Development of mammary gland in cattle : a review

Abstract

This report reviews the findings concerning development and function of mammary gland in the cow. These processes are regulated by several hormones, and variations of hormonal concentrations and of tissue sensitivity, which are mediated by changes in the number and affinity of their specific binding sites are presented. In cattle, as in many ruminants, the growth of mammary gland takes place from foetal life to the first lactation. At birth, mammary gland consists of a restricted immature duct system and a stromal portion, and begins to grow allometrically. At puberty, growth is isometric (equal to general body growth). During the oestrus cycle, oestrogens are generally considered to induce ductal growth, while progesterone induces alveolar growth. The normal lobuloalveolar development requires synergistic stimulation between ovarian steroids and prolactin. During pregnancy, elevated secretions of oestrogens and progesterone increase allometric mammary growth. In the first half of pregnancy, the growth of mammary gland is restricted to replacement of adipose tissue by ductal tissue. The final development, in the second half of pregnancy, results in an extensive lobuloalveolar proliferation. Increase in mammary enzymatic activities during the weeks preceding parturition is temporally related to the increase in serum oestrogen concentrations. In cattle, the dramatic decrease of progesterone concentration, 48 h to 24 h before parturition, leads to final differenciation of epithelial mammary cells. Increases in prolactin, growth hormone and glucocorticoïd blood levels, at parturition, induce the onset of milk secretion. Inhibition of prepartum prolactin discharge, with ergocryptine, suppress induction of lactogenesis. However, once lactation has been established, prolactin and ovarian steroids are not necessary for its maintenance. It is well documented that growth hormone is a galactopoïetic hormone. This effect is probably the consequence of an increment in nutrient supply and blood flow in mammary gland. The Insulin like Growth Factor 1 (IGF 1), produced by liver under growth hormone control, could also act directly at mammary cell level and mediate the growth hormone action.