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P. W. Keeley, B. E. Reese; The Role of Photoreceptor Afferents in the Dendritic Differentiation of Cone and Rod Bipolar Cells in the Mouse Retina. Invest. Ophthalmol. Vis. Sci. 2009;50(13):2723.
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Retinal bipolar cells can be classified by the type of photoreceptor, cone or rod, that forms synaptic connections with their dendrites in the OPL. To assess possible interactions between bipolar cells and their afferents upon dendritic differentiation, we have analyzed the morphology of cone and rod bipolar cells in two genetically engineered mouse models: the "coneless" mouse, in which all cones die during early postnatal development, and the "conefull" mouse, in which all rods have been re-specified to become cones.
Gustducin-GFP mice, in which the Type 7 cone bipolar cell and the rod bipolar cell express GFP, were crossed with coneless (CL) and conefull (CF) mice. Retinas were used as wholemounts to inject single bipolar cells, and as sections to label synaptic ribbons and cone pedicle active sites with antibodies to CtBP2 and PNA. Dendritic field area, number of terminal branches, depth of stratification, and connectivity were determined for each cell.
The dendritic field areas of cone bipolar cells were similar in CL, CF, and WT conditions, as were the total number of terminal branches. These cells restricted their dendrites to the innermost half of the OPL, where cone pedicles normally reside, regardless of the identity of cells in the ONL. In both CL and CF retinas, however, the terminal branches appeared less clustered than in WT retinas, yet were positioned adjacent to ribbon synapses, suggesting that in the CL retina, they are innervated by rod spherules. Rod bipolar cells also had dendritic field areas and terminal branch numbers that were comparable in all conditions. Rod bipolar cells extended branches throughout the OPL in all three conditions; however, in CF retinas, their dendritic stalks were coarser and their terminals, now terminating at cone pedicle active sites, were less punctate than in WT and CL retinas.
These results indicate that the overall dendritic structure of cone or rod bipolar cells is formed independent of cues from their normal afferents. The detailed dendritic terminal endings (in particular, their distribution and shape), however, requires normal connectivity with afferents.
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