Fig. 9. Sequential segregation of lineages from founder cells in the DM sheet. Schematic representation of the stage-dependent generation of restricted progeny from the DM epithelium. DM precursors act as stem cells with self-renewal ability in the epithelium. They express homogeneously both PAX3 and PAX7, divide with an orientation parallel to the apico-basal thickness of the epithelium, and give rise to progeny that remain confined to the DM and contribute to its mediolateral growth as long as the epithelium lasts. The DM epithelium begins its dissociation into the ID, which reveals an aggregated conformation. Most ID precursors either self-renew for one day and/or segregate asymmetrically into ID and PMPs or ID and dermis (intermediate restricted progeny). Finally, a significant proportion of ID cells generate both PMP and dermis at E5, suggesting that these still keep a stem-like character (see text for details).