Cell subpopulations within proliferative and differentiating compartments of epidermis.

The heterogeneous population of newborn rat keratinocytes was separated into different subgroups according to their cell size. The relation between cell size, position in the cell cycle, RNA content, and proliferative potential in culture was examined. A reserve stem cell population of Go/G1 cells, low in RNA, giving rise to colonies of undifferentiated phenotype in cell culture, has been separated from more differentiated transit basal cells. In the fractions of the larger cells, several subgroups, probably corresponding to different stages of differentiation, were identified: G2M cells with low RNA content, large S-phase cells rich in RNA, and small Go/G1 cells low in RNA. The clonogenic cells from these fractions have limited growth potential and give rise to moderately or terminally differentiated colonies. The selective sorting of stem cell populations may be useful for elucidating the mechanism of carcinogenesis in epidermis and other proliferative tissues. Analysis of the relative proportions of cell subpopulations represents a novel approach leading to the refinement of the concepts of epidermal structure in physiological and pathological states. It also could, by extension, shed new light on the behavior of other proliferative tissues.