Below the surface of the skin is the hair root, which is enclosed within a hair follicle. At the base of the hair follicle is the dermal papilla. The dermal papilla is feed by the bloodstream which carries nourishment to produce new hair. The dermal papilla is a structure very important to hair growth because it contains receptors for male hormones, which including androgens. Androgen hormones include testosterone, androsteinedione, and dihydrotestosterone (DHT), which are made in men’s bodies in large amounts, and in women's bodies in small amounts. Androgens regulate hair growth and in scalp hair androgens may cause the hair follicle to get progressively smaller and the hairs to become finer in individuals who are genetically predisposed to this type of hair loss.
Anagen is the active growth phase of hair follicles. The cells in the root of the hair are dividing rapidly, adding to the hair shaft. Our hair usually grows about one-half inch per month, with each hair having a growth phase of two to six years. The amount of time the hair follicle stays in the anagen phase is genetically determined. At the end of the anagen phase an unknown signal causes the follicle to go into the catagen phase.
The catagen phase is a short transition stage that occurs at the end of the anagen phase. It signals the end of the active growth of a hair. This phase lasts for about 2-3 weeks while a club hair is formed. A club hair is formed during the catagen phase when the part of the hair follicle in contact with the lower portion of the hair becomes attached to the hair shaft. This process cuts the hair off from its blood supply and from the cells that produce new hair. When a club hair is completely formed, about a 2 week process, the hair follicle enters the telogen phase.
The telogen phase is the resting phase of the hair follicle. At any given time, 10%-15% of all hairs are in the telogen phase. This phase lasts for about 100 days for hairs on the scalp and much longer for hairs on the eyebrow, eyelash, arm and leg.
During this phase the hair follicle is completely at rest and the club hair is completely formed. Pulling out a hair in this phase will reveal a solid, hard, dry, white material at the root. About 25-100 telogen hairs are shed normally each day due to the telogen phase. During this phase, the hair "rests" for a period of time, then falls out, and the follicle from which it sprang soon starts growing a new strand. This cycle continues usually well into our senior years. However, in those with a genetic predisposition to hair loss, androgen hormones interfere with this natural process. In those who are genetically susceptible, when testosterone comes in contact with enzymes residing in the hair cell, it is converted into the more potent androgen DHT, which then binds with receptors deep within the hair follicle.
Over time, an excess build-up of DHT in the follicle causes it to begin shrinking, which in turn alters the natural resting and growth phases of the hair. Some of the follicles eventually die, while others are rendered incapable of producing or maintaining healthy hair growth. The end result is hair loss and a condition that is medically known as androgenic alopecia. |