|
Lactation
Lactation is the
process of milk secretion and is prolonged as long as milk is
removed from the gland on a regular basis. Prolactin is required to
maintain milk secretion, and oxytocin is required to produce
let-down to allow infants to extract milk from the gland. An example
of the swollen ducts and alveoli that occur when milk is not removed
from the gland is shown in figure 4F. The volume of milk secreted
may be adjusted to the requirements of the infant by a local factor
secreted into milk, the so-called "feedback inhibitor of lactation."
OXYTOCIN, MILK
EJECTION, AND SUCKING
Milk removal
form the breast is accomplished by the contraction of myoepithelial
cells, the processes of which from a basketlike network around the
alveoli where milk is stored, in concert with sucking by the infant.
When infants are suckled, afferent impulses from sensory stimulation
of never terminals in the areolus travel to the CNS, where they
promote the release of oxytocin from the posterior pituitary. In
women, oxytocin release often is associated with such stimuli as the
sight or sound , or even the thought, of their infants, indicating a
significant psychological component in this neuroendocrine reflex.
The oxytocin is carried through the bloodstream to the mammary
gland, where it niteracts with specific receptors on myoepithelial
cells, initiating their contraction and expelling milk from the
alveoli into the ducts and subareolar sinuses. The passage of milk
through the ducts is facilitated by longitudinally arranged
myoepithalial cell processes, the contraction of which shortens and
widens the ducts, allowing for free flow of milk to the nipple. The
process by wich milk is forcibly moved out of the alveoli is called
milk ejection or let-down and is essential to milk removal from the
lactating breast.
During correct
suckling, the nipple and much of the areola are drawn well into the
infant's mouth so that a long teat reaching nearly to the infant's
soft palate is formed. The mammary sinuses extend into this teat.
Milk is removed not so much by suction as by the stripping motion of
the tongue against the hard palate. This motion carries milk through
the teat and into the infant's mouth. The sinuses refill as the
continued action of oxytocin forces milk from the alveoli into the
ducts.
Mechanisms also
exist for preventing unwanted loss of milk from the mammary gland
when it is not being suckled. Smooth-muscle and elastic fibers form
a tight aphincter at the end of the teat in most animals.
Also, in goats, but not in humans,
evidence shows the presence of smooth muscle around the larger ducts
under sympathetic control. Also, sympathetic reactions resulting in
the release of epinephrine seem to have a cerebral component that
inhibits milk ejection. All of these factors probably operate to
ensure that effective milk removal is accomplished only by the
suckling young under condition of favorable interaction with the
mother.
In addition to
its action on the mammary gland, oxytocin causes uterine
contractions, especially in the term and postpartum uterus.Women who
put their infants to the breast soon after delivery often experience
uterine cramping that probably aids in uterine involution.
SUMMARY
Milk
secretion is a robust process that proceeds normally in at least 85%
of women postpartum. Anecdotal evidence suggests that, with
assistance in the techniques of breastfeeding, at least 97% of women
can successfully breastfeed their infants. The causes of lack of
success in breastfeeding are not well understood because, at least
in Western societies, when infants fail to thrive on the breast,
formula substitution is easy. Although this article is not the place
to discuss possible pathologic mechanisms, breastfeeding failure
usually occurs at approximately the first week postpartum, and a
much better understanding of the mechanisms by which milk secretion
is initiated during this period may help researchers to understand
why some women have severe problems with lactation. The general
understanding of the mechanisms of milk secretion. The general
understanding of the mechanisms of milk secretion is fairly good,
but the regulatory mechanisms at the cellular and molecular levels
have not been given adequate attention and are ripe for future
investigation. Other arease that require attention are the behavior
correlates of breastfeeding and the transfer of drugs and toxins
into milk. The latter may have a long-term impact on infant health
and should receive increased attention.
(The Pediatric Clinic of North America)
BREASTFEEDING 2001, PART I:
THE EVIDENCE FOR BREASTFEEDING
Volume 48.Number1.February 2001
| 
