is ejected from
breasts, not just the suckled one. During
nursing, oxytocin also stimulates the recently emptied uterus
to contract, helping it to return to (nearly) its prepregnant size.
Breast milk has advantages for the infant:
Its fats and iron are better absorbed and its amino acids are
metabolized more efficiently
than those of cow’s milk.
It has a host of beneficial chemicals
, including IgA, comple-
ment, lysozyme, interferon, and lactoperoxidase, that pro-
tect infants from life-threatening infections. Mother’s milk
also contains interleukins and prostaglandins that prevent
overzealous inflammatory responses, and a glycoprotein that
deters the ulcer-causing bacterium (
Helicobacter pylori
) from
attaching to the stomach mucosa.
Its natural laxative effect helps to cleanse the bowels of
ne-um), a tarry green-black paste containing
sloughed-off epithelial cells, bile, and other substances. Since
meconium, and later feces, provides the route for eliminat-
ing bilirubin from the body, clearing meconium as quickly
as possible helps to prevent
physiological jaundice
(see the
Related Clinical Terms section). Breast milk also encourages
bacteria (the source of vitamin K and some B vitamins) to
colonize the large intestine.
When nursing is discontinued, the stimulus for prolactin re-
lease and milk production ends, and the mammary glands stop
producing milk. Women who nurse their infants for six months
or more lose a significant amount of calcium from their bones,
pregnancy stimulate the hypothalamus to release prolactin-
releasing factors (PRFs). ±e anterior pituitary gland responds
by secreting
. (±is mechanism is described below.)
A²er a delay of two to three days following birth, true milk
production begins.
During the initial delay (and during late gestation), the
mammary glands secrete a yellowish fluid called
trum). It has less lactose than milk and almost no fat,
but it contains more protein, vitamin A, and minerals than true
milk. Like milk, colostrum is rich in IgA antibodies. Since these
antibodies are resistant to digestion in the stomach, they may
help to protect the infant’s digestive tract against bacterial infec-
tion. Additionally, these IgA antibodies are absorbed by endo-
cytosis and subsequently enter the bloodstream to provide even
broader immunity.
A²er birth, prolactin release gradually wanes, and con-
tinual milk production depends on mechanical stimulation of
the nipples, normally provided by the suckling infant. Mech-
anoreceptors in the nipple send afferent nerve impulses to the
hypothalamus, stimulating secretion of PRF. ±is results in a
burstlike release of prolactin, which stimulates milk production
for the next feeding.
±e same afferent impulses also prompt hypothalamic re-
lease of oxytocin from the posterior pituitary via a
feedback mechanism
. Oxytocin causes the
let-down reflex
the actual ejection of milk from the alveoli of the mammary
(Figure 28.19)
. Let-down occurs when oxytocin binds
to myoepithelial cells surrounding the glands, a²er which milk
Let-down reflex
Milk is ejected
through ducts
of nipples.
sends efferent
impulses to the
pituitary where
oxytocin is stored.
Anterior pituitary
secretes prolactin
to blood.
Hypothalamus releases prolactin-
releasing factors (PRF)
to portal circulation.
Oxytocin is
released from the
posterior pituitary
and stimulates
myoepithelial cells
of breasts to contract.
Prolactin targets
mammary glands
of breasts.
Milk production
Positive feedback
Stimulation of
mechanoreceptors in
nipples by suckling
infant sends afferent
impulses to the
Figure 28.19
Milk production and the positive feedback mechanism of the milk
let-down reflex.
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