[Molecular approach of hair biology]

The pilo-sebaceous unit is the site of numerous cell interactions, involving epithelial cells--hair follicle and sebaceous gland--, fibroblasts--dermal papilla--, nerve fibres, melanocytes, endothelial cells, and the immune system--Langerhans cells and lymphocytes--. From these timely and spacely regulated interactions results a complex process, namely the hair cycle comprising three main phases: anagen, catagen and telogen. Anagen is the growing phase of the follicle, catagen is a phase of involution, and telogen is a resting phase. Although the structure and the histology of the follicle is well known today, the molecular events controlling the hair cycle, namely the anagen-catagen and telogen-anagen transitions, remain obscure. The development of in vitro and in vivo models, however, has given some insights on the role growth factors such as IGF-1, TGF alpha, HGF, and EGF, as well as androgens such as the 5 alpha-dihydrotestosterone, cytokines such as IL-1 and potassium channel openers such as minoxidil. Many other mediators are likely to be discovered in the next future, the toughest task being to understand the integration of the diverse signals resulting in the harmonious development of the hair follicle.