Fig. 4

Development of the mammary gland in mammals. Mammogenesis begins during early fetal development. After birth, mammary ducts elongate through cell proliferation. At the onset of puberty, high concentrations of growth hormone, insulin-like growth factor in plasma stimulate mammary duct proliferation to form terminal end buds (TEBs) at the tips of the ducts. Under the influence of estrogen, TEBs actively proliferate to form ductal branches, which fill the mammary fat pad. After this stage, the TEBs regress. During pregnancy, progesterone and prolactin promote lobuloalveolar development to form alveolar buds. At the onset of lactation, mature alveoli capable of producing and secreting milk are formed. Suckling of the nipple by the neonate results in the contraction of the myoepithelial cells around the alveoli, causing the milk to be ejected through the ducts into the nipple. Upon weaning, lactation stops and the mammary gland undergoes involution through apoptosis and autophage to its nonlactating state. E2 = estrogen; GH = growth hormone; IGF-I = insulin-like growth factor-I; LN = lymph node; P4 = progesterone; PL = placental lactogen; PRL = prolactin