THF BIOLO (A OFHAIR C. RO\\TH is so remarkablx complex that it is truIx extraordinarx so little goes xw rong wxith the hair follicle. Numer-icallx. the opportunity for disorder is immense. for on the scalp alone there are 100.000 to 130.000 individual follicles. w hile oxver the remaindcer of the bodv there max be several times that number. In humans each follicle undergoes an asxynchronous and variable grow-th cycle xhich is geneticallx programmed to groxw scalp hair to about 1 meter. wxhile keeping facial. body. axillary. and pubic hair appropriatelx stylizedand at a conxenientlx manageable length. Similar to the epithelium of the small intestine and the myeloproliferatixe cells of the bone marroxw. groxwing (anagen hair matrix is one of the most rapidly proliferating tissues in the body. The single pool of stem cells xwhich forms the matrix of the hair bulb. in prodigy of controlled differentiation. produces an elaboratelx structured and orderlx concentric arrax of six morphologically different cell lavers: the medulla. cortex. and cuticle of the hair as w-ell as the three laxers of the inner sheath.\Ioreoxer. in pigmented hairs. the bulbar melanocvtes are integrated wx ith only two of the six proliferating cell lines-supplxving pigment to the cells of the cortex and medulla but not to the cells of the hair cuticle or the inner sheath.'As the follicle cycles into a resting phase telogen). these melanocxytes reduce their clendritic proc-esses and apparently remain in a dormant stateuintil called upon to once again become functional. w-ith the onset of nexx growxth. 2 In the upper part of the follicle the external root sheath is continuous xxith the epidermal component of the follicle. the infundibulum. wxhich extends from the lexel of the epidermis to the opening of the sebaceous duct. Within this area the epithelial cell of the hair sheath is indistinguishable from the epidermal keratinocxvte. It is this infundibular zone wxhich is subject to manx of the same enxironmental influences as is the epidermis and xwhich presum-ably gixes rise to some of the basal cell carcinomas. keratoacanthomas. and infundibular Kepidermoid) cvsts. wxhich w-ill not be considered here. Each renexwal or cxcle of hair growxth requires the inductixe effect of a dermal papilla. At the onset of each nexx cxycle there is a recondensation of