SECTION 31: DISORDERS OF HAIR FOLLICLES AND RELATED DISORDERS

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).

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.

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.

CATAGEN Apoptosis-driven phase between telogen and anagen phase. 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.

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

LABORATORY EXAMINATIONS

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

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.

SCALP BIOPSY Offers insight into pathogenesis of alopecia.

HAIR LOSS: ALOPECIA ICD-10: L63-L66

• 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 ...