124
UNIT 1
Organization of the Body
4
and become squamouslike. Te ability of transitional cells to
change their shape (undergo “transitions”) allows a greater vol-
ume of urine to flow through a tubelike organ. In the bladder, it
allows more urine to be stored.
Check Your Understanding
5.
Stratified epithelia are “built” for protection or to resist
abrasion. What are the simple epithelia better at?
6.
Some epithelia are pseudostratified. What does this mean?
7.
Where is transitional epithelium found and what is its
importance at those sites?
For answers, see Appendix H.
Glandular Epithelia
Define gland.
Differentiate between exocrine and endocrine glands, and
between multicellular and unicellular glands.
Describe how multicellular exocrine glands are classified
structurally and functionally.
A
gland
consists of one or more cells that make and secrete a
particular product. Tis product, called a
secretion
, is an aque-
ous (water-based) fluid that usually contains proteins, but there
is variation. For example, some glands release a lipid- or steroid-
rich secretion.
Secretion is an active process. Glandular cells obtain needed
substances from the blood and transform them chemically into
a product that is then discharged from the cell. Notice that the
term
secretion
can refer to both the gland’s
product
and the
pro-
cess
of making and releasing that product.
Glands are classified according to two sets of traits:
Where they release their product—glands may be
endocrine
(“internally secreting”) or
exocrine
(“externally secreting”)
Relative cell number—glands may be
unicellular
(“one-
celled”) or
multicellular
(“many-celled”)
Unicellular glands are scattered within epithelial sheets. By
contrast, most multicellular epithelial glands form by invagina-
tion (inward growth) of an epithelial sheet into the underly-
ing connective tissue. At least initially, most have
ducts
, tubelike
connections to the epithelial sheets.
Endocrine Glands
Because
endocrine glands
eventually lose their ducts, they are
o±en called
ductless glands
. Tey produce
hormones
, messen-
ger chemicals that they secrete by exocytosis directly into the
extracellular space. From there the hormones enter the blood
or lymphatic fluid and travel to specific target organs. Each hor-
mone prompts its target organ(s) to respond in some character-
istic way. For example, hormones produced by certain intestinal
cells cause the pancreas to release enzymes that help digest food
in the digestive tract.
Endocrine glands are structurally diverse, so one description
does not fit all. Most are compact multicellular organs, but some
individual hormone-producing cells are scattered in the diges-
tive tract lining (mucosa) and in the brain, giving rise to their
collective description as the
diffuse endocrine system
. Endocrine
secretions are also varied, ranging from modified amino acids
to peptides, glycoproteins, and steroids. Since not all endocrine
glands are epithelial derivatives, we defer consideration of their
structure and function to Chapter 16.
Exocrine Glands
All
exocrine glands
secrete their products onto body surfaces
(skin) or into body cavities. Te unicellular glands do so di-
rectly (by exocytosis), whereas the multicellular glands do so via
an epithelium-walled duct that transports the secretion to the
epithelial surface. Exocrine glands are a diverse lot and many
of their products are familiar. Tey include mucous, sweat, oil,
and salivary glands, the liver (which secretes bile), the pancreas
(which synthesizes digestive enzymes), and many others.
Unicellular Exocrine Glands
Te only important examples of
unicellular
(or one-celled) glands are
mucous cells
and
goblet
cells
. Unicellular glands are sprinkled in the epithelial linings of
the intestinal and respiratory tracts amid columnar cells with
other functions (see Figure 4.3c).
In humans, all such glands produce
mucin
(mu
9
sin), a com-
plex glycoprotein that dissolves in water when secreted. Once
dissolved, mucin forms mucus, a slimy coating that protects and
lubricates surfaces. In
goblet cells
the cuplike accumulation of
mucin distends the top of the cell, making the cells look like a
glass with a stem (thus “goblet” cell,
Figure 4.4
). Tis distortion
does not occur in
mucous cells
.
Multicellular Exocrine Glands
Compared to the unicellular
glands,
multicellular exocrine glands
are structurally more
complex. Tey have two basic parts: an epithelium-derived
duct
and a
secretory unit
(
acinus
) consisting of secretory cells. In all
but the simplest glands,
supportive connective tissue
surrounds
the secretory unit and supplies it with blood vessels and nerve
fibers, and forms a
fibrous capsule
that extends into the gland
and divides it into
lobes
.
Multicellular exocrine glands can be classified by structure
and by type of secretion.
Structural classification.
On the basis of their duct struc-
tures, multicellular exocrine glands are either simple or com-
pound
(Figure 4.5)
.
Simple glands
have an unbranched
duct, whereas
compound glands
have a branched duct. Te
glands are further categorized by their secretory units as (1)
tubular
if the secretory cells form tubes; (2)
alveolar
(al-
ve
9
o-lar) if the secretory cells form small, flasklike sacs (
al-
veolus
5
“small hollow cavity”); or (3)
tubuloalveolar
if they
have both types of secretory units. Note that the term
acinar
(as
9
ĭ-nar; “berrylike”) is used interchangeably with alveolar.
Modes of secretion.
Multicellular exocrine glands se-
crete their products in different ways, so they can also be
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