Page 528 - Anatomy and Physiology of Farm Animals, 8th Edition

Anatomy and Physiology of the Mammary Glands / 513

Food and Drug Administration (FDA) production. Serotonin is a neurotransmit-
ter produced by mammary epithelial cells
VetBooks.ir approved rbST for use in dairy cows and can contribute to a decline in milk
under the trade name Posilac® and in the
production in cows. Several species also
first 20 years of commercial use in the
United States, over 35 million dairy produce a feedback inhibitor of lacta-
cows received rbST. In recent years, an tion (FIL). This small protein is produced
increase in consumer demand for “rbST‐ in the mammary gland while it is produc-
free” dairy products has been observed. ing milk; as the gland fills with milk and
The safety of milk and other dairy prod- the concentration of FIL rises, FIL inhib-
ucts from rbST‐treated cows has been its milk production by the mammary
extensively studied by the FDA as well as gland. Other components of milk may
several other regulatory authorities and have similar inhibitory effects, and rou-
scientific review panels around the tine removal of these inhibitory factors is
world. Several key points must be made: necessary to promote the continuation of
(1) trace amounts of both bST as well as lactation.
IGF‐1 can be found in milk regardless of
whether cows have been treated with
rbST; (2) treatment with rbST does not Milk Ejection or Letdown
change the composition of milk in a
biologically relevant manner; (3) rbST is Milking or nursing alone can empty only
a protein and therefore digested when the cisterns and largest ducts of the udder.
consumed orally; and (4) bST is not In fact, any negative pressure causes the
biologically active in humans because ducts to collapse and prevents emptying
the three‐dimensional structure of the of the alveoli and smaller ducts. Thus,
human and bovine hormones differ such the dam must take a biologically active
that the human receptor does not recog- (although totally unconscious) part in
nize the bovine hormone. While many milking to force milk from the alveoli into
will argue that consumption of dairy the cisterns. This is accomplished by active
products from cows treated with rbST is contraction of the myoepithelial cells sur-
correlated with an increased risk of can- rounding the alveoli (milk ejection, or
cer or other diseases, it is unclear if this milk letdown). These myoepithelial cells
association is more than a coincidence contract when stimulated by oxytocin, a
or if it is the result of other naturally hormone released from the neurohypoph-
occurring components within the milk ysis of the pituitary as a result of a neu-
(e.g., high calcium levels). roendocrine reflex. The afferent side of the
Routine milking or suckling to remove reflex includes visual or auditory stimuli as
milk from the mammary glands is essen- well as sensory nerves from the mammary
tial for continued milk production. When glands, particularly the nipples or teats.
milk removal is stopped abruptly, a num- Afferent information reaches the hypo-
ber of changes in the udder occur: after thalamus, which regulates the release of
24 hours, the alveoli become distended to oxytocin from the neurohypophysis (see
a maximum, and the capillaries are full of Chapter 13). Suckling the teats by the
blood; between 36 and 48 hours, there is a young is the usual stimulus for the milk
decrease in the number of patent (open) ejection reflex, but whether milk is with-
capillaries, and the alveoli do not respond drawn from the teat or not, the milk ejec-
to intravenous oxytocin. While this abrupt tion reflex produces a measurable increase
cessation of milk removal is often regarded in the pressure of milk within the cisterns
as the most effective method for inter- of the udder.
rupting lactation (“drying up” a cow), The milk ejection reflex can be condi-
there are several mechanisms within the tioned to stimuli associated with milking
mammary tissue that can inhibit milk routine, such as feeding, barn noises, or