Hair Growth and Loss

A given hair grows in a hair cycle consisting of three stages—anagen, catagen, and telogen. In the anagen23 stage, stem cells from the bulge in the follicle multiply and travel downward, pushing the dermal papilla deeper into the skin and forming the epithelial root sheath. Root sheath cells directly above the dermal papilla form the hair matrix. Here, sheath cells transform into hair cells, which synthesize keratin and then die as they are pushed upward away from the papilla.

In the catagen24 phase, epithelial root sheath cells below the bulge undergo apoptosis. The follicle shrinks, the dermal papilla is drawn up toward the bulge, and the hair loses its anchorage. When the papilla reaches the bulge, the hair goes into a resting stage called the telogen25 phase. The hair may fall out during catagen or telogen. About 50 to 100 scalp hairs are lost daily. Eventually, anagen begins anew and the cycle repeats itself.

In a young adult, scalp follicles typically spend 6 to 8 years in anagen, 2 to 3 weeks in catagen, and 1 to 3 months in telogen. About 90% of the scalp follicles at any given time are in anagen. Scalp hairs grow at a rate of about 1 mm per 3 days (10-18 cm/yr) during this phase.

Hair grows fastest from adolescence until the 40s. After that, an increasing percentage of follicles are in the catagen and telogen phases rather than the growing anagen phase. Follicles also shrink and begin producing wispy vellus hairs instead of thicker terminal hairs. Thinning of the hair, or baldness, is called alopecia26 (AL-oh-PEE-she-uh). It occurs to some degree in both sexes and may be worsened by disease, poor nutrition, fever, emotional trauma, radiation, or chemotherapy. In the great majority of cases, however, it is simply a matter of aging.

Pattern baldness is the condition in which hair is lost from only some regions of the scalp rather than thinning

25telo = end 26alopecia = fox mange

Saladin: Anatomy & 6. The Integumentary Text © The McGraw-Hill

Physiology: The Unity of System Companies, 2003

Form and Function, Third Edition

204 Part Two Support and Movement uniformly across the entire scalp. It results from a combination of genetic and hormonal influences. The relevant gene has two alleles, one for uniform hair growth and a baldness allele for patchy hair growth. The baldness allele is dominant in males and is expressed only in the presence of the high level of testosterone that is characteristic of men. In men who are either heterozygous or homozy-gous for the baldness allele, testosterone causes the terminal hair of the scalp to be replaced by thinner vellus, beginning on top of the head and later the sides. In women, the baldness allele is recessive. Homozygous dominant and heterozygous women show normal hair distribution; only homozygous recessive women are at risk of pattern baldness. Even then, they exhibit the trait only if their testosterone levels are abnormally high for a woman (for example, because of a tumor of the adrenal gland, a woman's only source of testosterone). Such characteristics in which an allele is dominant in one sex and recessive in the other are called sex-influenced traits.

Excessive or undesirable hairiness in areas that are not usually hairy, especially in women and children, is called hirsutism.27 It tends to run in families and usually results from either masculinizing ovarian tumors or hypersecretion of testosterone by the adrenal cortex. It is often associated with menopause.

Contrary to popular misconceptions, hair and nails do not continue to grow after a person dies, cutting hair does not make it grow faster or thicker, and emotional stress cannot make the hair turn white overnight.

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