The hair follicle undergoes lifelong 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 in this phase.

EXOGEN Active process of hair shaft shedding.

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.