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Yong N. Li, John E. Dowling; A Correlation Study between a Bipolar Cell’s Morphology and Its Photoreceptor Connectivity in the Zebrafish Retina. Invest. Ophthalmol. Vis. Sci. 2012;53(14):4307.
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Bipolar cells (BCs) are usually classified based on axonal stratification within the inner plexiform layer (IPL) and dendritic tree morphology. Does a bipolar cell’s axonal stratification and dendritic tree morphology indicate its photoreceptor connectivity? In this study, we identified the photoreceptor connections of 322 BCs found in the zebrafish retina and examined their axonal stratification and dendritic tree morphologies.
Pulled glass electrode tips loaded with DiI crystals were inserted and broken off into whole-mounted retinas of SWS1-GFP, SWS2-GFP, LWS-GFP and XOPS-EGFP transgenic zebrafish and their intercrosses (Takechi et al., 2003, 2008; Tsujimura et al., 2010; Fadool, 2003; Li et al., 2009). BCs were fully labeled by DiI within two weeks. Serial confocal images of BCs were taken and their dendrites and axon terminals analyzed. Their photoreceptor connectivity was determined based on the zebrafish cone mosaic coupled with transgenically labeled photoreceptor terminals.
We divided the IPL into 6 strata - 1-3 located distally and presumably are the OFF-sublaminae, strata 4-6 are proximal and the ON-sublaminae. Mono-, bi- and tristratified axonal terminal patterns were observed making a total of 28 different stratification combinations. BC axonal terminals that stratified on the same level could have different connectivity with the photoreceptor terminals - for example, with RGB cones, GBUV cones, RG cones and rods, etc. The exception was the monostratified BCs with a large axonal terminal on stratum 6 of the IPL - most of these cells connected with rods and R cones. On the other hand, BCs with similar dendritic morphologies (size, dendritic pattern, number of terminal boutons, etc.) usually connected with the same combination of photoreceptors.
BCs with the same axonal stratification patterns can have heterogeneous photoreceptor connectivity. BCs with the same dendritic tree morphology usually have the same photoreceptor connectivity, although their axons may stratify on different levels.
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