Keratinocyte proximity and contact can play a significant role in determining mesenchymal stem cell fate in human tissue.

Bone marrow-derived human mesenchymal stem cells (hMSCs) possess multipotent differentiation capabilities and are a potent source of paracrine factors. We show how the epidermal keratinocyte can direct hMSC differentiation selectively. Keratinocytes and hMSCs were either cocultured in physical contact (contact cocultures), or separated without physical contact using a transwell insert (noncontact cocultures). We also delivered hMSCs into an ex vivo human excisional wound where subpopulations of the hMSCs were either in contact or were physically separated from the epidermal keratinocytes. In comparison to control hMSCs that were not cocultured, contact cocultured hMSCs adopted an epithelial morphology and expressed keratinocyte markers while noncontact cocultured hMSCs, surprisingly, adopted phenotypes that resembled myofibroblast and early neural lineage, both of which are of dermal origin. Cell fusion was not a requirement in in vitro contact cocultures, as determined by fluorescence-activated cell sorting (FACS) and fluorescence in situ hybridization analysis (FISH). To the best of our knowledge, this work provides the first example of hMSC differentiation into different lineages depending on their proximity to a single cell type.