Culture of hair matrix and follicular keratinocytes.

In recent years, a variety of in vitro models for the cultivation of hair follicles and their constituents have been developed. Outer root sheath (ORS) keratinocytes (KC) have been mainly studied in explant cultures, planted on bovine eye lens capsules, collagen substrata, 3T3 cell feeder layers, or dermal equivalents, yielding outgrowth of a multilayered stratified epithelium with some biochemical and ultrastructural characteristics of keratinocytic differentiation. More recently, ORS KC cultures have also been initiated from single cell suspensions, and organotypic cultures have been obtained by recombination with dermal cells, inducing a higher degree of epidermal differentiation. Presumptive human hair matrix cells have been isolated from plucked anagen hair follicles and have been successfully propagated on 3T3 cell or normal human fibroblast feeder layers, giving rise to multilayered stratified KC cultures. In contrast, only preliminary data exist concerning the cultivation of bulge cells that have been suggested to represent follicular stem cells. In conclusion we dispose of several in vitro models today to cultivate ORS KC and hair matrix cells that have increased our knowledge on the regulation of the human hair cycle by soluble factors and dermal-epidermal interactions. Further comparative studies on ORS KC, bulge cells and matrix cells have to be carried out to confirm the distinct character of these hair KC subsets.