The formation of complex stratified epithelial barriers in mammals is essential for animal survival. During mouse embryonic development the skin epithelium is generated from a simple single-layered epithelium which stratifies and differentiates to form a multi-layered epidermis just before birth. But the mechanisms underlying epidermal stratification and how cells switch their cell fate from basal to suprabasal cells are still unknown. Recently, we proposed a two-phase model of epidermal stratification, with the first major phase starting around embryonic day E13.5 when the epithelium gets multi-layered. Our current data showed that while the tissue growth of the embryo is linear, the epidermal progenitors go through a burst in proliferation around E13.5 compared to earlier and later time points. The data suggest that the sudden increase in cell number pushes the cells to go suprabasally and form new layers. To study the switch of cell fate from the basal to the suprabasal layers, we generated a reporter mouse for the suprabasal marker keratin 10 (Krt10) fused to the bright fluorescent protein mNeonGreen. Using this reporter mouse for time-lapse imaging of the developing skin epidermis enables us to follow single cells during their commitment to a suprabasal fate. To identify the signalling pathways that govern epidermal stratification, we used single-cell RNA sequencing (scRNA-Seq) of the developing skin every day from E12.5 until E16.5. The data suggest the involvement of the Notch signalling pathway during the second phase of epidermal stratification. Consistent with the scRNA-Seq data, the Notch downstream target HES1 was highly expressed in the nuclei of suprabasal cells and ~2% of basal progenitors at E15.5. Collectively, we propose a working model where the HES1-expressing basal cells delaminate and migrate suprabasally to maintain epidermal homeostasis.

ESDR098 - A burst of progenitor proliferation drives epidermal stratification

50th Annual Meeting of the European Society for Dermatological Research. VIRTUAL MEETING