122
UNIT 1
Organization of the Body
4
short distance into every body opening that is directly continu-
ous with the skin. Te outer layer, or epidermis, of the skin is
keratinized
(ker
9
ah-tin
0
īzd), meaning its surface cells contain
keratin
, a tough protective protein. (We discuss the epidermis in
Chapter 5.) Te other stratified squamous epithelia of the body
are
nonkeratinized
.
Stratified Cuboidal and Columnar Epithelia
Stratified cuboi-
dal epithelium
is quite rare in the body, mostly found in the
ducts of some of the larger glands (sweat glands, mammary
glands). It typically has two layers of cuboidal cells.
Stratified columnar epithelium
also has a limited distribu-
tion in the body. Small amounts are found in the pharynx, the
male urethra, and lining some glandular ducts. Tis epithelium
also occurs at transition areas or junctions between two other
types of epithelia. Only its apical layer of cells is columnar. Be-
cause of their relative scarcity in the body, Figure 4.3 does not
illustrate these two stratified epithelia (but see
A Brief Atlas of
the Human Body
, Plates 8 and 9).
Transitional Epithelium
Transitional epithelium
forms the
lining of hollow urinary organs, which stretch as they fill with
urine (Figure 4.3f). Cells of its basal layer are cuboidal or co-
lumnar. Te apical cells vary in appearance, depending on the
degree of distension (stretching) of the organ. When the organ
is distended with urine, the transitional epithelium thins from
about six cell layers to three, and its domelike apical cells flatten
several cell layers are present; hence “pseudostratified.” Te short
cells are relatively unspecialized and give rise to the taller cells.
Tis epithelium, like the simple columnar variety, secretes or ab-
sorbs substances. A ciliated version containing mucus-secreting
cells lines most of the respiratory tract. Here the motile cilia propel
sheets of dust-trapping mucus superiorly away from the lungs.
Stratified Epithelia
Stratified epithelia contain two or more cell layers. Tey regen-
erate from below; that is, the basal cells divide and push apically
to replace the older surface cells. Stratified epithelia are con-
siderably more durable than simple epithelia, and protection is
their major (but not their only) role.
Stratified Squamous Epithelium
Stratified squamous epi-
thelium
is the most widespread of the stratified epithelia (Fig-
ure 4.3e). Composed of several layers, it is thick and well suited
for its protective role in the body. Its free surface cells are squa-
mous, and cells of the deeper layers are cuboidal or columnar.
Tis epithelium is found in areas subjected to wear and tear, and
its surface cells are constantly being rubbed away and replaced
by division of its basal cells. Because epithelium depends on
nutrients diffusing from deeper connective tissue, the epithelial
cells farther from the basement membrane are less viable and
those at the apical surface are o±en flattened and atrophied.
²o avoid memorizing all its locations, simply remember that
this epithelium forms the external part of the skin and extends a
(d)
Pseudostratified columnar epithelium
Description:
Single layer of cells of differing
heights, some not reaching the free surface;
nuclei seen at different levels; may contain
mucus-secreting cells and bear cilia.
Function:
Secrete substances, particularly
mucus; propulsion of mucus by ciliary action.
Location:
Nonciliated type in male’s
sperm-carrying ducts and ducts of large
glands; ciliated variety lines the trachea, most
of the upper respiratory tract.
Photomicrograph:
Pseudostratified ciliated
columnar epithelium lining the human trachea (800
m
).
Trachea
Cilia
Pseudo-
stratified
epithelial
layer
Basement
membrane
Figure 4.3
(continued)
Epithelial tissues.
(d)
Simple epithelium. (For a related image, see
A Brief Atlas of the Human Body
, Plate 6.)
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