STRUCTURE OF CLONAL AND POLYCLONAL CELL ARRAYS IN CHIMERIC MOUSE RETINA

One of the most striking results of recent cell-lineage studies of vertebrate retina is the marked variability in the size and types of clones marked by retroviral transfection and dye injection of embryonic progenitor cells. Is this variability due to microenvironmental modulation of cell determination, to lineage restriction, or to experimental perturbation of the progenitor cells? We have taken advantage of species-specific DNA probes to mark groups of lineage-related cells in experimental mouse chimeras. This method of marking cells has two distinct advantages over previous methods: direct manipulation of progenitor cells is avoided, and clones are established at an earlier stage of retinal development. The most notable feature of retinal cohorts in chimeras is their structural uniformity-each is a solid radial array that contains the same ratio of major cell types as the retina itself. This is true even of the smallest monoclonal cohorts, which contain fewer than 200 cells. Our result provides compelling empirical support for the hypothesis that the murine retina is made up of hundreds of relatively homogeneous radial units, each derived from single retinal precursor cells. This finding is inconsistent with micro-environmental modulation of clone structure early in development. We raise the possibility that the heterogeneity among clones marked by dye injection and transfection is due to progressive lineage restriction or to experimental perturbation of the retinal progenitor cells.