22
U N I T 1
Cell and Tissue Function
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 strati-
fied squamous epithelial cells that are better able to with-
stand the irritating effects of cigarette smoke.
Pseudostratified epithelium
is a type of epithelium in
which all of the cells are in contact with the underly-
ing intercellular 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 sur-
face 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.
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 special-
ized to produce a fluid secretion. This process is usually
accompanied by the intracellular synthesis of macromol-
ecules. 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 synthe-
size, 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.
Simple squamous
Simple cuboidal
Simple columnar
Transitional
Stratified squamous
Pseudostratified columnar
ciliated
FIGURE 1-15.
Representation of the various epithelial tissue
types.
Cell
junctions
Microvilli
Apical surface
Epithelial cells
Basement membrane
Connective tissue
Nerve fiber
Blood vessel
N
W
FIGURE 1-14.
Typical arrangement of epithelial cells in relation
to underlying tissues and blood supply. Epithelial tissue has no
blood supply of its own but relies on the blood vessels in the
underlying connective tissue for nutrition (N) and elimination
of wastes (W).
