Accessory Structures of the Skin: Hair Growth and Types

Hair growth follows three distinct phases. The anagen phase is the active growth period lasting two to seven years, during which hair grows at approximately 0.3 mm per day. The catagen phase is a brief two to three-week transition when the follicle becomes inactive and condenses. Finally, the telogen phase is a dormant resting period lasting two to four months before the cycle repeats and new hair pushes out the old.

Vellus hair is pale-colored, fine body hair found in children and adult females. Terminal hair is longer, thicker, and heavily pigmented, appearing on the scalp, eyelashes, and eyebrows. During puberty, terminal hair develops on the face and chest in males and in axillary and pubic regions in both sexes, replacing vellus hair in those areas.

Hair consists of dead keratin-filled cells produced by basal cells in the hair bulb. As new keratinocytes are deposited at the hair bulb, they are pushed upward and keratinized, forming the visible hair shaft. Because the external hair is completely dead, it can be cut or shaven without damaging the living hair structure, as the cut is entirely superficial.

On average, 50 hairs are lost and replaced per day as part of the normal growth cycle. Hair loss occurs when more hair is shed than replaced, which can result from hormonal or dietary changes, radiation therapy, chemotherapy, or aging. Understanding normal shedding helps distinguish typical hair loss from pathological conditions requiring medical attention.

Hair color results from melanin produced by melanocytes in the hair papilla, with genetic factors determining the type and amount of melanin present. As a person ages, melanin production decreases, causing hair to lose its color and become gray and white. This natural process reflects declining melanin synthesis rather than structural changes to the hair itself.

Most chemical hair removers act superficially on the external hair shaft. However, electrolysis and laser hair removal target the hair bulb to prevent regrowth. Since the hair bulb contains living basal cells responsible for producing new hair, destroying it prevents the hair growth cycle from restarting, making these methods more permanent than surface-level removal.

The hair bulb contains basal cells that continuously produce keratinocytes through rapid cell division. As new cells are deposited at the hair bulb, they push the existing hair shaft upward through the follicle toward the skin surface. During this upward movement, cells undergo keratinization, transforming into the dead, keratin-filled cells that form the visible hair shaft.