In the mammalian cortex, cell-fate choices are determined by the orientation of the mitotic spindle. Previous studies have shown that when the mitotic spindle of a cell orientates parallel to the apical surface of the neural tube, two progenitor cells are generated, but when it orientates perpendicularly, a neuron and a progenitor are produced in a stem cell mode of division. Whether these findings can be extrapolated to cell divisions in the rest of the nervous system, particularly in the spinal cord, is unknown. Using a novel, time-lapse imaging assay, Kate Storey and co-workers now reveal() that in the chick spinal cord the mitotic spindle orientation does not correlate with a switch from progenitor-only to neuron plus progenitor-generating divisions. However, it does distinguish stem cell modes from terminal modes of division that produce only neurons. The birth of neurons from stem cell divisions is captured for the first time and, surprisingly, the relationship between spindle orientation and cellular identity appears different in the spinal cord as compared with the cortex.
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