ABSTRACT
Development o f the mammary gland is unique as it occurs primarily after birth, and in distinct stages that are closely linked to the reproductive status o f the animal. Initially, the small ductal network present in the neonate gradually extends into the subtending mammary fat-pad until puberty, when rapid ductal elongation and bifurcation fill the fat-pad in response to ovarian steroid hormones. During each oestrous cycle and throughout early pregnancy, side branching o f the ducts occurs. In a full pregnancy, mammary epithelium undergoes prolifera tion followed by differentiation, and alveolar buds progress into alveoli and later lobuloalveoli. These lobuloalveoli are the milk producing structures o f the mammary gland, and represent the functional endpoint of mammopoiesis. Following weaning, involution occurs, and apoptosis o f epithelia, together w ith extracellular matrix remodelling, results in the return to a mammary architecture resembling that o f the nulliparous gland. This process o f controlled proliferation, differentiation and apoptosis can occur repeatedly throughout the lifespan of the animal (Fig. 1).
