Long-term live imaging reveals sexual dimorphism in the stem cell hierarchy of mouse epidermis

The epidermis is a stratified epithelium comprising proliferative basal cells and differentiated suprabasal cells that are continuously renewed throughout life. Conflicting theories have been proposed regarding the heterogeneity of basal cell layer, with some studies proposing that all proliferative cells have self-renewal potential during homeostasis while others posit a model in which stem cell differentiation occurs through an intermediate progenitor state with strictly limited proliferative potential. To elucidate the heterogeneity and pattern of cell fate, we performed intravital timelapse imaging to track the fate of individual lineage-labeled cells over the long term in the ear skin interfollicular epidermis. Quantitative analysis of clonal dynamics supports a two-compartment model of epidermal homeostasis, with basal cells organized in a stem-progenitor cell hierarchy. Critical to these conclusions was the discovery of a sexual dimorphism in epidermal fate, with female mice showing a highly synchronized pattern of differentiation and delamination. Single-cell RNA sequencing, transcriptional profiling, and clonal analysis following castration revealed that male sex hormones promote SC differentiation and delamination. As well as providing insight into the functional identity of the SC population during homeostasis, our study highlights sex-specific differences in the regulation of epidermal stem and progenitor cell fate.