ABSTRACT
A major evolutionary advantage of mammals is the investment by females toward the survival of their offspring by providing milk to their infants via the mammary gland (Newburg 2013). Mammary glands are epidermal prominences evolved from ancient apocrine glands of the skin that are specialized to provide fl uid for the infant. The maturation and activation of the gland is under hormonal control. Mammary glands experience three developmental stages: embryonic, pubertal, and mature adult. In the embryonic stage, milk ducts form from overlying ectoderm; ectodermal cells migrate along the ducts to coalesce into epithelial placodes, which differentiate into the mammary bud, mammary sprout, and the rudimentary gland. The next qualitative maturation of the mammary gland occurs in puberty, when branching morphogenesis and lumen formation are the major events. This matures into a complex secretory organ of epithelial cells, adipocytes, vascular endothelial cells, and a variety of immune cells. Upon pregnancy, mammary glands undergo further morphological change and differentiation as they prepare for lactation (Watson and Khaled 2008). Lactation, the production of milk, is strictly controlled by hormones, such as progesterone, estrogen, prolactin, growth hormone, adrenocorticotropic hormone (ACTH), thyroid stimulating hormone (TSH), and many others (Capuco and Akers 2009). The stages of modifi cation of the adult breast for lactation are initiated during pregnancy, culminate with lactation, and end at weaning with involution of the breast, reversing many of the stages of lactogenesis until the next pregnancy.
