Endogenous retinoids in the hair follicle and sebaceous gland

The document from 2012 reviews the critical role of retinoic acid (RA) in the development and maintenance of the hair follicle and sebaceous gland, which are part of the pilosebaceous unit (PSU). It emphasizes that both deficiency and excess of vitamin A, from which RA is derived, can lead to dysfunction in these structures. The review details the complex metabolism of vitamin A to RA within the PSU, the localization of RA synthesis, and the regulation of RA levels, including feedback mechanisms. It also discusses the structure of the PSU, cell differentiation, and the identification of stem cell populations. The document reports that alterations in RA signaling can result in alopecia, affecting the hair cycle and lipid metabolism in the skin. It notes that RA synthesis varies throughout the hair cycle and may be crucial for stem cell maintenance. Increased RA synthesis enzymes and binding proteins have been observed in hair loss diseases, and dietary vitamin A reduction has prevented hair loss in mouse models. In the sebaceous gland, RA is important for sebum production, with excess leading to gland atrophy. The document concludes that maintaining precise RA levels is essential for PSU function and calls for further research to understand RA's specific mechanisms and its potential in treating hair loss diseases and acne. The work was supported by grants from the National Institutes of Health, the National Alopecia Areata Foundation, and the North American Hair Research Society.