Interfollicular epidermal differentiation is gradualistic rather than stepwise with GRHL3 controlling progression from stem to transition cell states

Although interfollicular epidermal (IFE) differentiation is thought to be stepwise as reflected in sharp boundaries between the basal, spinous, granular and cornified layers, this prediction has not been studied at a single cell resolution. We used single cell RNA-seq to show that IFE differentiation is best described as a single step gradualistic process with a large number of transition cells between the basal and spinous layer. RNA-velocity analysis identifies a commitment point that separates the plastic basal and transition cell state from the unidirectionally differentiating cells. We also show that GRHL3, best known for promoting IFE terminal differentiation, has a major function in suppressing epidermal stem cell expansion and the emergence of an abnormal stem cell state by suppressing Wnt signaling in stem cells.