Epithelial Tissue.
Epithelial tissue covers the body’s
outer surface and lines the internal closed cavities (including blood vessels)
and body tubes that communicate with the exterior (gastrointestinal,
respiratory, and genitourinary tracts). Epithelium also forms the secretory
portion of glands and their ducts.
Origin and Characteristics
Epithelial tissue is derived from
all three embryonic layers. Most epithelia of the skin, mouth, nose, and anus
are derived from the ectoderm. Linings of the respiratory tract,
gastrointestinal tract, and glands of the digestive system are of endodermal
origin. The endothelial lining of blood vessels originates from the mesoderm.
Many types of epithelial tissue retain the ability to differentiate and undergo
rapid proliferation for replacing injured cells.
The cells that make up epithelium
have three general characteristics:
• They are
characterized by three distinct surfaces: a free surface or apical surface, a
lateral surface, and a basal surface.
• They are
closely apposed and joined by cell-to-cell adhesion molecules (CAMs), which form specialized cell junctions.
• Their
basal surface is attached to an underlying basement membrane (Fig. 4.18).
The characteristics and geometric
arrangement of the cells in the epithelium determine their function. The free
or apical surface is always directed toward the exterior surface or lumen of an
enclosed cavity or tube, the lateral surface communicates with adjacent cells and is characterized by
specialized attachment areas, and
the basal surface rests on the basement membrane anchoring the cell to the surrounding connective tissue.
Epithelial tissue is avascular (i.e.,
without blood vessels) and must therefore receive oxygen and nutrients from
the capillaries of the connective tissue on which the epithelial tissue rests
(see Fig. 4.18). To survive, epithelial tissue must be kept moist. Even the
seemingly dry skin epithelium is kept moist by a nonvitalized, waterproof layer
of superficial skin cells called keratin, which prevents evaporation of
moisture from the deeper living
cells.
Basement Membrane.
Underneath all
types of epithelial tissue is an extracellular matrix, called the basement membrane.
A basement membrane consists of the basal lamina and an underlying reticular
layer. The terms basal lamina and basement membrane are often
used interchangeably. Epithelial cells have strong intracellular protein
filaments (i.e., cytoskeleton) that are important in transmitting
mechanical stresses from one cell
to another.
Cell Junctions and Cell-to-Cell
Adhesions. Cells of epithelial
tissue are tightly bound together by specialized junctions. These specialized
junctions enable the cells to form barriers to the movement of water, solutes,
and cells from one body compartment to the next. Three basic types of
intercellular junctions are observed in epithelial tissues: continuous tight
junctions, adhering junctions, and gap junctions (Fig. 4.19).
Continuous tight or occluding junctions (i.e., zonula
occludens), which are found only in epithelial tissue, seal the surface
membranes of adjacent cells together. This type of intercellular junction
prevents materials such as macromolecules in the intestinal contents from
entering the intercellular space.
Adhering junctions represent sites of strong adhesion between
cells. The primary role of adhering junctions may be that of preventing cell
separation. Adhering junctions are not restricted to epithelial tissue; they
provide adherence between adjacent cardiac muscle cells as well. Adhering
junctions are found as continuous, beltlike adhesive junctions (i.e., zonula
adherens) or scattered, spotlike adhesive junctions, called desmosomes (i.e.,
macula adherens). A special feature of the adhesion belt junction is that
it provides an anchoring site to the cell membrane for microfilaments. In
epithelial desmosomes, bundles of keratin-containing intermediate filaments (i.e.,
tonofilaments) are anchored to the junction on the cytoplasmic area of the
cell membrane. A primary disease of desmosomes is pemphigus, which is caused by
a buildup of antibodies to desmosome proteins.
Affected people have skin and
mucous membrane blistering. Hemidesmosomes,
which resemble a
half-desmosome, are another type of junction. They are found at the base of
epithelial cells and help attach the epithelial cell to the underlying
connective tissue.
Gap or nexus junctions involve the close
adherence of adjoining cell membranes with the formation of channels that link
the cytoplasm of the two cells. Gap junctions are not unique to epithelial
tissue; they play an essential role in many types of cell-to-cell
communication. Because they are low-resistance channels, gap junctions are
important in cell-to-cell conduction of electrical signals (e.g., between
cells in sheets of smooth muscle or between adjacent cardiac muscle cells,
where they function as electrical synapses). These multiple communication
channels also enable ions and small molecules to pass directly from one cell to
another.
Types of Epithelium
Epithelial tissues are classified
according to the shape of the cells and the number of layers that are present: simple,
stratified, and pseudostratified. The terms squamous (thin
and flat), cuboidal (cube shaped), and columnar (resembling a
column) refer to the cells’ shape
(Fig. 4.20).
Simple Epithelium.
Simple epithelium
contains a single layer of cells, all of which rest on the basement membrane. Simple
squamous epithelium is adapted for filtration; it is found lining the blood
vessels, lymph nodes, and alveoli of the lungs. The single layer of squamous
epithelium lining the heart and blood vessels is known as the endothelium.
A similar type of layer, called the mesothelium, forms the serous
membranes that line the pleural, pericardial, and peritoneal cavities and cover
the organs of these cavities. A simple cuboidal epithelium is found on the
surface of the ovary and in the thyroid. Simple columnar epithelium lines
the intestine. One form of a simple columnar epithelium has hairlike
projections called cilia, often with specialized mucus-secreting cells
called goblet cells. This form of simple columnar epithelium lines the
airways of the respiratory tract.
Stratified and Pseudostratified
Epithelia. Stratified
epithelium contains more than one layer of cells, with only the deepest layer
resting on the basement membrane. It is designed to protect the body surface. Stratified
squamous keratinized epithelium makes up the epidermis of the skin. Keratin
is a tough, fibrous protein found as filaments in the outer cells of skin. A
stratified squamous keratinized
epithelium is made up of many epithelium
is found on moist surfaces such as the mouth and tongue. Stratified cuboidal and columnar epithelia are found in the
ducts of salivary glands and the larger ducts of the mammary glands. In
smokers, the normal columnar ciliated epithelial cells of the trachea and
bronchi are often replaced with stratified squamous epithelium cells that are
better able to withstand the irritating effects of cigarette smoke.
Pseudostratified epithelium is a type of epithelium in which all of the
cells are in contact with the underlying inter-cellular matrix, but some do not
extend to the surface. A pseudostratified ciliated columnar epithelium with
goblet cells forms the lining of most of the upper respiratory tract. All of
the tall cells reaching the surface of this type of epithelium are either
ciliated cells or mucus-producing goblet cells. The basal cells that do not
reach the surface serve as stem cells for ciliated and goblet cells.6 Transitional
epithelium is a stratified epithelium characterized by cells that can change
shape and become thinner when the tissue is stretched. Such tissue can be
stretched without pulling the superficial cells apart. Transitional epithelium
is well adapted for the lining of organs that are constantly changing their volume, such as the urinary
bladder.
Glandular Epithelium. Glandular epithelial tissue is formed by cells specialized to produce a fluid secretion. This process is
usually accompanied by the intracellular synthesis of macromolecules. The
chemical nature of these macromolecules is variable. The macromolecules
typically are stored in the cells in small, membrane-bound vesicles called secretory
granules. For example, glandular epithelia can synthesize, store, and
secrete proteins (e.g., insulin), lipids (e.g., adrenocortical
hormones, secretions of the sebaceous glands), and complexes of carbohydrates
and proteins (e.g., saliva). Less common are secretions that require
minimal synthetic activity, such as those produced by the sweat glands.
All glandular cells arise from
surface epithelia by means of cell proliferation and invasion of the underlying
connective tissue, and all release their contents or secretions into the
extracellular compartment. Exocrine glands, such as the sweat glands and
lactating mammary glands, retain their connection with the surface epithelium
from which they originated. This connection takes the form of epithelium-lined
tubular ducts through which the secretions pass to reach the surface. Exocrine
glands are often classified according to the way secretory products are
released by their cells. In holocrine-type cells (e.g., sebaceous
glands), the glandular cell ruptures, releasing its entire content into the
duct system. New generations of cells are replaced by mitosis of basal cells. Merocrine-
or eccrine-type glands (e.g., salivary glands, exocrine glands of
the pancreas) release their glandular products by exocytosis. In apocrine secretions
(e.g., mammary glands, certain sweat glands), the apical portion of the
cell, along with small portions of the cytoplasm, is pinched off the glandular
cell. Endocrine glands are epithelial structures that have had their
connection with the surface obliterated during development. These glands are
ductless and produce secretions (i.e., hormones) that move directly into
the bloodstream.
