Anatomy of the breast – Development of the breast
The breast is a modified apocrine sweat gland. The breasts develop from the bilateral mammary ridges of the embryo that extend along the ventral surface from the base of the forelimb bud to a point medial to the base of the hindlimb bud. The breast buds develop from the middle portion of the upper third of the mammary ridges, and the rest of the ridges disappear. Failure of portions of the ridge to involute may result in accessory breast tissue anywhere along the milk line (the line along which the mammary ridges extend in embryonic life), extending from the axilla to the inguinal region. At birth, only a rudimentary duct tree is present in both the sexes.
At puberty, levels of estrogen and progesterone hormones increase to initiate breast development from the rudimentary duct tree. The ductal elements proliferate and the terminal buds at the ends of the branching ducts differentiate into tufts of blind-ending ductules. A complex tree-like structure develops, with branching ducts, TDLUs, and lobules surrounded by connective tissue. Hormones and growth factors act upon stromal and epithelial cells to regulate mammary gland development, maturation, and differentiation. Estrogen mediates development and elongation of ductal tissue; progesterone facilitates ductal branching and lobulo-alveolar development. Prolactin hormone regulates milk secretion during and after pregnancy.