LACTATION LACTOGENESIS HUMAN MILK

Two essential hormones - prolactin and oxytocin
During the second stage of lactogenesis, the breast becomes capable of milk production. For the ongoing synthesis and secretion of human milk, hormonal signals must be received by the mammary gland continually. These signals are in direct response to stimulation of the nipple and areola (mammae) and are then relayed to the central nervous system. This cyclical process of milk synthesis and secretion is termed lactation. Lactation occurs with the help of 2 hormones, prolactin and oxytocin. While prolactin and oxytocin act independently on different cellular receptors, their combined actions are essential for successful lactation.

Prolactin
Milk synthesis occurs in the mammary gland epithelial cells in response to prolactin activation of epithelial cell prolactin receptors. Prolactin, a hormone synthesized by the anterior pituitary, is structurally similar to GH. It stimulates mammary glandular ductal growth and epithelial cell proliferation and induces milk protein synthesis. The significance of prolactin can be seen in the inhibition of lactogenesis using bromocriptine and other dopamine analogues, which are prolactin inhibitors.

Oxytocin
The other important hormone involved in the milk ejection or letdown reflex is oxytocin. When the neonate is placed at the breast and begins suckling, oxytocin is released. The suckling infant causes stimulation of the touch receptors that are densely located around the nipple and areola. The tactile sensations create impulses that, in turn, activate the dorsal root ganglia via the intercostals nerves (4, 5, and 6). These impulses ascend the spinal cord, creating an afferent neuronal pathway to both the paraventricular nuclei of the hypothalamus where oxytocin is synthesized and secreted and the pituitary gland. The stimulation of the nuclei causes the release of oxytocin down the pituitary stalk and into the posterior pituitary gland, where oxytocin is stored.

Once the posterior pituitary gland is stimulated by the afferent impulses created by the infant's suckling, oxytocin is released in a pulsatile fashion to adjacent capillaries, traveling to the mammary myoepithelial cell receptors that in turn stimulate the cells to contract. Oxytocin causes the contraction of the myoepithelial cells that line the ducts of the breast. These smooth muscle–like cells, when stimulated, expel milk from alveoli into ducts and subareolar sinuses that empty through a nipple pore.

Milk secretion and synthesis - directly correlated
The regulation of milk synthesis is quite efficient. Milk synthesis is remarkably constant, at around 800 mL/d. The actual volume of milk secreted, however, may be adjusted to the requirement of the infant by feedback inhibitor of lactation, a local factor secreted into the milk; therefore, the rate of synthesis of milk is related to the degree of emptiness or fullness of the breast. The emptier breast produces milk faster than the fuller one.