Asymmetric division of epidermal progenitor cells through perpendicular spindle orientation promotes stratification. (A) Epidermis is a multi-layered structure whose inner most layer, the basal layer (BL), contains progenitor cells that can orient their mitotic spindle parallel or perpendicular to the underlying basement membrane (BM) to undergo symmetric (SCD) or asymmetric division (ACD) respectively. Asymmetric division of a basal cell gives rise to a differentiated suprabasal/spinous layer (SL) cell and a proliferative basal cell. Cells of the SL further differentiate and migrate outward to give rise to the granular layer (GL) and the cornified layer (CL). (B) Coupling between the apical Par polarity complex and NuMA in mitosis is proposed to direct apical-basal spindle orientation to drive ACD in the epidermal basal cells.