross section, appearing as thin lines with a bulge at the nucleus.
- A simple squamous epithelium
is so thin as to be barely visible by light microscopy.
- 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.
- Cuboidal epithelial cells typically appear square
or rectangular in cross section, with a round, centrally placed nucleus.
- A cuboidal epithelium is typical
of glandular ducts.
- "Columnar" signifies that the cells at
the free surface are considerably taller than wide.
- By convention, cells are called columnar if their
height is at least twice their width, and cuboidal if not so tall.
- A columnar epithelium is often
associated with secretion or absorption.
- "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.
TOP OF PAGE
TYPES OF EPITHELIAL TISSUE
The most common types of epithelial tissue are regularly associated with
particular functions and locations.
Stratified squamous epithelium consists of flattened (squamous) cells
on the surface overlying multiple layers of cells that are usually cuboidal.
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.
- 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, the lining of the upper GI tract (e.g., cheek and esophagus), and the lining of the vagina.
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).
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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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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.
A pseudostratified columnar epithelium is characteristic of the respiratory
tract and of the ducts in the male reproductive
system.
Cuboidal epithelium is characterized by boxy (cuboidal) cells comprising the surface layer of the epithelium; if the epithelium is
stratified, the deeper layers are also usually cuboidal. Nuclei in cuboidal cells are usually round. (Note that an epithelium
is named for the shape of cells in the outermost layer, on the epithelial surface. Both stratified squamous and stratified
cuboidal epithelia typically have cuboidal cells in deeper layers.)
Cuboidal epithelium is commonly encountered in glandular
ducts. Depending on location and cellular specialization, cuboidal epithelial cells may be active (pumping material
toward or away from the lumen) or passive (not altering the duct contents).
Small ducts typically have a simple cuboidal epithelium. Larger
ducts may have a stratified cuboidal epithelium.
Simple squamous epithelium consists of a single, very thin layer of flattened
(squamous) cells.
Simple squamous epithelium may be located at sites of rapid diffusion,
such as the lining of lung alveoli,
the lining of blood vessels (called endothelium),
and at sites where very little activity is occuring, such as Bowman's
capsule in the kidney and the lining of major body cavities (called 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.
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 called endothelium.
(The inner lining of the cornea is also called "corneal endothelium," but that is a different specialized tissue.)
The major body cavities (peritoneal, pleural, pericardial)
are lined by a mesodermally derived simple squamous epithelium that is called
mesothelium.
Glandular epithelium is specialized for secretion Glandular
epithelium forms more-or-less complex invaginations of epithelial tissue that
lie deep to the surface epithelium that lines 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.
For more on glands, click here.
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 epithelially-lined 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 cell's 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.)
