Page 526 - Anatomy and Physiology of Farm Animals, 8th Edition

Anatomy and Physiology of the Mammary Glands / 511

Lipid droplets secretory epithelium and the ductile network
of the mammary glands must precede lac-
VetBooks.ir Lumen togenesis. The initial extensive develop-

ment of the mammary gland is usually
associated with puberty (the beginning of
sexual maturity) and the subsequent cyclic
changes in the ovarian hormones, estrogen
and progesterone. Estrogen particularly
promotes the growth of the ductile system
at each estrus, while progesterone, acting
with estrogen, is required for growth and
anatomic development of secretory alveoli.
Normal secretion of growth hormone and
glucocorticoids are also required for com-
plete mammary gland development.
Nucleus Rough endoplasmic During pregnancy, prolonged exposure
reticulum
of the mammary glands to progesterone
Figure 29-7. Secretion of milk lipids, milk promotes a more extensive anatomic
proteins, and lactose by epithelial cells lining the development of the secretory alveoli.
alveoli of the mammary glands. Proteins and While progesterone stimulates anatomic
lactose are released together in secretory vesicles development, it inhibits the functional
by exocytosis (arrows). development of the secretory epithelium.
Progesterone also inhibits the production
also produce secretory vesicles that contain of intracellular enzymes necessary for nor-
both milk proteins (caseins) and lactose. As mal milk secretion. This inhibitory effect
caseins are synthesized and packaged in of progesterone is lost just prior to parturi-
these vesicles, they self‐aggregate into parti- tion, and this is one factor promoting
cles termed micelles. The inclusion of one lactogenesis.
specific type of casein (κ‐casein) in this Prolactin, a protein hormone, is synthe-
aggregation increases the solubility of the sized and secreted by lactotropes in the
micelle so that milk proteins remain in adenohypophysis. Release of prolactin is pri-
solution after their release from the cell. marily regulated by a humoral inhibitory
The lactose within the secretory vesicle factor from the hypothalamus, which is
generates an osmotic force to draw water believed to be the catecholamine dopamine.
into the vesicle from the cytosol of the cell. In the absence of this inhibitory factor, there
The secretory vesicles, each containing a is a continuous and relatively high rate of
mixture of micelles, lactose, and water, are prolactin release. In most domestic animals,
transported to the apical surface of the cell blood levels of prolactin gradually increase
and released into the alveoli by exocytosis late in gestation, with an abrupt increase at
(Fig. 29‐7). Because of the various mecha- the time of parturition. During late gesta-
nisms by which lipids, proteins, and lac- tion, prolactin receptors in mammary glands
tose are secreted from alveolar cells, milk also increase under the influence of rising
is considered to be a combination of apo- estrogen. Prolactin promotes both anatomic
crine and merocrine secretions. and functional development of the secretory
epithelium of mammary glands to promote
Lactogenesis milk secretion, so the abrupt rise at parturi-
tion is necessary for lactogenesis. Under the
Lactogenesis is the establishment of milk influence of prolactin, secretory cells lining
secretion, and galactopoiesis is the contin- alveoli produce intracellular enzymes neces-
ued production of milk by the mammary sary for milk secretion. The functional
glands. Growth and development of the development of alveolar secretory cells is