The hair: all you need to know

The hair: some figures

100 000

is the average number of hairs on a human scalp

1

is the number of centimeters of average hair growth per month. Hair that is 40cm long hair is thus 3 years old.

10 and 20

the number of hair growth cycles during a lifetime

100 and 200

physiological hair loss per day (varies depending on the season)

The anatomy of the hair

The hair is broken down into two parts:

• the root, which is fixed inside the skin and made up of living cells.
• the visible hair shaft, which contains keratinised cells.

The hair shaft has three layers:

• Cuticle (or outer layer) protects the hair and makes sure it is shiny and soft.
• Cortex is the middle layer, and contains the melanin that manage hair pigmentation. The hair's strengh and resistance depends on disulphide bridges, keratin and keratin-associated proteins in the cortex.
• Medulla (or inner layer) is a substance containing cells with no nucleus, and is not always found in the hair shaft. Even today, scientists are still unsure about the nature of its properties.

95% keratin

The hair contains 95% keratin, a helix-shaped fibrous protein contained in a rich sulphur matrix. This protein consists of eight amino acids (including cysteine) that form disulphide bridges between molecules to ensure the hair stays strong, resistant and supple. The intake of protein rich in sulphur protexts the quality of the hair.

The hair life cycle in 4 steps

The hair cycle is a renewal process for the lower part of the hair shaft. This complete regeneration varies between 10 and 20 cycles throughout a liftetime, depending on individuals. These growth cycles are continuous and split into 4 phases: anagen, catagen, telogen and exogenous.

• During the anagen phase, hair follicle cells proliferate in the epithelial compartments, and the hair is in active growth. This phase lasts between 2 and 6 years.
• The catagen phase is a period of transition. The hair stops growing because proteins and pigments are no longer produced. The follicle gets ready to renew itself. It is a rest phase that lasts from 2 to 3 weeks.
• The telogen phase is a rest phase for the hair follicle during which the hair begins to pull itself loose, in preparation for its elimination. This dead hair elimination process concerns 10 to 15% of the hair, and is spread over a period of 3 months.
• The exogenous phase is an intermediate period between when one hair falls out and another begins to grow. The appearance of hair density seems to depend on the length of this phase (no hair is present).