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M Kuzmanovic, GS McGillem, V Sarthy; Morphology of a Presumptive On-type Cone Bipolar Cell in the Mouse Retina . Invest. Ophthalmol. Vis. Sci. 2002;43(13):739.
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Purpose: During a screen for Enhanced Green Fluorescent Protein (EGFP)-expressing cells in transgenic mouse lines, we have come across mice in which EGFP is expressed in two sub-populations of bipolar cells in the inner nuclear layer (INL). The aim of this study was to characterize EGFP-positive cone bipolar cells based on general morphology and known immunological markers. Methods: A construct consisting of EGFP linked to 0.3 kb GFAP promoter was obtained. Transgenic mice were generated and screened by PCR. Retinas from transgenics were isolated, fixed in 4% paraformaldehyde, and examined as whole mounts using confocal microscopy to determine the pattern of EGFP expression. In addition, 10 µm frozen sections were cut from retinas from transgenic animals and double labeled with antibodies to known retinal markers to determine the stratification pattern of these cells. Results: Across the retina, EGFP-labeling was restricted to populations of cells in the inner nuclear layer with bipolar morphology. One of these cell types was the rod bipolar cell that has been described in detail in many species including the mouse. The other labeled bipolar cell was a presumptive On-type bipolar cell that had an axonal arborization of 20-30 µm projecting to stratum 4 of the IPL. Double labeling with antibodies to the glutamate transporter GLT-1 showed that the axonal processes of these cells were restricted to stratum 4. ChAT staining further suggested that the synaptic terminals of these cells come into contact with the On-type, Starburst Amacrine cell processes localized to stratum 4. Conclusion: The close proximity to ChAT positive processes of the axonal arborization of the cone bipolar cell suggests that this bipolar cell may be pre-synaptic to the On-type Starburst amacrine cell. This transgenic line which contains EGFP-expressing rod and cone bipolar cells can be expected to be highly useful for detailed studies of bipolar synaptic circuitry in the mouse retina.
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