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HairComments

Hair

The presence of hair is a distinguishing feature of mammals. Men and women have the same density of hair, but testosterone makes the hair more apparent on men. The primary functions of hair are protection and attraction.

Each hair consists of a diagonally positioned shaft, hair and bulb. The shaft is the visible, but dead, portion of the hair projecting above the surface of the skin. The bulb is the enlarged base of the root within the hair follicle. Each hair develops from stratum basale cells within the bulb of the hair, where nutrients are received from dermal blood vessels. As the cells divide, they are pushed away from the nutrient supply toward the surface, and cellular death and keratinization occur.

In a healthy person, hair grows at the rate of approximatel 1 mm every 3 days. As the hair becomes longer, however, ti enters a resting period where there is minimal growth. The life span of a hair varies from 3 to 4 months for an eyelash to 3 to 4 years for a scalp hair. Each hair lost is replaced by a new hair that grows from the base of the follicle and pushes the old hair out. Between 10 and 100 pairs are lost daily.

3 layers can be observed in hair that is cut in cross section. The inner medula is composed of loosely arranged cells separated by numerous air cells. The thick cortex surrounding the medula consists of hardened, tightly packed cells. A cuticle covers the cortex and forms the toughened outer layerof the hair. Cells of the cuticle have serrated edges that give it a scaly appearance under the microscope.

Hair color is determine by type and amount of pigment produced in stratum basale at base of hair follicle. Varying amounts of melanin produce from blond to brunette to black. More melanin, darker. Trichosiderin, a pigment with an iron base, produces red hair. Gray or white hair is lack of pgiment and air spaces within layers of shaft of hair. Texture of hair is based on cross-sectional shape: straight hour round, wavy hair oval, kinky hair flat.

Sebaceous glands and arrectores pilorum are attached to hair folicle. Arrectores pilorum muscles involuntary, responded to thermal or pyschological stiumli. When contract, hair pulld into more vertical position to make goose bumps. Three kinds of hair:

Lanugo: fine, silky fetal hair appears during last trimester of development seen only on premature infants
Vellus: shrort, fine hair replacing lanugo. Abundant in children and women just barely extended from the hair follicules.
Terminal hair: coarse, pigment (except in elderly people) and sometimes curly. It includes scalp, pubic, eyelash hair.
Angora hair is terminal hair growing continually as in scalps and faces of mature males.
Definitive hair grows to a certian lengths and stops, including eyelashes.