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Editors Selection IGR 23-2

Basic Research: In-vivo imaging retinal ganglion cells

Comment by Chris Leung on:

26166 In vivo imaging of retinal ganglion cell axons within the nerve fiber layer, Kanamori A; Catrinescu MM; Traistaru M et al., Investigative Ophthalmology and Visual Science, 2010; 51: 2011-2018

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The advent of experimental models for in-vivo imaging of retinal ganglion cells (RGCs) has facilitated the investigation of molecular mechanism and treatment of glaucoma and other optic neuropathies. Most animal models, however, are limited in visualizing and following dendritic and axonal degeneration. Kanamori et al. (644) performed an elegant study showing concurrent degeneration of RGCs and their axonal fibers. They stained the axonal bundles with intravitreal injection of CMFDA, a fluorescent chloromethyl derivative for intracellular labeling, and retrograde-labeled the RGCs with Alexa Flour 488 dextran. With longitudinal imaging using a confocal scanning laser ophthalmoscope, they demonstrated that there was concomitant reduction in the number of RGC bodies and axonal bundles at five to seven days after optic nerve transection in rats.

There is evidence showing that RGC degeneration begins with progressive dendritic shrinkage, followed by loss of the axon and the cell body

The main limitations of CMFDA for axonal labeling are rapid fading and focal staining. Fading of the florescent dye occurred within a few days of injection. For this reason, dye injection to a group of control eyes was required to adjust for spontaneous degradation over time in the measurement of florescent signals. The authors carefully ruled out the possibilities of retinal damage, DMSO (a diluent for CMFDA) toxicity, and low dye concentration as potential causes for the focal staining. They proposed that CMFDA entered the RNFL through the inner limiting membrane at the site of injection. A key challenge in the analysis of progressive RGC damage is to correlate the region of retrograde-labeled RGC bodies to the stained axonal bundles. Visualizing axonal changes is pertinent in examining RGC degeneration as the presence of the cell body does not represent an intact RGC. Likewise, imaging dendritic shrinkage would be equally important. There is evidence showing that RGC degeneration begins with progressive dendritic shrinkage, followed by loss of the axon and the cell body. The conventional approach of counting the cell body may not as reliable as measuring the rates of axonal loss and dendritic shrinkage as indicators of RGC degeneration.

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