The hair cycle
Each follicle passes, independently of its neighbours, through regular cycles of growth and shedding. There are three phases of follicular activity (Fig. 13.2).
1 Anagen. The active phase of hair production.
2 Catagen. A short phase of conversion from active growth to the resting phase. Growth stops, and the end of the hair becomes club-shaped.
3 Telogen. A resting phase at the end of which the club hair is shed.
The duration of each of these stages varies from region to region. On the scalp (Fig. 13.3), said to contain an average of 100 000 hairs, anagen lasts for up to 5 years, catagen for about 2 weeks, and telogen for about 3 months. As many as 100 hairs may be shed from the normal scalp every day as a normal consequence of cycling. The proportion of hairs in the growing and resting stages can be estimated by looking at plucked hairs (a trichogram).
On the scalp, about 85% are normally in anagen and 15% in the telogen phase. The length of hair is determined by the duration of anagen; e.g. the hairs of the eyebrows have shorter cycles than those of the scalp.
Each hair follicle goes through its growth cycles out of phase with its neighbours, so there is no moulting period. However, if many pass into the resting phase (telogen) at the same time, then a correspondingly large number will be shed 2–3 months later (see Telogen effluvium, below).
There are important racial differences in hair. Asians tend to have straight hair, Negroids woolly hair and Europeans wavy hair. These differences are associated with different cross-sectional shapes (round, flattened, etc.). Mongoloids have less facial and body hair than Mediterranean people who also have more hair than northern Europeans.