SummaryNeurons in the mammalian neocortex arise from asymmetric divisions of progenitors residing in the ventricular zone. While in most progenitor divisions, the mitotic spindle is parallel to the ventricular surface, some progenitors reorient the spindle and divide in oblique orientations. Here, we use conditional deletion and overexpression of mouse Inscuteable (mInsc) to analyze the relevance of spindle reorientation in cortical progenitors. Mutating mInsc almost abolishes oblique and vertical mitotic spindles, while mInsc overexpression has the opposite effect. Our data suggest that oblique divisions are essential for generating the correct numbers of neurons in all cortical layers. Using clonal analysis, we demonstrate that spindle orientation affects the rate of indirect neurogenesis, a process where progenitors give rise to basal progenitors, which in turn divide symmetrically into two differentiating neurons. Our results indicate that the orientation of progenitor cell divisions is important for correct lineage specification in the developing mammalian brain.

During mammalian neurogenesis, progenitor cells can divide with the mitotic spindle oriented parallel or perpendicular to the surface of the neuroepithelium. Perpendicular divisions are more likely to be asymmetric and generate one progenitor and one neuronal precursor. Whether the orientation of the mitotic spindle actually determines their asymmetric outcome is unclear. Here, we characterize a mammalian homolog of Inscuteable (mInsc), a key regulator of spindle orientation in Drosophila. mInsc is expressed temporally and spatially in a manner that suggests a role in orienting the mitotic spindle in the developing nervous system. Using retroviral RNAi in rat retinal explants, we show that downregulation of mInsc inhibits vertical divisions. This results in enhanced proliferation, consistent with a higher frequency of symmetric divisions generating two proliferating cells. Our results suggest that the orientation of neural progenitor divisions is important for cell fate specification in the retina and determines their symmetric or asymmetric outcome.

The developmental potential of hematopoietic progenitors is restricted early on to either the erythromyeloid or lymphoid lineages. The broad developmental potential of Pax5−/− pro‐B cells is in apparent conflict with such a strict separation, although these progenitors realize the myeloid and erythroid potential with lower efficiency compared to the lymphoid cell fates. Here we demonstrate that ectopic expression of the transcription factors C/EBPα, GATA1, GATA2 and GATA3 strongly promoted in vitro macrophage differentiation and myeloid colony formation of Pax5−/− pro‐B cells. GATA2 and GATA3 expression also resulted in efficient engraftment and myeloid development of Pax5−/− pro‐B cells in vivo. The myeloid transdifferentiation of Pax5−/− pro‐B cells was accompanied by the rapid activation of myeloid genes and concomitant repression of B‐lymphoid genes by C/EBPα and GATA factors. These data identify the Pax5−/− pro‐B cells as lymphoid progenitors with a latent myeloid potential that can be efficiently activated by myeloid transcription factors. The same regulators were unable to induce a myeloid lineage switch in Pax5+/+ pro‐B cells, indicating that Pax5 dominates over myeloid transcription factors in B‐lymphocytes.