Hair has no vital function in humans yet its psychological importance is enormous. There are several different types of hair in the human body. Scalp hair, sexual hair arriving after puberty e.g. beard, genital, axillary and some body hair, and general body. Hair follicles show intermittent activity i.e. each hair grows to a maximum length, is retained for a time and is eventually shed and replaced. An adult man has roughly 5 million hairs, 1 million on the head and 100,000 on the scalp, the rest being on the body.
The duration of hair follicle activity producing a hair (anagen) differs for hairs in different parts of the body. On the scalp, the hair grows roughly 1cm per month and can keep growing for up to three years. Shaving has no effect on the rate of growth. Anagen is followed by a period of change called catagen lasting two weeks followed by a period of no growth called Telogen. At the end of this period of roughly six months, the hair is lost and replaced by a new hair; anagen starts again!
Types of Hair
The human skin has two types of hair, terminal hair which is longer, coarser and pigmented on the scalp, eyebrows and eyelashes and vellus hair which is soft, rarely more than 2cm long and generally fairer. After puberty, secondary sexual hair develops from vellus hair in response to hormone changes.
Different races have different patterns and types of hair growth. Mongoloids tend to have coarse, straight hair. Negroids curly hair and Caucasoids, a range of textures and curl. These differences are in part due to the shape and size of the hair shaft and partly due to the shape of the hair follicle. There is also a huge variation between individuals.
Hair changes With Age
At puberty, terminal hair replaces vellus hair starting in the pubic regions. On average this occurs at about 13 in boys and about 11½ in girls. Although there is considerable variation axillary hair growth starts about 2 years after pubic hair growth.
Facial hair in boys appears at the same time as axillary hair, starting at the corners of the upper lip, spreading to complete the moustache and then to the cheeks and beard.
Other vellus hairs on the neck, chest, back and limbs may also continue to change to terminal hairs depending on the sex of the individual and may continue to increase until the early ’40s. Certain follicles of the scalp may regress to vellus hairs with age to produce only fine, short vellus hairs. This condition of patterned hair loss called androgenetic alopecia is inherited and requires the effects of male hormones.