Müller glia cells reorganize reaggregating chicken retinal cells into correctly laminated in vitro retinae

Müller cells, that belong to the family of radial glia cells, have central functions during retinogenesis. They form a stabilizing scaffold, they are candidate targets for the mediation of extraneous retinogenetic factors, and they are an important source for retina-borne retinogenetic factors. Reaggregate cultures allow the analysis of retinogenesis from dispersed cells to fully laminated tissues. Reaggregating cells from the embryonic chick retina reassemble to reversed laminated cellular spheres including constituents of all retinal layers, yet the outer nuclear layer is represented by internal rosettes. Using spheroids, we tested whether Müller cells have a decisive function in establishing retinal polarity and in determining the lamination pattern. To this end, we established confluent monolayers of highly enriched Müller cells derived from E6 or E13 chicken retinas, and then let dispersed E5.5 retinal cells reaggregate either in the absence of these monolayers or on top of them. In the presence of Müller cells, the reversed lamina polarity of rosetted spheroids progressively transformed within a week into correctly laminated retinal spheres, whereas all initial rosettes vanished. Moreover, photoreceptors formed a regular outer nuclear layer, as visualized by the rod-specific CERN901 antibody. In correctly laminated spheroids, staining for vimentin and glutamine synthetase was much more pronounced than in rosetted spheroids; in particular, a well-established inner limiting membrane stood out wherever the retinal lamination was complete. Because these effects can be similarly achieved by supernatants derived from Müller cells, direct cell-cell contacts or cellular replenishment from the monolayer do not account for these effects. We conclude that Müller cells are involved in the establishment of a correct retinal lamination and in the arrangement of the cells in the reaggregate cultures. In particular, rosette formation is counteracted and the formation of an inner limiting membrane is induced. Because rosettes are objects of concern in several ophthalmological defects, these results are highly relevant, both biomedically and also for normal retinogenesis.