and are not visible on the surface of the skin. Figure 2 shows the structure of a normal human hair. The invisible part of the human hair is enclosed by two root sheaths, the inner (IRS) and outer (ORS) root sheaths. Hair follicle stem cells lie in the bulge of the ORS and give rise to keratinocytes and other cell types [67, 68]. The main task of mature keratinocytes is to produce keratin for the growing hair. Human hair cycles through distinct stages, which can be partially discriminated by the shape of the hair bulb. Darkly pigmented hockey stick shaped bulbs with distinct root sheaths typify anagen hair bulbs which build up new hair roots in the growth phase. Nearly all hair (85%–90%) shows this state. Telogen hair has no or an insufficient root sheath, a club-shaped bulb and is less pigmented than anagen bulbs. At this stage, the hair follicle is resting and regenerating for the production of a new hair. The catagen hair bulb indicates a very short transition phase between the two previous ones where the bulb is narrowing and the hair is atrophying [69].
