During vertebrate spinal cord development, specific neuronal subtypes develop from distinct domains of neural progenitor cells. The p2 progenitor domain in the zebrafish ventral spinal cord, for example, produces V2a and V2b interneurons. But are these V2 neurons generated by cell divisions that produce progenitor cells and neurons or by asymmetric cell divisions of`pair-producing' progenitors? On , Kimura and co-workers report that these interneurons form by the latter process via Delta-Notch signalling between sister neurons. Using time-lapse microscopy of a transgenic zebrafish line in which GFP expression begins in the p2 progenitor cells just before their final cell divisions and using neuron-specific markers, the researchers show that virtually all the GFP-labelled progenitors divide once to produce V2a/V2b neuron pairs. However,both paired cells adopt the V2b fate following forced activation of Notch signalling. This mechanism for cell fate determination resembles that seen during Drosophila neurogenesis but, the researchers note, unlike in Drosophila, the orientation of the division axis in the final asymmetric division in vertebrate neurogenesis is not fixed.
