Most sebaceous glands develop embryologically from hair germs, but a few free glands arise from the epider-mis. Those associated with hairs lie in the obtuse angle between the follicle and the epidermis (Fig. 13.1). The glands themselves are multilobed and contain cells full of lipid, which are shed whole (holocrine secretion) during secretion so that sebum contains their remnants in a complex mixture of triglycerides, fatty acids, wax esters, squalene and cholesterol. Sebum is discharged into the upper part of the hair follicle. It lubricates and waterproofs the skin, and protects it from drying; it is also mildly bacteriocidal and fungistatic. Free sebaceous glands may be found in the eyelid (meibomian glands), mucous membranes (Fordyce spots), nipple, peri-anal region and genitalia.
Androgenic hormones, especially dihydrotestos-terone, stimulate sebaceous gland activity. Human sebaceous glands contain 5α-reductase, 3α- and 17α-hydroxysteroid dehydrogenase, which convert weaker androgens to dihydrotestosterone, which in turn binds to specific receptors in sebaceous glands, increasing sebum secretion. The sebaceous glands react to mater-nal androgens for a short time after birth, and then lie dormant until puberty when a surge of androgens produces a sudden increase in sebum excretion and sets the stage for acne.