Human Hair Follicles Display a Functional Equivalent of the Hypothalamic-Pituitary-Adrenal (HPA) Axis and Synthesize Cortisol

The study demonstrated that human scalp hair follicles exhibited a functional equivalent of the hypothalamic-pituitary-adrenal (HPA) axis, capable of synthesizing and secreting cortisol. When stimulated by corticotropin-releasing hormone (CRH), these hair follicles up-regulated proopiomelanocortin (POMC) transcription and increased immunoreactivity for adrenocorticotropic hormone (ACTH) and α-melanocyte-stimulating hormone (α-MSH). The follicles also modulated the expression of their receptors and secreted significant levels of cortisol, which was further enhanced by CRH. Additionally, CRH influenced hair growth parameters such as hair shaft elongation and melanin production. The study concluded that hair follicles possess HPA axis-like regulatory feedback systems, as evidenced by the down-regulation of follicular CRH expression by hydrocortisone, a glucocorticoid receptor agonist.