What is a hair?

The hair, or hair shaft, is formed at its root, found in the epidermis, known as the pilosebaceous follicle. The hair is primarily comprised of a specific protein known as keratin, as well as lipids from sebaceous glands that provide protection for the hair. It also contains melanin, the pigment that is responsible for the hair's natural color.

The hair shaft has three parts: the cuticle, the cortex and the medulla.

1. The cuticle is the outer layer of the hair shaft. It is made up of scales - thin plates of keratin layered one over another like the tiles on a roof. It provides impermeable protection.
2. The cortex makes up 90% of the hair shaft's weight. It is made up of keratin, organized to give the hair strength and elasticity. The cortex also contains melanin, which is responsible for the hair's color.
3. The medulla is a narrow tube at the center of the hair shaft. Sometimes it is not present in very fine hair.

We have an average of 200 to 300 hairs per cm² of our scalp, making a total of around 100,000. We naturally lose between 50 and 100 hairs per day. The hair develops in stages that make up the its growth cycle. Each pilosebaceous follicle will go through an average of 25 hair cycles in its lifetime. During a cycle, the hair grows, remains, degenerates and falls out over 3 phases:

1. Anagen phase = the growth phase, lasting 3 to 6 years. Some 85% of hair is in this phase at any time.
2. Catagen phase = the transition phase (regression of the hair) which lasts 2 to 3 weeks and affects 1% of the hair.
3. Telogen phase = the shedding phase, which lasts 2 to 3 months and affects 14% of the hair.