Keratinocyte stem cells: a commentary

For many years it has been widely accepted that stem cells play a crucial role in adult tissue maintenance. The concept that the renewing tissues of the body contain a small subcompartment of self-maintaining stem cells, upon which the entire tissue is dependent, is also now accepted as applicable to all renewing tissues. Gene therapy and tissue engineering are driving considerable interest in the clinical application of such hierarchically organized cellular compartments. Recent initial observations have provided a tantalizing insight into the large pluripotency of these cells. Indeed, scientists are now beginning to talk about the possible totipotency of some adult tissue stem cells. Such work is currently phenomenologic, but analysis of data derived from genomics and proteomics, identifying the crucial control signals involved, will soon provide a further impetus to stem cell biology with far reaching applications. The epidermis with its relatively simple structure, ease of accessibility, and the ability to grow its cells in vitro is one obvious target tissue for testing stem cell manipulation theories. It is crucial, however, that the normal keratinocyte stem cell is thoroughly characterized prior to attempting to manipulate its pluripotency. This commentary assesses the data generated to date and critically discusses the conclusions that have been drawn. Our current level of understanding, or lack of understanding, of the keratinocyte stem cell is reviewed.