Hair formation, keratinization and elimination: the hair cycle

Over the course of an individual's life, about twenty hair cycles take place asynchronously in order to maintain a correct hair mass.

This hair cycle is composed of three distinct phases:

- The anagen phase, or hair growth phase

- The catagen phase, or hair regression phase

- The telogen phase, or the hair's resting phase

Anagen phase

The anagen phase, or hair growth phase, concerns about 80 to 90% of the hair mass and lasts an average of 5 years for women and 3 for men.

The hair is born in the hair follicle and grows thanks to a mechanism of massive division of the cells of the root.

It is the cells of the dermal papilla that trigger the genesis of the hair by emitting a hormonal signal to the stem cells of the bulge* which then mutate into progenitor cells that will themselves divide from mother cells into daughter cells to create the new hair.

As soon as they are formed, the daughter cells split, the new cells driving the previous ones out of their respective nuclei, making them die and become rigid. This process of accumulation of hardened dead cells constantly pushing each other along the hair follicle is called keratinization, a phenomenon that will lead to the formation of hair as we can see it with the naked eye.

(* cluster of keratinocytes, cells synthesizing keratin, attached to the outer follicular sheath.)

Catagen phase

The catagen phase, or phase of hair regression, lasts about 1 month and concerns about 3% of the hair mass.

The hair growth ends, the hair follicle stops its activity causing the death of the cells of the hair bulb and its retraction towards the surface of the scalp.

Telogen phase

The telogen phase, or resting phase of the hair, lasts an average of 2 to 7 months and affects about 10 to 20% of the hair mass. The hair is dead, shrinks, thins and tends to lighten, but remains attached to its follicle while its bulb is dormant. At the end of this period of rest, the hair falls under the push of a new hair shaft which then begins its life cycle.