ABSTRACT
Vitamin A has been shown to have an essential role in the differentiation, growth and regulation of the human hair cycle. The use of both systemic and topical retinoids is implicated in both the pathogenesis and treatment of various hair disorders. Of particular importance is the direct impact that retinoids have in the dynamic hair growth cycle. Retinoic acid has been shown to directly affect the anagen initiation phase of the hair cycle and direct the differentiation of embryonic and induced pluripotent stem cells into keratinocytes to produce normal hair follicles. Retinoids also have a critical role in the up- or down-regulation of protein pathways depending on their mode of transmission. Systemic retinoids have been shown to cause hair disorders including acute telogen effluvium, alopecia areata and acquired progressive kinking of the hair. Conversely, the use of topical retinoid agents has been reported as an effective treatment option for androgenetic alopecia and alopecia areata, whilst systemic retinoid agents have been reported as an effective option for frontal fibrosing alopecia and monilethrix.
