In the fourth week of embryonic development, a pair of epidermal thickenings, named mammary ridges, develops on either side of the anterior aspect of the trunk and extends from the future axillae to the future inguinal region and to the medial aspect of the thigh (Fig. 7).¹⁴ In humans, unlike some other mammals, the mammary ridges disappear, except on the breasts.⁶ The involution occurs in a manner similar to the purported regression of apocrine glands and ducts elsewhere in most of the skin. The mammary glands and their associated ducts begin their proliferation downward in the fifth to sixth week of embryonic life (Fig. 8) and in a fashion similar to that of folliculosebaceous-apocrine units throughout the rest of the skin. In the sixth week, germs like those described previously for all folliculosebaceous-apocrine units in the skin appear on the undersurface of developing ectoderm of the breast. Those germs evolve in a mode similar to germs of the folliculosebaceous-apocrine units distributed elsewhere. In the skin of the breast, authentic folliculosebaceous-apocrine structures develop in the usual manner, but toward term the apocrine units seem to involute. The situation is different on the nipple, where the germs eventuate only in infundibulosebaceous-apocrine units; there are no true hair follicles on the nipple. Apocrine glands that persist in the subcutaneous fat of the breasts are known as mammary glands, and the ducts of them are called lactiferous ducts. Those ducts enter infundibula on the nipple, just as ducts of apocrine glands in the adjacent axilla and on several other sites enter infundibula.

At first, the germs that will become infundibulosebaceous-apocrine units on the nipple resemble folliculosebaceous-apocrine units, but as soon as they extend into the primitive dermis (Fig. 9), they become round and then more elongated aggregations of epithelial cells that in the substance of the breast develop into apocrine units. In short, there are three different kinds of germs on a breast, namely, infundibulosebaceous-apocrine germs on the nipple and both folliculosebaceous-apocrine germs and eccrine germs in the remainder of the skin of it. Hair follicles, sebaceous glands and ducts, apocrine glands and ducts, and eccrine glands and ducts are present throughout the skin of the breast, just as they are throughout the rest of the skin. Infundibulosebaceous-apocrine germs and folliculosebaceous-apocrine germs on the breast are typified at their periphery by columnar cells whose nuclei are crowded and aligned in a palisade (Fig. 3, 9). Note in figures 3 and 9 that both kinds of germs are enveloped partially by primitive mesenchymal cells. What is known conventionally as the mammary pit of the infundibulosebaceous-apocrine (mammary) unit is nothing more than the ostium of an infundibulum. The mammary germ, referred to also as the mammary bud, extends progressively deeper into the dermis and, by the 10th week, begins to branch. More branching of germs (secondary buds) is apparent by the 12th week. For the remainder of gestation, the buds lengthen and continue to branch within the subcutaneous fat. The apocrine ducts on the nipple canalize and become lactiferous ducts.¹⁴

At birth, the infundibulosebaceous-apocrine units consist of 15 to 25 lactiferous ducts, in continuity with mammary glands, that enter an infundibulum whose ostium is a mammary pit. Proliferation of cells of mesoderm that surrounds the infundibula on the nipple results in formation, within a few weeks after birth, of a distinctive protuberance, to wit, a nipple. That distinctive structure is composed, in part, of circular and longitudinal bundles of smooth muscle in conjunction with collagenous and elastic connective tissue and with lactiferous ducts that course through it.⁹ In similar fashion, mesenchymal cells in the immediate vicinity of folliculosebaceous-apocrine units in the reticular dermis differentiate to form smooth muscle fascicles of hair erection. The nipple also contains numerous sebaceous glands, each duct of which, just like each lactiferous duct, enters the base of an infundibulum.

In summary, the development of the breast is essentially the development of skin and subcutaneous tissue on a particular anatomic site. Germs that give rise to folliculosebaceous-apocrine units are present throughout the skin of the breast (just as they are throughout the rest of "glabrous skin") and they are very similar to the germs that give rise to the infundibulosebaceous-apocrine unit of the nipple (Fig. 10). Both of those germs are very different from germs that eventuate in eccrine units. Because there are no true hair follicles (composed of a bulb, stem, and isthmus) on the nipple, the units there are rightly termed infundibulosebaceous-apocrine ones. The apocrine glands of the skin in the axilla, genitalia, and external auditory canal are indistinguishable morphologically from those apocrine glands designated mammary glands, and the apocrine ducts on the former three sites are indistinguishable morphologically from those apocrine ducts called lactiferous ducts. In sum and in short, apocrine units on different anatomic sites, such as the external auditory canal, the axilla, and the breasts, develop in similar fashion and have similar histologic attributes, despite their having vastly different physiologic functions.