The human skin as a hormone target and an endocrine gland

Hormones influence the development and function of human skin which also produces and releases hormones. Recently attention has been focused on identifying and understanding the complex endocrine properties of human skin, such as expression and function of specific hormone receptors, synthesis of hormones from major classes of compounds used by the body for general purposes, organized metabolism, activation, inactivation and elimination of the hormones in specialized cells of the tissue, exertion of biological activity and release of tissue hormones in the circulation. Specifically, hormones exert their biological effects on the skin through interaction with high-affinity receptors, such as several receptors for peptide hormones and neurotransmitters, steroid and thyroid hormones. Hormones exhibit a wide range of biological activities on the skin with distinct effects caused by growth hormone/insulin-like growth factor-I, neuropeptides, sex steroids, glucocorticoids, retinoids, vitamin D, peroxisome proliferator-activated receptor ligands, eicosanoids, melatonin and serotonin. Human skin produces, activates or inactivates metabolically numerous hormones which are probably important for skin functions but also for functions of the entire human organism, such as sex hormones, especially in aged individuals, insulin-like growth factor and -binding proteins, neuropeptides, prolactin, catecholamines, retinoids, steroids, vitamin D and eicosanoids. These functions are undertaken in most cases by different skin cell populations in a coordinated way, indicating the endocrine autonomy of the skin. Characteristic examples are the metabolic pathways of the corticotropin-releasing hormone/propiomelanocortin axis, steroidogenesis, vitamin D and retinoids. The human skin is, thus, the largest, peripheral endocrine organ.