Classification of Hair

Human hair varies with respect to texture, color, and length. Lanugo hairs cover most of the fetal skin and shed perinatally. These lightly pigmented hairs are fine in texture.

Thicker and darker than lanugo hairs, vellus hairs cover most of the glabrous skin surface except palms, soles, palmar and plantar surfaces of fingers and toes, inner aspect of the prepuce, and the glans penis.

Darkly pigmented terminal hairs are long and thick and are most commonly located on the scalp and the facial areas of men. These hairs can reach a preprogrammed length based on length of the anagen phase. Ultimately, terminal hairs undergo involution and convert to catagen and then to the telogen phase. The mechanism and factors that induce the hair to terminate growth and involute are largely unknown.

Some hair follicles have the ability to produce different types of hair at various times. For example, hair follicles on an adult scalp that normally produce terminal hairs can, in individuals with androgenetic alopecia, undergo miniaturization and produce short, fine, and lightly pigmented hairs. Similarly, hair follicles on the male beard, which until puberty produce only vellus hairs, are capable of producing terminal hair thereafter.