In the developing vertebrate CNS, `deep' cells differentiate into neurons whereas undifferentiated superficial epithelial cells continue to proliferate. The rate of neuronal differentiation depends on the balance between these two cell types, which are generated by asymmetric divisions of the superficial cells. Now, Jeremy Green and co-workers reveal that the conserved polarity protein PAR-1 promotes these asymmetric divisions in the neural plate of Xenopus embryos by controlling spindle orientation (see ). PAR-1, which is basolaterally localised in epithelia, is required for the differentiation of deep cells. By grafting marked superficial cells that express activated PAR-1 onto untreated embryos, the researchers show that PAR-1 drives the generation of deep cells from the superficial epithelium. Depletion experiments indicate that PAR-1 is normally required for vertically orientating epithelial mitotic spindles, thereby ensuring a sufficient number of asymmetric cleavages. Importantly, the effect of PAR-1 on spindle orientation not only generates deep cells, note the researchers, but also promotes neurogenesis by partitioning these cells away from anti-neurogenic, apically localised atypical protein kinase C.
