The science of body hair
Human hair is made up of two distinct structures:
1. The visible shaft, protruding from the the skin. Hair fiber is composed of keratin, a form of hard protein that also comprises the structure of toenails and fingernails.
2. The hair follicle beneath the surface of the skin. The “bulse” or “bulb” is located in the outer root sheath, at the base of the follicle and is the living part of the hair. It supplies the entire hair follicle with new cells, and takes part in healing the epidermis after a wound.
Morphologically, there are three types of hair on your body.
Vellus hair, also referred to as “peach fuzz”, is soft, fine, and short. It is only lightly pigmented, and is not cosmetically important. Vellus hair covers the body of children and adults, with most women having the hair on their chest, back, and face. The growth of vellus hair is not affected by hormones, unlike terminal hair, but it can be darker and more noticeable in some women than others, especially those with darker complexions. Vellus hair shafts do not have a melanin layer, and the follicles do not have adjacent sebaceous glands. Vellus hair provides some insulation, and helps the body maintain a steady temperature.
Terminal hair is the coarser, thicker, pigmented hair found on scalp, mustache, beard, underarms, and pubic area. It is longer than vellus hair, and is influenced by hormones, most of all by testosterone. Around puberty, the rising levels of hormones causes some vellus hair to become terminal hair. This is more prevalent in men, where terminal hair starts growing on the face, stomach, shoulders, chest, legs and back. Terminal hairs have a medulla at the innermost part, and large, darkly pigmented hair fibers. Terminal hairs provide protection and cushioning.
Intermediate hair shares properties with both terminal and vellus hairs. It has less pigment than terminal hair, and has a smaller hair follicles which penetrates less below the skin surface. During the balding process, the scalp follicles often shrink, stop producing terminal hair, and sometimes produce intermediate hair. As balding continues, the follicles continue to shrink until they start producing vellus hair.