Coexistence of Widespread Clones and Large Radial Clones in Early Embryonic Ferret Cortex

Cell lineage analysis in rodents has shown that the cerebral cortex is formed from both widespread and large radial clustered clones representing partly distinct lineages and producing differing cell types. Since previous cell lineage analysis of the ferret cortex using retroviral libraries showed that most neurons labeled at E33-E35 formed widespread clones, we determined whether clones labeled earlier in neurogenesis showed a greater tendency to form coherent radial clones. Clones labeled at E27-E29 occasionally consisted of widespread multineuron clones (13% of PCR-defined clones), but commonly consisted of small clusters of two to four neurons (65%). Moreover, 6/21 hemispheres contained a single, much larger (6-150 cells) radial cluster. Although large clusters were observed in 28% of experiments, they contained many neurons, accounting for 38% of retrovirally labeled cells. The large clusters showed at most few widely scattered sibling cells, either by histological analysis or by PCR analysis, suggesting that radial and widespread clones coexist but are lineally separate at early stages of corticogenesis. Coexistence of large radial and widespread neuronal clones appears to be an evolutionarily conserved mechanism for cortical neurogenesis.