The breast of the human newborn is formed through 10 progressive fetal stages that begin in the sixth week of fetal development.1 The mammary gland originates from the milk streaks, bilateral ectodermal thickenings that extend from the axilla to the groin. The ectoderm over the thorax invaginates into the surrounding mesenchyme, with subsequent epithelial budding and branching.2,3 During the later part of pregnancy, this fetal epithelium further canalizes and ultimately differentiates to the end-vesicle stage seen in the newborn.1 At term birth, the breast has six to eight widely patent ducts that empty at the nipple. Recent anatomical studies confirm a parallel bundle of an additional 25 smaller ducts with distinct openings at the nipple surface.4 All of these initial ducts contain one layer of epithelium and one layer of myoepithelial cells, terminating in a dilated blind sac. These so-called "ductules" are the precursors of future lobuloalveolar structures, the ultimate milk-producing units of the breast. Interestingly, despite the large number of actual ducts that drain onto the nipple, one fourth of the breast is drained by one duct and its branches and one half of the breast is drained by only three ducts.4 Similar to the development of the ductal system, the subareolar lymphatic plexus also develops from the ectoderm.5

From birth until 2 years of age, there is wide individual variability in the morphologic and functional stages in the breast, with some neonates having more well-developed lobular structures and others with more secretory epithelial phenotypes.6 The degree of morphologic differentiation does not correlate with functional ability. The ability of the entire ductal structure to respond to secretory stimuli may even occur in the rudimentary ductal systems.6 Ultimately, in normal infant development, the differentiated glandular structures involute and only small ductal structures are left remaining within the stroma.7

During childhood, the ductal structures and stroma grow isometrically at a rate similar to that of the rest of the body until puberty.6,8 The lymphatics grow simultaneously with the duct system, maintaining connection with the subareolar plexus.5 As in the fetal stage, there are no morphologic differences between the sexes.6

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  • Category: Women's diseases