Formation, keratinization and elimination of the hair: the hair cycle 

Throughout the course of a person’s life about twenty hair cycles occur asynchronously in order to maintain the right hair mass.

This hair cycle includes three distinct phases:

-      The anagene phase, or growth phase

-      The catagen phase, or transitional phase

-      The telogen phase, or rest phase

Anagen phase

The anagen phase or hair growth phase involves 80 to 90% of the hair on the head and lasts approximately 5 years for women and 3 years for men.

The hair emerges from the hair follicle and grows using a massive cell division mechanism in the root.

The cells of the dermal papilla trigger hair growth by emitting a hormonal signal to the stem cells in the bulge* which then mutate into progenitor cells that separate themselves from mother cells to become daughter cells and create a new hair.

As soon as they are formed, these daughter cells split into two dispelling the former cells of their nucleus, causing them to die and become rigid. This accumulation process of dead cells, continually repelling one another along the hair follicle, is called keratinization, a phenomenon that ends in the formation of the hair that is visible to the naked eye.

(* pile of keratinocytes, cells that synthesize keratin, attached to the outer follicle.)

Catagen phase

The catagen phase, or transition phase of the hair, lasts for approximately 1 month and involves 3% of the hair on the head.

Hair growth comes to an end, the hair follicle ceases its activity causing the death of cells in the hair bulb and the retraction of the hair towards the surface of the scalp.

Telogen phase

The telogen phase, or the rest phase, lasts for an average of 2 to 7 months and affects approximately 10 to 20% of the hair mass. The hair dies, shrinks, becomes finer and usually becomes lighter in color, but remains attached to its follicle while the bulb is dormant. At the end of this rest period, the hair is pushed out of the scalp by the new hair shaft growing in and beginning its own hair cycle.