6.5. Lactogenesis

Colostrum or first milk accumulates in the epithelial cells before parturition. It is concentrated with antibodies and passively immunizes the young to a variety of antigenic factors. The onset of lactation begins at the end of the last third of gestation.

Lactogenesis is manifested by a process of differentiation of mammary cells, an increase in the blood uptake of precursor molecules, an increase in the enzymatic activity responsible for the synthesis of the components of milk, in this way the mammary cells acquire the capacity to synthesis and secretion of milk.

There is an increase in the size of the alveoli due to the accumulation of fluid in the alveolar lumen, this is externally evidenced by the increase in the volume of the udder several weeks before calving. The onset of lactation is the result of a complex set of interrelated hormonal effects. Pituitary hormones, hypothalamic releasing factors and inhibitors, growth factors and, pancreatic, adrenal, ovarian, and placental hormones are involved.

The anterior pituitary appears to be decisive for lactogenesis because hypophysectomy in mammals prevents the initiation of lactation. In the cow, the endocrine events that appear to trigger the onset of active milk secretion are a decrease in progesterone of placental origin, an increase in prolactin, growth hormone, glucocorticoids, and estrogens that occur before calving.

Insulin is required in vitro to elicit a lactogenic response in breast tissue. It causes the non-secretory epithelium to undergo cell division. Although its specific role in vivo is unknown, its administration causes the mammary cells to undergo a high rate of cell division in late pregnancy. Both insulin and Insulin-like Growth Factors (IGFs) can participate in the uptake of glucose by the mammary cells necessary for the synthesis of lactose. Glucocorticoids are required in vitro for the complete initiation of milk secretion. They seem to be involved in the development of the endoplasmic reticulum and other structural changes required for protein synthesis, and the transcription of genes for the synthesis of casein and a-lactalbumin. Glucocorticoid concentrations in blood are fairly low during most of the pregnancy but increase markedly during the last few days of prepartum.