1. The hair

A normal scalp contains 100 to 150,000 hairs. The speed of hair growth varies with age and season, and also has a hereditary component. It averages 1 cm per month. In a lifetime, we can therefore produce around 10 m of hair, or 1,000 km end-to-end. In France, brown hair predominates (50%), followed by gray (20%), blond and black (10%), red (5%), dark brown and white (2.5%). Diameter is around 80 μm.

Hair is divided into two parts: the root, which is the non-visible, living part, and the shaft, the visible part (figure 1 ). The latter is subdivided into 3 concentric zones, from the central to the outer part:

• the marrow, made up of anucleate cells, which have no vital properties;

• the cortex: the cells in this area...