Wnt1 expression accelerates differentiation of organotypic cultures. NIK cells were transferred to culture conditions that promote epidermal stratification and development. Samples were taken at the timepoints indicated (12, 16 and 30 days), embedded, sectioned and stained with H&E (A) to reveal their z-axis morphology. The arrows indicate the pattern of growth typical of these cultures, relying on progenitor cells to feed the basal transit amplifying population that differentiate upwards into the stratified layers. Similar samples were processed for filaggrin-staining (B), and cell outlines were visualized with an antibody to E-cadherin.