In order to appreciate fully the character of the breast and to understand profoundly the nature of it, comprehension of embryologic development is essential. This essay seeks capture, in words and pictures, the chronological course by which the breast develops, and to compare the sequence with that of the rest of the skin and subcutaneous tissue. In brief, the skin consists of epidermis and dermis; the subcutaneous "soft tissues" reside beneath the dermis and consist of the panniculus adiposa (subcutaneous fat, hypoderm), fascia, skeletal muscle, and bone.

Development of the skin and subcutaneous fat is a continuous process that is repeatable, more or less, on all anatomic sites. The morphologic changes are a reflection of an extraordinary dynamic that reflects gestational age. ⁵

The epidermis is derived from ectoderm that covers, thinly, the entire surface of an embryo, whereas the dermis and subcutaneous fat are derived from mesoderm in a pad that underlies surface ectoderm. ^6–8 At first the epidermis consists of a single layer of epithelial cells, the dermis of a mucinous gel, and the subcutaneous fat of rudimentary mesenchymal cells (Fig. 1). By the seventh week of embryonic life, two layers of surface epithelial cells have come into being, namely, a superficial layer of glycogen-rich cells designated the periderm and a deep layer of cuboidal cells called the basal layer. Over time, the cells of the periderm are sloughed into amniotic fluid, a process that usually is completed by about the 20th week of gestation. In the 11th week, the basal layer begins to proliferate, resulting in formation of a tier of larger epithelial cells. Those cells, devoid of glycogen, are seen by electron microscopy to possess features of keratocytes, to wit, cytoplasmic tonofilaments that attach to desmosomes at intercellular junctions. After the fifth month of intrauterine life, basal cells proliferate even more rapidly, keratohyaline granules become more numerous in the upper part of the now nearly fully evolved epidermis, and cells near the surface lose their nuclei as they show signs increasingly of cornification, a process that is completed during the sixth month of gestation. During the first trimester, the dermo-epidermal interface becomes punctuated by subtle undulations formed by alternation of rete ridges of epidermis and papillae of dermis; by the sixth month of fetal life, those papillae come to look increasingly like tiny nipples. At this juncture, a thin papillary dermis made up of delicate bundles of collagen can be distinguished readily from a much wider reticular dermis composed of thicker bundles of collagen. By the eighth week of gestation, melanocytes, having traveled to the skin from the neural crest, appear at the junction of dermis and epidermis, but they do not manufacture melanosomes until sometime between the fourth and sixth month,^6–8 at which time they transfer those organelles through dendrites to keratocytes.

The subcutaneous fat takes on its own character, but at a slower pace. After the 24th week, fat cells (adipocytes, lipocytes) develop from primitive mesenchymal cells situated beneath the dermis.⁹ During the course of this transformation, fibrous septa that house large blood vessels and nerves emerge and, by virtue of the intersection of struts of fibrous tissue, lobules of the panniculus adiposa are created.