P5.56 ROLE OF THYROID HORMONE IN REPIGMENTATION OF TERMINAL HAIR AND ROLE OF FOLLICULAR MELANOCYTE IN HAIR CYCLE REGULATION

Pedro Redondo, Pedro Lloret, Julio del Olmo.

Department of Dermatology. University Clinic of Navarra, Pamplona, Spain

Introduction: The hair graying trait correlates closely with chronological aging and occurs in all individuals, regardless of gender or race. The best model to study the hair cycle is the highly standardized C57BL/6 model of depilation-induced HF cycling. Here we discuss the role of thyroid hormone in the hair darkening of two patients with hair graying. In addition, we investigated the effects triiodo-L-thyronine on mouse hair follicles (C57BL/6) in vivo, in order to understand the role of this thyroid hormone on HF melanocyte homeostasis.

Methods and results: The two patients both showed an increase in T3 through exogenous administration, one due to hypothyroid coma and the other for a decompensation due to his disease (thyroid carcinoma followed by replacement hormone with thyroid hormone). C57BL/6J mice with telogen hair follicles were treated topically with 0.5 μg T3 daily for 6 days, and then 100% of the tested animals entered anagen. In the control animals, a spontaneous anagen started on day 10, and 100% of them were in anagen at day 16. There was a dose-dependent increase in skin pigmentation in the T3-treated animals.

Discussion: Treatment of cultured human follicles with T3 prolongs their survival but does not increase their hair growth rate, and they transform into catagen prematurely versus controls. An in vitro study demonstrated that TGF-b, phorbol esters, bFGF, and bovine calf serum reduce the growth of the follicle and play an essential role in the induction of the catagen phase of the human hair cycle. TGF-b, like bFGF, favors the entry of the hair into catagen, making it shorter and finer. This effect may possibly be explained by the fact that both TGF-b and bFGF stimulate the expression/production of c-kit. Melanocytes express c-kit and therefore may regulate the hair cycle: pigmentation and entry/exit into anagen-catagen. T3 is also necessary for the culture of melanocytes, as occurs with the other cells derived from the neural crest. We hypothesize that the melanocyte may be the target cell for the action of these substances (including thyroid hormone) on the pilosebaceous follicle.