The hair follicle: the hidden part of hair

The human hair follicle is a skin appendage which has a very complex structure, with more than twenty different cell types distributed into six main compartments: the connective tissue sheath, the dermal papilla, the outer root sheath, the inner root sheath, the shaft and the sebaceous gland. The pigmentation unit, responsible for hair color, is made of fully active melanocytes. This complex appendage has a unique characteristic of renewing itself under a cyclical, asynchronous and stochastic way, out of a double reservoir of pluripotent stem cells. The pigmentation unit also renews itself out of a melanocyte progenitor reservoir which progressively declines with time, provoking the hair whitening process. Finally, the shape of the hair shaft is programmed from the bulb. The hair follicle thus behaves as a fully autonomous skin appendage with its own regulatory network and cycle. This article is aimed at making the reader aware of the hair follicle complexity. Only its understanding can help designing specific cosmetic actives.