Hair Discussions - Hair Metamorphosis

Scalp hair fibers grow from 100,000 to 350,000 follicles which are reported to occupy the human scalp; however, not all the follicles are productive. In each producing follicle, the duration of the hair's life cycle is influenced by age, pathology and a wide variety of physiological factors. The life cycle is divided into the anagen (active), catagen (transitional) and telogen (resting) phases.

The anagen phase is the period of active hair growth where protein synthesis and keratinization are continuously occurring. In normal subjects, this phase lasts for up to five years, although longer durations have been documented.

The cessation of the anagen phase is characterized by a transitory phase known as catagen. This phase lasts for two to three weeks. Following the catagen phase, the hair enters the telogen or "resting" phase. In normal subjects, telogen hair is retained within the scalp for up to 12 weeks before the emerging new hair dislodges it from its follicle.

During the anagen phase, protein's thesis is the main distinction of the hair bulb. In the telogen phase, the dermal papilla undergoes renewal. It is at this time that structural characteristics can be modified. The new hair should be identical to its predecessor, but with advancing age, and in some pathological states, a strict copy is not maintained. In these circumstances, the hair may become finer and shorter, modifying the esthetic profile. Since these effects occur over several hair cycles, years may elapse before the affected individual recognizes the difference.

Like skin cells, hair grows and is shed regularly. Shedding anywhere from 50 to 100 hairs per day is considered normal. The average rate of growth is about 1/2 inch a month. It is now known that hair grows fastest in the summer, slowest in the winter, speeds up under heat and friction, but slows down when exposed to cold. Hair grows the best between the ages of 15 to 30. But, hair growth begins to wind down sometime between the ages of 40 and 50. Progressive hair loss begins naturally in both sex about age 50, accelerating in the 70s. About 40 percent of Caucasian men lose hair to some extent by age 35.