During the third month of gestation,^6,10,13 the germs that will eventuate in folliculosebaceous-apocrine units appear in discrete foci on the undersurface of the covering ectoderm (Fig. 2). Those collections of germinative cells are situated immediately above a cluster of mesenchymal cells, each germ and its incipient papilla being a unit that moves together through the evolving dermis (and on some anatomic sites even further into the developing subcutaneous fat), all the while differentiating into hair follicles, sebaceous glands and ducts, and apocrine glands and ducts. Each of the germs is crescentic (Fig. 2), at the periphery of each of them being cells whose nuclei are crowded, columnar, and aligned in a palisade. The crescent of germinative cells rapidly transforms into a structure like that of a knob, thereby heralding descent of that structure, imminently, into the primitive dermis (Fig. 3). From one side of the evolving column of epithelial cells that now bears a vague resemblance to a mature hair follicle, three protuberances, known as bulges, develop, the top bulge being an anlagen of the apocrine gland and duct, the middle bulge an anlagen of the sebaceous gland and duct, and the lowest bulge an anlagen of sites of attachment for fascicles of smooth muscle of hair erection. Those muscles originate from cells nearby in the primitive mesenchyme of the future dermis (Fig. 4). That mesenchyme also gives rise to nerves and blood vessels (arterioles, venules, and capillaries) that permeate and infuse the dermis.

At the base of the club-like developing follicle, a bulb comes into being. It houses matrical cells that differentiate into a central hair shaft, an inner sheath, and an outer sheath. The putative follicle is enveloped by a thin basement membrane that, in turn, is surrounded by perifollicular connective tissue sheath whose constituents are just like those of the papillary dermis. Above the future follicle is a funnel-shaped invagination of surface epidermis known as the infundibulum. Infundibular epidermis and surface epidermis represent a single epithelium and are indistinguishable morphologically from one another.

Very fine hairs (lanugo), the product of maturation of matrical cells in the center of an embryonic follicular bulb, appear first on the eyebrows and upper lip at the end of the 17th week, and by the 20th week they cover the entire body.¹⁰ Lanugo hairs are lost by the time of birth, or shortly thereafter, at which time some of them, like those on the scalp, are replaced by hairs that are coarser, longer, and darker, being known then as terminal hairs. Most lanugo hairs on the body are succeeded by hairs that are thinner, shorter, and lighter, so-called, vellus hairs.

Sebaceous glands come into being as a result of proliferation of cells of the middle bulge of a follicle¹⁰ and are made up of discrete lobules that, when fully formed, are composed at the periphery of a single layer of immature cells and of a core of progressively mature cells that exhibit ever more prominent vacuolated cytoplasm, the vacuoles representing droplets of lipid. The nucleus in a fully mature sebocyte has a scalloped outline consequent to the effects of pressure on it by the lipid droplets. In brief, the cells most laden with lipid are found closest to the sebaceous duct, whereas cells devoid of readily discernible cytoplasmic vacuoles are positioned furthest from the duct at the periphery of lobules. As those latter cells mature, they move from the periphery of a lobule toward a sebaceous duct that enters the base of an infundibulum. In the process, the cells become so laden with lipid that eventually they explode to produce a secretion designated holocrine. At about 15 weeks of gestational life, sebaceous secretion begins to pass sluggishly through the sebaceous duct and up the infundibulum onto the surface of the skin where it constitutes the main component of what is termed vernix caseosa. In post-pubescents, the secretion of sebaceous glands is the major contributor to the gemisch known as sebum.

Apocrine glands derive from the uppermost bulge that comes off the infundibulum and grows as a thin column of cells through the reticular dermis and often into the subcutaneous fat (Fig. 5).¹³ By 24 weeks, the column becomes coiled at its base. Most of the coiled portion is the apocrine gland, but the upper part of it, as well as the straight portion that extends through the dermis to the infundibulum, is the apocrine duct. The apocrine duct enters the mid portion of the infundibulum just above the entrance of the sebaceous duct, which passes into the base of the infundibulum. When canalization of the cords of epithelial cells occurs as a result of necrosis ("apoptosis") of cells in the center of them, a lumen comes into being in both the gland and the duct.

The apocrine gland is characterized by cells that, at the apex of their luminal surface, display cytoplasmic blebs, an appearance that has been likened to their being decapitated, thereby earning this mode of releasing the product of secretion the name "decapitation secretion" or "pinching off secretion." The blebs also have been dubbed "snouts." Those three terms are synonyms for apocrine secretion. Myoepithelial cells encircle the apocrine gland but not its duct, which consists of two layers of cuboidal epithelial cells.

It is claimed that apocrine glands at first develop over most of the body, but in the latter part of fetal life they involute, except in certain regions, such as the axillae, mons pubis, prepuce, scrotum, labia minora, external auditory canal, and breast.⁶ At puberty, those glands in the axilla and the genitalia begin to secrete, producing a milky-colored complex of lipids and proteins that are modified by bacterial activity into odorous compounds^6,9 that contain pheromones and seem to function primarily as adjuncts to social and sexual communication. In the skin of the external auditory canal, the secretion of the apocrine glands contributes to the gemisch known as cerumen.