Quantitative estimates of mammary growth during various physiological states: a review.

The parenchymal portion of the mammary gland is immature at birth and begins to grow at a faster rate than the whole body shortly before occurrence of puberty. This accelerated or allometric growth rate is maintained for several estrous cycles, then returns to a growth rate equal to general body growth. Allometric growth of the mammary gland returns at conception and continues in most species for a variable period after parturition. Elevated secretion of estradiol and progesterone throughout pregnancy drives the allometric mammary growth during pregnancy. However, mammary growth during lactation in cows is independent of ovarian secretions and prolactin. Mammary cell numbers during lactation eventually decline as milk production decreases. Concurrent pregnancy reduces mammary cell numbers during lactation, but during the dry period concurrent pregnancy markedly increases mammary cell numbers over those in nonpregnant animals. Dry periods that are short reduce the increments in mammary cell numbers, which normally occur during early stages of the next lactation. Because numbers of mammary epithelial cells are a major determinant of milk yield, understanding the mechanisms that stimulate mammary epithelial cell numbers has the potential to lead to new methods for increasing efficiency of milk production.