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S. C. Lee, E. J. Cowgill, A. Al-Nabulsi, B. E. Reese; Homotypic Regulation of Cone Bipolar Cell Morphology in the Mouse Retina. Invest. Ophthalmol. Vis. Sci. 2010;51(13):6449.
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© ARVO (1962-2015); The Authors (2016-present)
Cone bipolar cells acquire their distinctive morphological appearance and dendritic field size in the absence of all cone photoreceptors, although they fail to form discrete clusters of terminal endings normally associated with pedicles. Here we look at whether homotypic cell density plays a role in determining the dendritic morphology of cone bipolar cells in the mouse retina.
Mice lacking the pro-apoptotic gene, Bax, show an increase in the size of the INL and GCL, without affecting the ONL. We crossed such Bax-/- mice with Gustducin-GFP expressing mice to identify the Type 7 cone bipolar cells, using Bax+/+ littermates as controls. DiI was injected into single bipolar cells to reveal the detailed dendritic morphology of each cell in wholemount preparations. Cone pedicles were labeled with fluorescein-conjugated peanut agglutinin (PNA), and confocal microscopy was used to reconstruct bipolar cell dendritic morphology. The dendritic field area and number of terminal branches were determined for each cell, as was the number of cone pedicles contacted. Cone pedicle and Type 7 cone bipolar cell densities were also determined for Bax-/- and control retinas, by sampling twelve fields (each 0.016mm2) per retina (n=4 mice per group).
There was no significant difference in retinal area between the two groups, and nor was there a significant difference in cone density based upon counts of PNA-labeled cone pedicles. Type 7 cone bipolar cells, by contrast, increased in density by 37% in Bax-/- compared to Bax+/+ retinas The dendritic field size of these bipolar cells in Bax-/- mice (n=31) was significantly reduced, by 23%, when compared to controls (n=41). There was also a significant reduction in the number of cone pedicles contacted per cell (Bax-/-: 6.6 pedicles; Bax+/+: 8.1 pedicles), but the average number of terminals per cone pedicle was not significantly different between Bax-/- (3.9 tips per pedicle) and Bax+/+ (4.1 tips per pedicle) animals.
These results demonstrate that the dendritic growth of cone bipolar cells is constrained by homotypic neighbors rather than being specified cell-intrinsically or commanded by afferent number. Homotypic density thereby constrains connectivity, limiting the pool of afferents from which a bipolar cell will sample.
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