Le sébum, produit par les glandes sébacées associées aux follicules pileux, est composé de cires, squalène et triglicérides. Il contient aussi de multiples molécules régulatrices endogènes et des facteurs exogènes, car l'unité pilosébacée est souvent colonisée par les micro-organismes. Le développement récent de cultures de sébocytes fonctionnels permettra de mieux comprendre divers aspects de la biologie des glandes sébacées.

Modern therapeutic approaches allow to control sebaceous secretion but knowledge about the sebaceous gland and its precise function within the pilosebaceous unit is still insufficient. Steroid hormones are the principal albeit not exclusive regulators of the sebaceous glands. Three phases may be distinguished in sebaceous physiology: secretion-production, stocking in the follicular reservoir, and excretion. Human “native” intracellular sebum, before secretion, is composed of squalene, waxes, and triglycerides. Once secreted, the sebum is colonised by various xenobiots whose development is controlled by several defensive humoral mechanisms and by the contact with ambient oxygen. Oxygen and micro-organisms transform “native” sebum, lysis of triglycerides to fatty acids being the most pronounced activity. Certain components of this complex mixture of molecules present in the sebum are clearly cytotoxic or irritant, provoking reactive follicular hyperkeratosis and comedone formation - the first step to acne. Some lipophilic organisms like Malassezia yeast may be highly antigenic and induce chronic inflammatory reactions like in seborrhoeic dermatitis. Demodex is an intrafollicular parasite feeding on sebum that frequently causes blepharitis. Sebum is also a vehicle transporting and transmitting several endogenous and exogenous molecules, including potential regulatory factors of hair follicles. Recent development of in vitro cultures of functional sebocytes should help to better understand several aspects of the sebaceous gland's biology.