Disorders of Hair Follicles and Related Disorders: Introduction
Human hair has little vestigial function:
Contributes to a psychological perception of beauty and attractiveness.
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.
Biology of Hair Growth Cycles
The hair follicle undergoes life-long cyclic transformations into three primary phases: anagen, catagen, and telogen (Fig. 31-1).
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 and age; determines the ultimate length of hair at a site. Anagen hair matrix has rapidly proliferating epithelial cells and is exquisitely sensitive to drugs, growth factors, hormones, stress, and immunologic and physical injury. Destruction of epithelial stem cells results in permanent hair loss. Anagen hairs have pigmented malleable proximal ends (Fig. 31-2A). About 85–99% of hairs will be in this phase, with some individual variation.
Hair mount (A) Anagen: note the malleable proximal ends and (B) Telogen: club hairs. [From Goldsmith LA et al. (eds.). Fitzpatrick’s Dermatology in General Medicine, 8th edition. New York: McGraw-Hill, 2012.]
Period of relative quiescence, prior to shedding. Telogen hairs are club hairs with depigmented rounded proximal ends (Fig. 31-2B). About 1–15% of hairs are in this phase at any given time.
Apoptosis-driven phase between telogen and anagen phase. Duration: few weeks. Only about 1% of hairs are seen in this phase.
Active process of hair shaft shedding.
Soft fine pigmented hair that covers much of fetus; usually shed before birth.
Fine, nonpigmented hair; growth not affected by hormones. Genetically determined to produce very small (but functionally fully active cycling) hair follicles located in the dermis.
Thick, pigmented hair found on scalp, ...