Margaret C.Neville,PhD


    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."


    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.


      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)


Volume 48.Number1.February 2001