Classic experiments in invertebrates suggest that stereotypic patterns of cell division generate specific cell types during development, but the extent to which stereotypic lineages play a part in the developing vertebrate CNS is an open question. Now, Michel Cayouette and co-workers report that stochasticity plays a major role in cell fate decisions in the developing rat retina (see p. 227). In vivo cell-lineage tracing studies show that vertebrate retinal progenitor cells (RPCs) yield retinal clones of varying size and cellular composition. Whether this variability reflects distinct but reproducible lineages among many different RPCs or stochastic fate decisions within a population of more equivalent RPCs is unclear. To find out, the researchers use videomicroscopy to follow the lineages of rat RPCs cultured at clonal density. Their analysis of the reconstructed lineages indicates that fixed probabilities determine the decision of the RPCs to multiply or differentiate. Thus, stochasticity plays a major part in the development of the retina and possibly also of other parts of the vertebrate CNS.

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