Lineage tracing in the vertebrate CNS: Neuronal subtype specification and a shared lineage for astrocytes and oligodendrocytes

Retinal progenitor cells have been shown to be multipotent throughout development. However, previous lineage studies did not address whether these multipotent progenitor cells were biased in their production of particular neuronal subtypes. Here, lentivirus-mediated gene transfer was used to mark single retinal progenitor cells and quantified the different subtypes of horizontal cells (HCs) in each clone. Clones with 2 HCs consistently contained a single HC subtype, either a pair of H1 or a pair of H3 cells. This suggests that a multipotent progenitor cell produces a mitotic cell fated to make a terminal division that produces two HCs of only one subtype. This bias in production of one HC subtype suggests a novel mechanism of cell fate determination in at least a subset of retinal cells that involves decisions made by mitotic cells that are inherited in a symmetric manner by both neuronal daughter cells. Furthermore, developing neural tissue undergoes a period of neurogenesis followed by a period of gliogenesis. The lineage relationships among glial cell types haven't been defined for the CNS. Here we use retroviruses to label clones of glial cells in the chick retina. We found that almost every clone had both astrocytes and oligodendrocytes. In addition, we discovered a novel glial cell type, with features intermediate between those of astrocytes and oligodendrocytes, which we have named the diacyte. Diacytes also share a progenitor cell with astrocytes and oligodendrocytes.