Clonal analysis reveals laminar fate multipotency and daughter cell apoptosis of mouse cortical intermediate progenitors

In developing cerebral cortex, most pyramidal-projection neurons are produced by intermediate progenitors (IPs), derived in turn from radial glial progenitors. Although IPs produce neurons for all cortical layers, it is unknown whether individual IPs produce multiple or single laminar fates, and the potential of IPs for extended proliferation remains uncertain. Previously, we found that, at the population level, early IPs (present during lower-layer neurogenesis) produce lower- and upper-layer neurons, whereas late IPs produce upper-layer neurons only. Here, we employed mosaic analysis with double markers (MADM) in mice to sparsely label early IP clones. Most early IPs produced 1-2 neurons for deep layers only. Less frequently, early IPs produced larger clones (up to 12 neurons) spanning lower and upper layers, or upper layers only. The majority of IP-derived clones (similar to 66%) were associated with asymmetric cell death after the first division. These data demonstrate that laminar fate is not predetermined, at least in some IPs. Rather, the heterogeneous sizes and laminar fates of early IP clones are correlated with cell division/death/differentiation choices and neuron birthdays, respectively.