Mammary Growth and Regression -Regulation of Milk Synthesis-

The mammary gland is an unique organ present only in the mammal. The mammary gland, grown and differentiated during pregnancy, begins to synthesize milk following parturition. The lactating mother continues actively to supply milk to the young for a certain period, but milk synthesis terminates at the late stage of lactation. In various species, the changing profile of milk production is known as the lactation curve. The increase or decrease in milk production has merits for both the young and the mother. At the termination of milk synthesis, the mammary gland undergoes regression and most mammary epithelial cells disappear from the gland through the process of apoptosis. The mammary gland returns the state identical to that before pregnancy. It is well known that prolactin regulates mammary gland functions; growth, differentiation, milk synthesis and regression. The mammary gland modulates sensitivity to prolactin by changing the number of prolactin receptors of the mammary epithelial cell. As described here, milk production is regulated primarily by the level of the prolactin receptor gene expression. It is believed that as a result of the law of the survival of the fittest, the mammary gland acquired the ability of regulating its own functions by modulating the prolactin receptor gene expression. Milk production of dairy cattle has been improved during the last two centuries. By manipulating prolactin receptor gene expression, it will be possible to further improve milk production in dairy cattle.