Anemia of Prematurity - Portal Neonatal

LACTATION Section 5 of 11
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.
BIOCHEMISTRY OF HUMAN MILK Section 6 of 11
Human milk is a unique, species-specific, complex nutritive fluid with immunologic- and growthpromoting
properties. This unique fluid actually evolves to meet the changing needs of the baby during
growth and maturation. The synthesis and secretion of milk by the mammary gland involve numerous
cellular pathways and processes (summarized in Image 4). The processing and packaging of nutrients
within human milk changes over time as the recipient infant matures. For example, early milk or
colostrum has lower concentrations of fat than mature milk but higher concentrations of protein and
minerals (see Image 5). This relationship reverses as the infant matures.