Isolation, characterization, and culture of epithelial stem cells.

It is well accepted that homeostasis of continuously renewing adult tissues, such as the epidermis, is maintained by somatic stem cells. These are undifferentiated, self-renewing cells, which also produce daughter transit amplifying (TA) cells that make up the majority of the proliferative cell population in the tissues. Although still proliferative in nature, it is thought that TA cells can undergo only a finite number of cell divisions before they commit to leave the proliferative compartment and move toward terminal differentiation. Stem cells, on the other hand, have been assumed to persist throughout the lifetime of the organism. We directly demonstrated the presence of putative stem cells in the proliferative compartment of murine epithelia in 1981 when we identified a small population of label-retaining cells (LRCs) in mouse stratified squamous epithelia. Since then, we have developed the method described here to isolate this population of epidermal stem cells (EpiSC). We showed that EpiSC are all keratin 14+ and thus of keratinocyte origin and not of mesenchymal or hematopoietic origin. We have also demonstrated that EpiSC can regenerate the epidermis, that they can permanently express a recombinant gene in the regenerated tissue, and that while the majority of EpiSC reside in the G1 phase of the cell cycle, they are not held out of the cell cycle, that they express proliferating genes and the mitotic cyclin B1 protein. Recently, we have shown that EpiSC have the capacity to alter their cell fate in vivo if placed into stress environments, i.e. after irradiation or wounding or when injected into a developing blastocyst environment. Thus being able to isolate EpiSC is critical for testing their use in cell and gene therapy.