Published online by Cambridge University Press: 29 September 2009
Introduction
The pilosebaceous unit consists of the hair follicle and associated sebaceous and apocrine glands. Hair follicles and sebaceous glands have cellular androgen receptors and react to circulating androgens. Physiological androgen levels induce secondary sexual hair development at puberty, and mild acne is a near universal accompaniment. Androgens also induce pattern hair loss the prevalence and severity increasing with age. Polycystic ovary syndrome (PCOS) is associated with androgen excess and may induce hirsutism and seborrhea and accentuate androgenetic alopecia and acne (Table 8.1). Acanthosis nigricans is a cutaneous marker of insulin resistance that is also associated with PCOS.
Physiology of the sebaceous gland
Sebaceous glands occur on all parts of the skin except on the glabrous skin of the palms and soles. They are most numerous on the face, scalp, and back occurring at a concentration of between 400 and 900 glands per square centimeter. Each of the several lobes of the gland has a duct lined with keratinizing squamous epithelium, and these join to form a main duct that enters the follicular canal. Glandular cells, which divide at the periphery, move towards the center of the glands and become increasingly filled with sebaceous material. During this process, cells undergo complete dissolution and discharge all cellular contents into the sebaceous duct. The lipid composition of sebum differs from epidermal lipid in that it contains wax esters and squalene that the former does not, although there are a similar percentage of glycerides (Cunliffe and Simpson 1998).
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