MATTHEW ROBERTS LABORATORY

Identification of candidate human epidermal stem cells

A Li, P J Simmons, P Kaur

It is well accepted that cell replacement in the epidermis of human skin, like a number of other continually-renewing tissues such as the bone marrow, is achieved through a hierarchy of proliferative cells. These include transit amplifying (TA) cells with a limited proliferative capacity, and a small number of stem cells (SCs) with greater proliferative potential which are ultimately responsible for the production of all mature keratinocytes. Human epidermal stem cells remain poorly characterised due to the absence of specific markers which distinguish these cells from TA cells.

We have recently demonstrated that human epidermal stem cells can be markedly enriched by fluorescence activated cell sorting (FACS), using antibodies to the a ₆ subunit of the adhesion receptor a _{6b 4} integrin and Mab 10G7, a novel Mab generated in this laboratory, which detects an as yet unidentified cell surface component with a characteristic oncofoetal pattern of expression. Human epidermal cells freshly isolated from neonatal foreskins were fractionated by means of two-colour FACS and the long-term proliferative capacity of various fractions determined by assaying total cell output at clonal density over serial passages until all growth potential was exhausted. We have shown that keratinocyte stem cells are restricted to a minor subpopulation of basal epidermal cells (~10 per cent) characterised by high levels of a ₆ expression and low levels of 10G7 antigen expression (a ₆^bri/10G7^dim). This is based on our observations that cells within this fraction had the greatest proliferative potential of all basal epidermal cells in culture over an extended period of 90 days on average. Further, a single cell within this fraction could give rise to about 5.8 x 10⁸ cells. We have also been able to establish that the basal epidermal cells of the human neonatal foreskin exhibit heterogeneity in terms of their proliferative capacity and can be placed into a hierarchy of proliferative cells on the basis of a ₆ and 10G7 expression . The ability of the candidate stem cell population to reconstitute epidermal tissue in vivo in xenogeneic grafts remains to be tested. We believe this study to be of central importance in skin biology and a major breakthrough in the future characterisation of human epidermal stem cells, given that this population is responsible for maintaining production of all keratinocytes and is also the target for transformation by a variety of carcinogens resulting in the development of malignant carcinomas. Our work will provide a basis for the identification of genes with a critical role in epidermal growth and differentiation. Further, this work represents an essential prerequisite to the development of cellular therapeutic approaches which use keratinocyte stem cells as vehicles for gene therapies.