150
UNIT ONE
ids. Such cells are usually found within columnar or cuboidal
epithelium, and one or more of these cells constitute a
gland.
Glands that secrete their products into ducts that open onto
surfaces, such as the skin or the lining of the digestive tract, are
called
exocrine glands.
Glands that secrete their products into
tissue fl
uid or blood are called
endocrine glands.
(Endocrine
glands are discussed in chapter 13.)
An exocrine gland may consist of a single epithelial cell
(unicellular gland), such as a mucous-secreting goblet cell,
or it may be composed of many cells (multicellular gland). In
turn, the multicellular forms can be structurally subdivided
into two groups—simple and compound glands.
A
simple gland
communicates with the surface by means
of a duct that does not branch before reaching the glandular
cells or secretory portion, and a
compound gland
has a duct
that branches repeatedly before reaching the secretory portion.
These two types of glands can be further classiF ed according
to the shapes of their secretory portions. Glands that consist
of epithelial-lined tubes are called
tubular glands;
those whose
terminal portions form saclike dilations are called
alveolar
glands
(acinar glands). Branching and coiling of the secretory
portions may occur as well.
Figure 5.11
illustrates several
types of exocrine glands classiF
ed by structure.
Table 5.3
summarizes the types of exocrine glands, lists their charac-
teristics, and provides an example of each type.
Exocrine glands are also classiF
ed according to the ways
these glands secrete their products. Glands that release fl uid
products by exocytosis are called
merocrine
(mer
o-krin)
glands.
Glands that lose small portions of their glandular
cell bodies during secretion are called
apocrine
(ap
o-krin)
glands.
Glands that release entire cells are called
holocrine
(ho
lo-krin)
glands.
After release, the cells containing accu-
mulated secretory products disintegrate, liberating their
secretions
(f
gs. 5.12
and
5.13)
.
Table 5.4
summarizes these
glands and their secretions.
RECONNECT
To Chapter 3, Movements Into and Out o± the Cell,
page 98.
Stratif
ed Columnar Epithelium
Stratif
ed columnar epithelium
consists of several layers of
cells
(f
g. 5.9)
. The superF
cial cells are elongated, whereas
the basal layers consist of cube-shaped cells. Stratified
columnar epithelium is found in part of the male urethra and
ductus deferens and in parts of the pharynx.
Transitional Epithelium
Transitional epithelium
(uroepithelium) is specialized to
change in response to increased tension. It forms the inner lin-
ing of the urinary bladder and lines the ureters and the superior
urethra. When the wall of one of these organs contracts, the tis-
sue consists of several layers of cuboidal cells; however, when
the organ is distended, the tissue stretches, and the physical
relationships among the cells change. While distended, the tis-
sue appears to contain only a few layers of cells
(f g. 5.10)
. In
addition to providing an expandable lining, transitional epithe-
lium forms a barrier that helps prevent the contents of the uri-
nary tract from diffusing back into the internal environment.
Up to 90% of human cancers are
carcinomas,
growths that origi-
nate in epithelium. Most carcinomas begin on surfaces that contact
the external environment, such as skin, linings of the airways in the
respiratory tract, or linings of the stomach or intestines in the diges-
tive tract. This observation suggests that the more common cancer-
causing agents may not deeply penetrate tissues.
PRACTICE
6
Describe the structure of each type of epithelium.
7
Describe the special functions of each type of epithelium.
Glandular Epithelium
Glandular epithelium
is composed of cells specialized to
produce and secrete substances into ducts or into body fl u-
(b)
(a)
Lumen
Stratified
columnar
epithelium
Connective
tissue
Basement
membrane
Free surface
of tissue
FIGURE 5.9
±StratiF
ed columnar epithelium consists of a superF
cial layer of columnar cells overlying several layers
of cuboidal cells (230×).
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