146 A two-phase model of epidermal stratification: Lessons from centrosomes

Abstract

The stratified skin epidermal barrier in mammals develops from a single-layered epithelium. The mechanisms initiating epidermal stratification and how the process is fueled are still not well-understood. Previous studies proposed a central role for perpendicular/asymmetric cell division orientation of the basal keratinocyte progenitors in epidermal stratification. Here, we used centrosomes, that organize the mitotic spindle, to test whether cell division orientation and stratification are linked. Genetic ablation of centrosomes from the developing epidermis led to the activation of the p53-dependent mitotic surveillance pathway resulting in reduced epidermal thickness and hair follicle arrest. Double mutant keratinocyte progenitors lacking both centrosomes and p53 rescued the epidermal phenotypes. Importantly, the mutant progenitors significantly altered their division orientation in the later developmental stages without affecting stratification or differentiation. Based on time-lapse imaging and analysis of tissue growth dynamics, our data suggested a two-phase model of epidermal stratification. The first and major phase of epidermal stratification is characterized by highly proliferating basal and suprabasal transit-amplifying cells as well as cell delamination, while the second phase may be uncoupled from the division orientation of the basal progenitors. The data provide insights for tissue homeostasis and hyperproliferative diseases that may recapitulate developmental programs.