CHAPTER 3
Epithelium and Glands
27
Epithelium
Introduction and Key Concepts for Epithelium
Epithelium covers nearly all body surfaces. The basic func-
tions of epithelial tissue are (1) protection of the body from
abrasion and injury (e.g., skin and esophagus); (2) absorption
of material from a lumen (e.g., tubules in kidney, small and
large intestines); (3) transportation of material along a surface
(e.g., cilia-mediated transport in the trachea); (4) secretion of
mucus, hormones, and proteins (e.g., glands); (5) gas exchange
(e.g., alveoli in the lung); and (6) lubrication between two sur-
faces (e.g., mesothelium of pleural cavity). Epithelium is an
avascular
tissue, which lacks a direct blood supply. Nutrients
are delivered by diffusion from blood vessels in the neigh-
boring connective tissue. Most epithelial
tissues are renewed
continuously.
ClassiF cation of Epithelial Tissues
Epithelium can be classiF
ed as
simple
or
stratiF
ed
based on the
number of layers of cells. If there is a single layer of cells, it is
referred to as
simple
epithelium. If there are two or more lay-
ers of cells, it is considered to be
stratif
ed
epithelium. Epithe-
lium is also classiF
ed according to the shape of the cells in the
most superF
cial layer. If the surface cells are fl attened in shape,
it is called
squamous epithelium
. If surface cells are cuboidal
in shape, it is called
cuboidal epithelium
. If the surface cells
are tall, with their height much greater than their width, it is
called
columnar
epithelium
. If the surface cells change shape
in response to stretching and relaxing, it is called
transitional
epithelium
(
urothelium
). As described below, these terms may
be variously combined to designate layers of cells and shapes
forming the superF
cial layer of the epithelium. In some cases,
the height of an epithelial cell represents the level of metabolic
activity. ±or example, epithelial cells lining the thyroid fol-
licle usually exhibit as simple cuboidal epithelium. However,
when the follicle cells are in a high metabolic state, they form
a simple columnar epithelium. By contrast, when the follicle
cells are in a low metabolic state, they form a simple squamous
epithelium.
SIMPLE SQUAMOUS EPITHELIUM
is
composed of one
layer of uniform fl at cells, which rest on the
basement mem-
brane
(±igs. 3-2A to
3-4). Apical surfaces are smooth, and the
width of the cells is greater than their height. The nuclei appear
fl attened and can easily be recognized following
hematoxylin
and
eosin staining
(H&E)
because of the
basophilia
(afF
nity
for blue stains) of the nucleic acids in the nuclei. This type of
epithelium is found
lining the posterior surface of the cornea;
lining blood vessels and lymphatic vessels (where it is called
endothelium
); lining the surface of the body cavities, includ-
ing the pericardial, pleural, and peritoneal cavities (where it is
called
mesothelium
); and lining the alveoli of the lungs.
SIMPLE CUBOIDAL EPITHELIUM
is composed of one
layer of uniform cuboidal cells, which rest on the
basement
membrane
(±igs. 3-5A,B and 3-6A,B). The cell’s height, width,
and depth are roughly equal. Nuclei are centrally placed and
spherical in shape. Some cuboidal cells have long and abun-
dant
microvilli
, which form a
brush border
on their apical
surfaces. Such cells are found in the proximal tubules of the
kidney. Other cuboidal cells have few, short microvilli which
do not form a brush border; these cells can be found in the
distal and collecting tubules of the kidney. Simple cuboidal
epithelium is mainly found lining most of the tubules in the
kidney and in some excretory ducts of glands.
SIMPLE COLUMNAR EPITHELIUM
is composed of one
layer of columnar cells resting on the
basement membrane
(±ig. 3-7A,B). The cell’s height is greater than the width. The
elongated ovoid nucleus is most often located in the basal
region of the cell. The apical surface of this epithelium may
reveal
microvilli
. Microvilli are often densely packed to form
a
brush border
and function to increase the apical surface
area of the cell to aid in absorption of fl
uid and other mate-
rial from a lumen. Simple columnar epithelium can be found
in the digestive tract, oviducts (fallopian tubes) in the female
reproductive system, and ductuli efferentes testis of the male
reproductive system.
PSEUDOSTRATI±IED COLUMNAR EPITHELIUM
is com-
posed of one layer of nonuniform cells that vary in shape and
height (±ig. 3-9A,B). Cells appear similar to stratiF
ed cells,
but all cells are in contact with the
basement membrane
. In
general, most cells are tall columnar cells, but there are also
some short
basal cells
, some of which are stem cells. The
most widespread type of pseudostratiF
ed columnar epithe-
lium is found in the respiratory tract and has long F
ngerlike,
motile structures called
cilia
on the apical surface of the cells.
Cilia aid in the transport of material across the surface of
epithelial cells. PseudostratiF
ed columnar epithelium is often
referred to as
respiratory epithelium
because it is found in
the linings of the respiratory tract, including the nasal cavity,
trachea, and primary bronchi.
STRATI±IED SQUAMOUS EPITHELIUM
contains several
layers
of cells, with cells in the superF
cial layer being fl
attened.
Only the
deepest layer of cells is in contact with the basement
membrane. This type of epithelium protects the body against
injury, abrasion, dehydration, and infection. This epithelium
may be
keratinized
or
nonkeratinized
, depending on functional
demands (±igs. 3-13A,B and 3-14A,B).
Keratinized stratif
ed
squamous epithelium
is found in the skin. The top layers consist
of either thick or thin keratinized cells (fl
attened, nonnucleated
dead cells) that are F
lled with tonoF
laments. The thickness of
the stratiF
ed squamous epithelium varies from region to region.
Nonkeratinized stratif
ed squamous epithelium
is similar to
keratinized squamous epithelium except that surface cells are
nucleated instead of nonnucleated. This type of epithelium
often covers wet surfaces and is found lining the oral cavity
(soft palate, cheeks, and fl
oor of the mouth), esophagus, vagina,
and true vocal cords.
STRATI±IED CUBOIDAL EPITHELIUM
is composed of two
or three layers of cuboidal cells with the basal layer of cells
often appearing nonuniform in distribution (±ig. 3-15A,B). It
is mainly found lining large ducts of exocrine glands. The cells
often have smooth apical surfaces and form barriers and ducts.
STRATI±IED COLUMNAR EPITHELIUM
is also composed
of two or three layers
of cells. The top layer is columnar in
shape and the basal layer is usually cuboidal in shape (±igs. 3-1B
and 3-16A,B). This is not a common type of epithelium and has
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