Chapter 4
Tissue: The Living Fabric
125
4
described functionally as
merocrine
,
holocrine
, or
apocrine
glands. Most are
merocrine glands
(mer
9
o-krin), which
secrete their products by exocytosis as they are produced.
Te secretory cells are not altered in any way (so think
“merely secrete” to remember their mode of secretion). Te
pancreas, most sweat glands, and salivary glands belong to
this class
(Figure 4.6a)
.
Secretory cells of
holocrine glands
(hol
9
o-krin) accumu-
late their products within them until they rupture. (Tey are
replaced by the division of underlying cells.) Because holocrine
gland secretions include the synthesized product plus dead cell
fragments (
holo
5
whole, all), you could say that their cells
“die for their cause.” Sebaceous (oil) glands of the skin are the
only true example of holocrine glands (Figure 4.6b).
Although
apocrine glands
(ap
9
o-krin) are present in other
animals, there is some controversy over whether humans
have this third gland type. Like holocrine glands, apocrine
glands accumulate their products, but in this case only just
beneath the free surface. Eventually, the apex of the cell
pinches off (
apo
5
from, off), releasing the secretory granules
and a small amount of cytoplasm. Te cell repairs its damage
and the process repeats again and again. Te best possibility
in humans is the release of lipid droplets by lactating mam-
mary glands, but most histologists classify mammary glands
as merocrine glands because this is the means by which milk
proteins are secreted.
Check Your Understanding
8.
What common secretion do all unicellular exocrine glands
produce?
9.
How are multicellular exocrine glands classified?
10.
Which gland type—merocrine or holocrine—would you
expect to have the highest rate of cell division? Why?
For answers, see Appendix H.
(b)
(a)
Microvilli
Secretory
vesicles
containing
mucin
Golgi
apparatus
Rough ER
Nucleus
Figure 4.4
Goblet cell (unicellular exocrine gland).
(a)
Photomicrograph of a goblet cell in
the simple columnar epithelium lining the small intestine (1640
3
).
(b)
Corresponding diagram.
Notice the secretory vesicles and well-developed rough ER and Golgi apparatus.
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