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BIOLOGY OF HAIR GROWTH CYCLES

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GLOSSARY OF TERMS

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

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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). (Used with permission from Lynn M. Klein, MD.)

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ANAGEN Growth phase; 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 to 99% of hairs will be in this phase, with some individual variation.

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Figure 31-2

Hair mount (A) Anagen: Note the malleable proximal ends and (B) Telogen: club hairs. (Reproduced with permission from Goldsmith LA, Katz SI, Gilchrest BA, et al, eds. Fitzpatrick's Dermatology in General Medicine. 8th ed. New York, NY: McGraw-Hill; 2012.)

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TELOGEN Period of relative quiescence, prior to shedding. Telogen hairs are club hairs with depigmented rounded proximal ends (Fig. 31-2B). About 1 to 15% of hairs are in this phase at any given time.

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CATAGEN Apoptosis-driven phase between telogen and anagen phase. Only about 1% of hairs are seen in this phase.

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EXOGEN Active process of hair shaft shedding.

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Types of Hair
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LANUGO HAIR Soft fine pigmented hair that covers much of fetus; usually shed before birth.

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VELLUS HAIR Fine, nonpigmented hair; growth not affected by hormones.

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TERMINAL HAIR Thick, pigmented hair found on scalp, eyebrow/eyelashes, beard, axillae, pubic area; growth is influenced by hormones.

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LABORATORY EXAMINATIONS

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HAIR PULL Scalp is gently pulled. Normally, three to five hairs are dislodged; shedding more hair suggests pathology.

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TRICHOGRAM Determines the number of anagen and telogen hairs and is made by epilating (plucking) 50 hairs or more and counting the number of anagen and telogen hairs.

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SCALP BIOPSY Offers insight into pathogenesis of alopecia.

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HAIR LOSS: ALOPECIA ICD-10: L63-L66

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  • Shedding of hair is termed effluvium or defluvium, and the resulting condition is called alopecia (Greek alópekia, "baldness").

  • Alopecia classified into:

  • Noncicatricial alopecia: No clinical sign of tissue inflammation, scarring, or atrophy of skin.

  • Cicatricial alopecia: Evidence of tissue destruction such as inflammation, atrophy, and scarring may be apparent.

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