The precise humoral and neural mechanisms involved in lactation are complex. Progesterone, estrogen, and placental lactogen, as well as prolactin, cortisol, and insulin, appear to act in concert to stimulate the growth and development of the milksecreting apparatus (Stuebe, 2014). With delivery, the maternal serum levels of progesterone and estrogen decline abruptly and profoundly. This drop removes the inhibitory inluence of progesterone on alpha-lactalbumin production and stimulates lactose synthase to increase milk lactose. Progesterone withdrawal also allows prolactin to act unopposed in its stimulation of alpha-lactalbumin production. Activation of calcium-sensing receptors (CaSR) in mammary epithelial cells downregulates parathyroid hormone-related protein (PTHrP) and increases calcium transport into milk (Vanhouten, 2013). Serotonin is also produced in mammary epithelial cells and has a role in maintaining milk production (Collier, 2012).
The intensity and duration of subsequent lactation are controlled, in large part, by the repetitive stimulus of nursing and emptying of milk from the breast. Prolactin is essential for lactation, and women with extensive pituitary necrosisSheehan syndrome�o not lactate (Chap. 58, p. 1133). Although plasma prolactin levels fall after delivery to levels lower than during pregnancy, each act of suckling triggers a rise in levels (Pang, 2007). Presumably a stimulus from the breast curtails the release of dopamine, also known as prolactin-inhibiting octor, from the hypothalamus. In turn, this transiently induces increased prolactin secretion.
The posterior pituitary secretes oxytocin in pulsatile fashion. This stimulates milk expression from a lactating breast by causing contraction of myoepithelial cells in the alveoli and small milk ducts (see Fig. 36-5). Milk ejection, or letting down, is a relex initiated especially by suckling, which stimulates the posterior pituitary to liberate oxytocin. he reflex may even be provoked by an infant cry and can be inhibited by maternal fright or stress (Stuebe, 2014).
Human milk contains several protective immunological substances, including secretory IgA and growth factors. The antibodies in human milk are speciically directed against maternal environmental antigens such as Escherichia coli (Iyengar, 2012). According to the Centers for Disease Control and Prevention (Perrine, 2015), breastfeeding decreases the incidence of ear, respiratory, and gastrointestinal infections; necrotizing enterocolitis; and sudden infant death syndrome.
Much attention has been directed to the role of maternal breast milk lymphocytes in neonatal immunological processes. Milk contains both T and B lymphocytes, but the T lymphocytes appear to difer from those found in blood. Speciically, milk T lymphocytes are almost exclusively composed of cells that exhibit speciic membrane antigens. These memory T cells appear to be an avenue for the neonate to beneit from the maternal immunological experience.