Modulation of the differentiated phenotype of keratinocytes of the hair follicle and from epidermis.

The differentiation capacity and its modulation by cell-cell and cell-matrix interactions of epithelial cells from epidermis (NEK), hair follicle outer root sheath (ORS) and hair matrix cells (HMC) in different experimental model systems is reviewed. Reformation of structurally intact and functioning hair follicles has been achieved with isolated neonatal mouse cells in vivo when follicular epithelia and dermal fibroblasts were transplanted. This structural reorganization has not yet been feasible in vitro in either surface or matrix embedded organotypic cocultures with mesenchymal cells. While the epithelial cells isolated from epidermis or hair follicle compartments formed similar stratified and keratinizing epithelia in vitro, their degree of differentiation was significantly different, declining from NEK over ORS to HMC. Differentiation was further reduced in HMC cultures in the presence of dermal papilla cells (DPC). Differentiation was analyzed by morphologic criteria and the biochemical analysis as well as immunohistochemical localization of differentiation products such as keratins, involucrin, filaggrin, integrins, basement membrane components and membrane antigens. The results demonstrate fully maintained differentiation capacity of skin and appendage-keratinocytes to reconstitute a squamous epithelium, reflecting their common origin from embryonic epidermis. For the induction of hair follicle-specific structural and functional characteristics, different and probably more complex interactive mechanisms may be required.