Unlike other epidermal structures which grow continuously, hair has a cyclical pattern of growth. The growing phase or anagen lasts an average of 1000 days on the scalp followed by an involutional phase known as catagen which is quite short, lasting only a few days. The hair then enters a resting phase, telogen, lasting about 100 days. In man, hair growth is normally asynchronous, with each individual hair following its own cycle independently of the others. The basal layer of the hair bulb from which the hair itself is produced is known as the matrix and contains melanocytes from which melanin pigment is incorporated into the hair. The type of melanin determines the colour and in grey or white hair, pigment production is reduced or absent.
The body surface with the exception of the palms, soles, the lips, and the genitalia, is covered with fine vellus hairs that do not have a medulla and are not pigmented. These hairs develop into longer coarse, medullated, terminal hair on the scalp and eyebrows. At puberty a similar change occurs in the pubic area and the axillae, also on the face and trunk, in the male. These changes are androgen dependent, even in females, but testicular androgen is required to produce beard growth and balding in men.
Racial characteristics and the genetic make-up of the individual determine the type and colour of the hair. Straight black oriental hair is clearly different from the nordic blonde type.
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