The growth of these structures is not continuous, but dependent on evolutionary and involutionary cycles, which follow one after the other. Each cycle consists of three phases, called, respectively:
- anagen (real growth that determines the lengthening)
- catagen (involution, growth block)
- and telogen (resting phase, in which the hair falls).
This growth pattern varies by species. In fur animals, for example, almost all hair follicles have a synchronized cycle, which allows a rapid fall and an equally rapid regrowth, essential for seasonal replacement. In humans, however, each hair follicle has a staggered growth cycle compared to adjacent ones; for this reason we do not realize the continuous process of renewal of body hair.
In hair and other body hair the duration of the various phases is different. In hair, for example, we have a very long anagen phase, which averages around 4 years. In contrast, the catagen and telogen phases are measured in weeks.
For the other body hairs, fortunately, the anagen phase is definitely shorter and for this reason they cannot reach such important lengths.
The average hair growth rate is 0.3-0.4 mm / day, and is higher in women.
When the catagen phase begins there is a wrinkling of the bulb, which tends to withdraw and leave the dermal papilla uncovered. The hair, on its part, withdraws, moving upwards. Melanogenesis is also stopped; for this reason the terminal part of the hair is white. When the root reaches the height of the insertion point of the erector muscle of the hair, we enter the telogen phase. However, in the bulb there remains a small area, called the hair germ, where there is some matrix.
The telogen phase is followed by a new anagen, thanks to which the bulb becomes enlarged again, tends to include the dermal papilla inside and starts the growth of a new hair. In the same follicle cohabit the new hair that is growing and the old hair, destined then to fall. At some point the old hair comes off, the new one continues to grow and the cycle starts again.
Alopecia ยป
