Human skin: An independent peripheral endocrine organ

The historical picture of the endocrine system as a set of discrete hormone -producing organs has been substituted by organs regarded as organized comm unities in which the cells emit, receive and coordinate molecular signals f rom established endocrine organs, other distant sources, their neighbors, a nd themselves. In this wide sense, the human skin and its tissues are targe ts as well as producers of hormones. Although the role of hormones in the d evelopment of human skin and its capacity to produce and release hormones a re well!, established, little attention has been drawn to the ability of hu man skin to fulfil the requirements of a classic endocrine organ. Indeed, h uman, skin cells produce insulinlike growth factors and binding proteins, p ropiomelanocortin derivatives, catecholamines, steroid hormones and vitamin D from cholesterol, retinoids from diet carotenoids, and eicosanoids from fatty acids. Hormones exert their biological effects on the skin through in teraction with high-affinity receptors, such as receptors for peptide hormo nes, neurotransmitters, steroid, hormones and thyroid hormones. In addition , the human skin is able to metabolize hormones and to activate and inactiv ate them. These steps are overtaken in most cases by different skin cell po pulations in a coordinated way indicating the endocrine autonomy of the ski n,. Characteristic examples are the metabolic pathways of the corticotropin -releasing hormone/propiomelanocortin axis, steroidogenesis, vitamin D, and retinoids. Hormones exhibit a wide range of biological activities on the s kin, with major effects caused by growth hormone/insulin-like growth factor -1, neuropeptides, sex steroids, glucocorticoids, retinoids, vitamin D, per oxisome proliferator-activated receptor ligands, and eicosanoids. At last, human skin produces hormones which are released in the circulation and are important for functions of the entire organism, such as sex hormones, espec ially in aged individuals, and insulin-like growth factor-binding proteins. Therefore, the human skin fulfils all requirements for being the largest, independent peripheral endocrine organ. Copyright (C) 2001 S. Karger AG, Ba sel.