Retroviral transduction of murine epidermal stem cells demonstrates clonal units of epidermal structure

It has been suggested that the number and position of epidermal stem cells are related to the units of columnar structure in the upper epidermal strata and that the cells of each unit are derived from a single stem cell. Studies of cell lineage in developing tissues have been facilitated by the use of retroviral transduction to provide inherited expression of a histochemically demonstrable foreign gene product. To provide direct evidence about the clonal nature of epidermal units, murine epidermal keratinocytes were transduced with a replication-deficient retroviral vector carrying the beta-galactosidase gene. Subepidermal injection of virus in vivo led to infrequent transduction with only transient presence of beta-galstaining keratinocytes within the epidermis. Transduction of keratinocytes in vitro and transplantation back to in vivo sites permitted demonstration of the transduced gene in clusters of cells within the reformed epidermis throughout a 12-wk period. The epidermis redeveloped an ordered columnar structure with restriction of transduced cells to individual columnar units. This clonal appearance is compatible with derivation of each epidermal unit from a single stem cell but is not compatible with a random pattern of cell proliferation. Transduced epidermal sheets that were recombined with oral mucosal connective tissue also redeveloped normal columnar structure with restriction of beta-gal staining to individual columnar units, These data suggest that the establishment of an epidermal stem cell pattern related to units of structure is an intrinsic property of the epithelium and is not dependent on regionally-specific connective tissue influences.