Control of planar divisions by the G-protein regulator LGN maintains progenitors in the chick neuroepithelium.

The spatio-temporal regulation of symmetrical as opposed to asymmetric cell divisions directs the fate and location of cells in the developing CNS. In invertebrates, G-protein regulators control spindle orientation in asymmetric divisions, which generate progeny with different identities. We investigated the role of the G-protein regulator LGN (also called Gpsm2) in spindle orientation and cell-fate determination in the spinal cord neuroepithelium of the developing chick embryo. We show that LGN is located at the cell cortex and spindle poles of neural progenitors, and that it regulates spindle movements and orientation. LGN promotes planar divisions in the early spinal cord. Interfering with LGN function randomizes the plane of division. Notably, this does not affect cell fate, but frequently leads one daughter of proliferative symmetric divisions to exit the neuroepithelium prematurely and to proliferate aberrantly in the mantle zone. Hence, tight control of planar spindle orientation maintains neural progenitors in the neuroepithelium, and regulates the proper development of the nervous system.