Late retinal progenitor cells show intrinsic limitations in the productionof cell types and the kinetics of opsin synthesis

The seven major cell classes of the vertebrate neural retina arise from a p ool of multipotent progenitor cells. Several studies suggest a model of ret inal development in which both the environment and the progenitor cells the mselves change over time (Cepko et al., 1996). To test this model, we used a reaggregate culture system in which a labeled population of progenitor ce lls from the postnatal rat retina were cultured with an excess of embryonic retinal cells. The labeled cells were then assayed for their cell fate cho ices and their kinetics of rod differentiation, as measured by opsin synthe sis. The kinetics of opsin synthesis remained unchanged, but fewer postnata l cells adopted the rod cell fate when cultured with embryonic cells. There was an increase in the percentage of bipolar cells produced by postnatal p rogenitor cells, indicating a possible respecification of fate. The increas e in bipolar cells could occur even after progenitor cells had completed th eir terminal mitoses. These alterations in cell fates appeared to be caused at least in part by a secreted factor released by the embryonic cells that requires the LIFR beta/gp130 complex for signaling. Finally, although surr ounded by 20-fold more embryonic cells, the postnatal cells did not choose to adopt any fates normally produced only by embryonic cells.