Hair has two separate structures: the hair shaft (the part we see above the skin) and the follicle (the part that is below the surface of the skin). The hair shaft is composed of strong structural protein called keratin. This is the same protein that makes up the nails and the outer layer of the skin (Fig. 1.1).
Below the surface of the skin is the hair “root” or hair follicle comprised of several structures: dermal papilla, hair bulb, hair shaft, sebaceous gland, and a tiny muscle.
Most hair-conditioning products attempt to affect the cuticle by making these scales lie flat, thereby imparting a silky feel to the hair.
Unlike other mammals that shed or grow hair according to the season, in humans hair growth and loss are random and not strictly but somewhat seasonal. At any given time, in humans a random number of hairs will be in various stages of growth and shedding.
The three phases of hair growth cycle are as follows: anagen,catagen, and telogen.
Each hair follicle goes through 10–20 cycles in a lifetime, while each cycle lasts a different time: anagen lasts 3–10 years, catagen 2–3 weeks, and telogen 3–4 months. About 25–100 hairs are shed normally each day (Fig. 1.2).
Some people have difficulty growing their hair beyond a certain length because they have a short active phase of growth. However, people with very long hair have a long active phase of growth.