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Disorders of Hair Follicles and Related Disorders: Introduction

  • 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.

Biology of Hair Growth Cycles

Glossary of Terms

Hair Follicle Cycle

The hair follicle undergoes life-long cyclic transformations into three primary phases: anagen, catagen, and telogen (Fig. 31-1).

Figure 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.)

Anagen

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.

Figure 31-2.

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.]

Telogen

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.

Catagen

Apoptosis-driven phase between telogen and anagen phase. Duration: few weeks. Only about 1% of hairs are seen in this phase.

Exogen

Active process of hair shaft shedding.

Types of Hair

Lanugo Hair

Soft fine pigmented hair that covers much of fetus; usually shed before birth.

Vellus Hair

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.

Terminal Hair

Thick, pigmented hair found on scalp, ...

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