Page 525 - Anatomy and Physiology of Farm Animals, 8th Edition

510 / Anatomy and Physiology of Farm Animals

atoms in length termed short‐chain fatty albumin and immunoglobulins produced
by the liver and lymphocytes, respectively.
acids. Such short‐chain fatty acids are not
VetBooks.ir generally found in adipose tissue through- proteolytic enzyme secreted by gastric
Rennin (also known as chymosin) is a
out the remainder of the body.
Lactose (milk sugar), the principal epithelial cells of very young mammals.
carbohydrate in milk, is a disaccharide Rennin changes the character of ingested
composed of the two monosaccharides milk from a liquid to a semisolid curd; this
glucose and galactose. Lactose is synthe- process is termed curdling or coagula-
sized in mammary glands and is typically tion. The change in character increases the
found only in the mammary glands and in time that milk is retained in the stomach,
milk. Secretory cells in mammary glands and this allows protein digestion to begin.
use glucose from the blood to synthesize Curdling results when rennin degrades one
galactose and then combine the galactose of the casein proteins responsible for
with more glucose to produce lactose; increasing the solubility of micelles, casein
therefore, glucose is essential for lactose protein aggregates in milk (discussed next).
synthesis. The extraction of glucose from Without this specific casein, milk proteins
blood by an actively secreting mammary precipitate with the calcium in milk to
gland is quite effective, so the glucose form curds.
concentration of venous blood leaving a
mammary gland is relatively low.
Recall that in ruminants blood glucose Milk Secretion
is primarily derived from gluconeogenesis
in the liver using propionic acid, a volatile The epithelial cells lining the alveoli of
fatty acid absorbed from the rumen, as a mammary glands are the cells primarily
substrate. Thus, propionic acid produced responsible for the secretion of milk. The
by ruminal microorganisms and fermenta- appearance of these cells varies as they
tive metabolism is the ultimate substrate synthesize and release the lipids, proteins,
for the production of lactose in ruminants. and lactose of milk. After the cells actively
Also, blood glucose is relatively low in secrete the constituents of milk and the
ruminants compared to other mammals, in lumen of the alveoli are filled with milk,
part because ruminants absorb very little the epithelial cells shrink and are described
glucose from the gastrointestinal tract. At as a simple low‐cuboidal epithelium
peak lactation of a high‐producing dairy (Fig. 29‐5). At this stage, their secretory
cow, the mammary glands use most of activity is relatively low. Shortly after the
the glucose produced by the liver for lac- stored milk is removed, the epithelial cells
tose production. If the need for glucose increase their secretory activity and begin
by the mammary glands cannot be met to refill the alveoli. Early in the secretory
by gluconeogenesis, and blood glucose phase the cells assume a more columnar
levels drop significantly, lactational appearance and then gradually reduce to
ketosis develops. While blood glucose cuboidal as milk fills the alveoli. Small,
levels are low, metabolic acids (pro- apparently nonfunctioning alveoli can be
duced in the liver from fatty acids) accu- found in dry mammary glands, and there is
mulate in the blood to produce a a relative increase in the amount of inter-
metabolic acidosis. stitial loose connective tissue (Fig. 29‐4).
The major milk proteins are the caseins. Milk lipids are synthesized and packaged
Amino acids found in blood are the pre- into secretory droplets, which are extruded
cursors for direct synthesis of the caseins from the luminal surface of the cell into the
by secretory cells within mammary glands. alveoli (Fig. 29‐7). As they are released, a
Other milk proteins include α‐lactalbumin membrane covering derived from the cell
and β‐lactoglobulins, which are produced membrane of the epithelial cell encases the
by cells of the mammary gland, and serum lipid droplets. The alveolar secretory cells