Telencephalic neural progenitors appear to be restricted to regional and glial fates before the onset of neurogenesis

The contribution of early cell lineage to regional fate in the mammalian fo rebrain remains poorly understood. Previous lineage-tracing studies using r etroviral methods were only begun at mid-neurogenesis and have suffered fro m region-specific retroviral silencing. We have been able to study cell lin eage in the telencephalon from the onset of neurogenesis by using ultrasoun d backscatter microscopy to label the forebrain neuroepithelium and a modif ied retroviral lineage library to overcome regional silencing. Our studies suggest that by embryonic day 9.5, forebrain clones are primarily restricte d to territories within anatomically demarcated regional boundaries, such a s the cortex, striatum and hypothalamus. In addition, we observed a subset of clones that appeared to be composed entirely of glia. These observations suggest that both regional and cell-type restrictions exist within progeni tor populations before the first forebrain cells become postmitotic.