- Hair follicles produce pigmented keratin fibers.
- Human hair has little vestigial function:
- Contributes to a psychological perception of beauty and attractiveness.
- Tactile sensation
- Protects the scalp, face, and neck from UV solar radiation
- Reduces heat loss through the scalp
- Psychology of hair: Alteration of the “normal” quantity of hair is often associated with profound psychological impact. Loss of scalp hair is considered abnormal in many societies, associating balding with old age (pattern hair loss) or impaired health (chemotherapy).
- Excess hair on the face (hirsutism, hypertrichosis) and extremities of women is often considered unattractive.
- Billions of dollars are spent annually in industrialized countries to care for hair and perceived abnormalities.
Life-long cyclic transformations of the hair follicle, which begins in utero (Fig. 32-1).
Hair growth cycle Diagrammatic representation of the changes that occur to the follicle and hair shaft during the hair growth cycle. A. Anagen (growth stage); B. Catagen (degenerative stage); C. Telogen (resting stage). (Courtesy of Lynn M. Klein, MD.)
Growth phase; lasts variable periods of time depending on body site, e.g., scalp, eyebrows. Duration: 1–6 years; average 3 years; varies with age; determines the ultimate length of hair at a site. Scalp, beard: long anagen. Eyebrows, eyelashes, axillary/pubic hair: anagen is short; telogen prolonged. Anagen hair matrix has rapidly proliferating epithelial cells; exquisitely sensitive to drugs, growth factors, hormones, stress; immunologic and physical injury. Destruction of epithelial stem cells results in permanent hair loss.
Hairs with pigmented malleable proximal ends.
Period of relative quiescence, last variable periods of body site.
Club hairs with depigmented rounded proximal ends.
Apoptosis-driven phase between telogen and anagen phase. Duration: few weeks.
Active process of hair shaft shedding.
Process of increased daily hair shaft shedding (normal scalp 25–100 hairs).
Soft fine pigmented hair that covers much of fetus; usually shed before birth. Between vellus and terminal hair in length and form.
Latin for “fleece.” Fine, nonpigmented hair (peach fuzz) that covers the body of children and adults; ...