Frontiers in sebaceous gland biology and pathology.

The development of experimental models for the in vitro study of human sebaceous gland turned down the theory of a phylogenetic relict and led to the identification of several, unknown or disregarded functions of this organ. Such functions are the production of foetal vernix caseosa, the influence of three-dimensional organization of the skin surface lipids and the integrity of skin barrier and the influence on follicular differentiation. In addition, the sebaceous gland contributes to the transport of fat-soluble antioxidants from and to the skin surface, the natural photoprotection, the pro- and antiinflammatory skin properties and to the innate antimicrobial activity of the skin. It is mainly responsible for skin's independent endocrine function, the hormonally induced skin ageing process, the steroidogenic function of the skin as well as its thermoregulatory and repelling properties and for selective control of the hormonal and xenobiotical actions of the skin. Interestingly, sebocytes, at least in vitro, preserve characteristics of stem-like cells despite their programming for terminal differentiation. This review reports on various sebaceous gland functions, which are currently under investigation, including its role on the hypothalamus-pituitary-adrenal-like axis of the skin, the impact of acetylcholine on sebocyte biology, the activity of ectopeptidases as new targets to regulate sebocyte function, the effects of vitamin D on human sebocytes, the expression of retinoid metabolizing cytochrome P450 enzymes and the possible role of sebum as vehicle of fragrances. These multiple homeostatic functions award the sebaceous gland the role 'brain of the skin' and the most important cutaneous endocrine gland.