The epidermis in the region of the future nipple becomes depressed, forming the mammary pit during the third trimester ( Fig. Although most sources agree these secondary processes end in rudimentary lobular structures or end buds, 3, 6, 23 some argue that the breast at birth does not contain any evidence of lobules, only ductal structures with surrounding stroma. 3, 6, 14, 23 Disagreement exists over the final morphology of the breast at birth.
Repeated branching of the secondary epithelial buds and canalization occur in the third trimester. 14 Concomitantly, the mesenchymal cells differentiate to form fibroblasts, smooth muscle cells, capillary endothelial cells, and adipocytes. 3 Two distinct populations of epithelial cells (central and basal) can be identified. 14 This core of cells continues to evaginate into the underlying stroma and becomes surrounded by a more cellular zone of fibroblast like cells within a collagenous mesenchyme.Īt the end of the first trimester of pregnancy, a well-defined mammary bud penetrating into the upper dermis can be observed. 6 Indentations along its basolateral margin appear, becoming sites for the future secondary mammary outgrowths. 10, 12, 13 Next, the primary mammary bud enlarges 14 and moves from a more dorsal to ventral position. Toward the end of the first trimester 21 the primary mammary buds begin to grow downwards into the underlying mesenchyme, under an inductive influence of regulatory factors secreted by the mesenchyme. 19, 20 These are rarely functioning but can occasionally be a cosmetic issue. 13, 17, 18 These supernumerary nipples can appear similar to pigmented macules or fully developed nipple and areola complexes. 6, 16 Supernumerary nipples (polythelia) occur in 2 to 5% of humans in a position from the groin to the axilla, supporting the existence of the mammary crest or ridge. Most of the mammary crest atrophies except for paired solid epithelial masses in the pectoral region at the fourth intercostal space, which form the primary mammary buds. (Reprinted with permission from Moore KL, Persaud TVN, Torchia MG, The Developing Human: Clinically Oriented Embryology. ( D–F) Similar sections showing successive stages of breast development between the 12 th week of gestation and birth. ( C) Transverse section of a mammary crest at the site of the developing mammary gland. ( B) Similar view at 6-week gestation showing the remains of the mammary crests. ( A) Ventral view of an embryo at 28-days gestation showing mammary crests.
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