Organotypic cocultures as models to study cell-cell and cell-matrix interactions of human hair follicle cells.

In the hair follicle complex interactions of specialized epithelial and mesenchymal cells as well as extracellular components are crucial for regulation of proliferation and differentiation. In order to mimic this situation in vitro, techniques to cultivate follicular cells such as outer root sheath (ORS) cells, hair matrix cells (HMC) and hair papilla cells (HPC) were developed. Human dermal fibroblasts (HDF) and HPC markedly enhanced proliferation of ORS cells when cocultured spatially separated in a two-chamber system, which was more pronounced with postmitotic than with mitotic mesenchymal cells. In organotypic cocultures of ORS cells on HDF- or HPC-populated collagen gels lifted at the air-medium interface, stratified epithelia developed largely reminiscent of the epidermis. The morphology was well structured and differentiation markers were expressed (e.g. suprabasal keratins, filaggrin, involucrin, basement membrane components). Both proliferation and differentiation were dependent on the presence of HDF or HPC. When grown embedded in extracellular matrix (Matrigel) without HDF, ORS cells formed large spheroids with inward directed differentiation. Also herein HDF or HPC sustained both cell growth and balanced, epidermistype differentiation. While in organotypic cocultures with HDF, HMC also organized into stratified epithelia, epidermis-type stratification was prevented by HPC. Similarly, in the presence of HDF, HMC in Matrigel formed keratinizing spheroids, whereas this was largely suppressed in the presence of HPC. However, hair-type differentiation was not observed, suggesting a crucial role of other yet unknown components or of the surrounding matrix.