Phases of Hair Growth

Hair growth occurs in the anagen, catagen, telogen and exogen phases. Growth follows a cyclical pattern of birth, growth, death and removal. Growth among people follows the same pattern; however, the duration of the growth period can be longer or shorter, depending on the individual. According to the International Society of Hair Restoration Surgery, growth occurs over a period of two to eight years. During this time, hair grows an average of 1/2 inch per month.

Anagen

Approximately 90 percent of hair is in the anagen phase at any one time. During the anagen, or growth phase, dormant hair follicles become active and produce hair for a period of two to eight years. Because hair grows at a steady rate of 1/2 inch per month, the length of the growth phase determines the maximum length that hair will grow.

Catagen

The catagen phase is a period of destruction. During this phase, hair stops growing and certain structures of the hair follicle begin to degenerate. The most important change occurs at the dermal papilla, the structure responsible for hair growth. The dermal papilla shrinks and rises to the surface of the skin. Melanin and keratin-producing cells also die during this phase, which lasts two to four weeks.

Telogen

The hair follicle rests during the telogen phase. This period lasts approximately two to four months, during which accumulated dead hair cells form a bulb at the root of the hair. Because the hair isn't attached to the dermal papilla during this phase, it can shed at any time. However, the bulb shape of the root usually holds the hair in place until the next phase.

Exogen

Many growth models combine the exogen and anagen phases together, since they occur simultaneously. The exogen phase is a phase of shedding. The new activity and growth of the anagen phase pushes the detached hair out of the hair follicle. Approximately 50 to 100 hairs shed per day, according to the International Society of Hair Restoration.

Disruption

Several factors can disrupt the hair-growth cycle, resulting in thinning and hair loss. The International Society of Hair Restoration lists medication, illness, poor nutrition, hormones, skin disease, stress, chemotherapy and radiation, thyroid disease and chemicals as potential causes for hair loss.