Hair is mainly composed of the protein keratin. Keratin assembles into rope-like intermediate filaments. The structure of these filaments provides strength to the hair shaft.
Hair growth begins under the skin in a hair follicle. The only “living” portion of the hair is found in the follicle. The hair that is visible is the hair shaft, which exhibits no biochemical activity and is considered dead. The base of the root is called the bulb, which contains the cells that produce the hair shaft. Other structures of the hair follicle include the oil producing sebaceous gland which lubricates the hair and the erector pili muscles, which are responsible for causing goose bumps.
Stages of the hair cycle
Between starting to grow and falling out, each hair passes through three distinct stages. These are so important that they have been given special names: anagen (the growing phase), catagen (the intermediate phase) and telogen (the shedding phase). Each strand of hair on the human body is at its own stage of development. Once the cycle is complete, it restarts and a new strand of hair begins to form.
The anagen phase or growth phase begins in the papilla and can last up to eight years. The span at which the hair remains in this stage of growth is determined by genetics. The longer the hair stays in the anagen phase, the faster and longer it will grow. During this phase, the cells in the papilla divide to produce new hair fibers, and the follicle buries itself into the dermal layer of the skin to nourish the strand.
The catagen phase or transitional phase, allows the follicle to, in a sense, renew itself. Signals sent out by the body determine when the anagen phase ends and the catagen phase begins. During this time, which lasts about two weeks, the hair follicle shrinks due to disintegration and the papilla detaches and “rests”, cutting the hair strand off from its nourishing blood supply.
During the telogen phase, or resting phase the hair and follicle remain dormant anywhere from 1–4 months. Ten to fifteen percent of the hairs on one’s head are in this phase of growth in any given time. The anagen phase begins again once the telogen phase is complete. The preceding hair strand is pushed up and out by the new, growing strand. This causes a normal hair loss commonly known as shedding.
The number of sessions depends on various parameters, including the area of the body treated, skin color, coarseness of hair and gender. Coarse dark hair on light skin is easiest to treat. Hair on darker skin is harder to treat. Finer hair is only sometimes affected. Certain areas (notably men’s faces) may require considerably more treatments to achieve desired results. In addition, since hair grows in several phases (anagen, telogen, catagen) and laser can only affect the currently active growing follicles (anagen), several sessions are needed to kill hair in all phases of growth.