Proliferation of the Mammary Duct System Begins at Puberty, with Ducts Under the Control of Estrogens, Growth Hormone, and Adrenal Steroids, and Alveoli Under the Control of Progesterone and Prolactin

Development of the mammary gland in postfetal life usually starts in concert with puberty. Cyclical ovarian activity results in the production of estrogen and progesterone. Estrogen, with growth hormone and adrenal steroids, is responsible for proliferation of the duct system.

Although the development of the mammary gland begins with the onset of puberty, the gland remains relatively undeveloped until the occurrence of pregnancy. In most domestic animals, udder development usually becomes evident by the middle of gestation; the secretion of milk often begins during the latter part of gestation (mainly from increas­ing prolactin secretion) and results in the formation of colostrum, as discussed later. By the end of pregnancy, the mammary gland has been transformed from a structure involving mostly stromal (connective tissue) elements to a structure that is filled with alveolar cells that are actively syn­thesizing and secreting milk. Groups of adjacent alveoli form lobules that further combine into larger structures called lobes. Connective tissue bands delineate the lobules and the lobes (Figure 39-5).

FIGURE 39-4 Hormones involved in the growth of the mammary gland and in the initiation of milk secretion in the hypophysectomized-ovariectomized- adrenalectomized rat. (From CowieAT: Lactation. In Austin CR, Short RV, editors: Reproduction in mammals, ed 2, vol 3, Hormonal control of reproduction, Cambridge, UK, 1984, Cambridge University Press.)

FIGURE 39-5 Drawings of sections of the mammary gland of the goat at three different times during pregnancy (which lasts approximately 150 days). A_r Note the small collections of ducts scattered throughout the stroma on the 35th day. B_r On the 92nd day the lobules of alveoli are forming in groups known as lobes; secretion is present in some of the alveolar Iumina_r and there is still considerable stromal tissue. C_r On the 120th day the lobules of alveoli are almost fully developed; the alveoli are full of secretion, and the stromal tissue is reduced to thin bands separating lobules and thicker strands between lobes. (From Falconer IR_r editor: Lactation, London, 1970_r Butterworths.)