Abstract
Purpose: :
To detect changes in retinal ganglion cell (RGC) morphology caused by chronic experimental glaucoma induced in mice by polystyrene bead injection.
Methods: :
Mice selectively expressing yellow fluorescent protein (YFP) under the Thy1 promoter (strain B6.Cg-Tg(Thy1-YFPH)2Jrs/J, The Jackson Laboratory, Bar Harbor, Maine) underwent polystyrene bead injection into the anterior chamber to produce chronic intraocular pressure (IOP) elevation for 1, 3, or 6 weeks. Individual RGCs were imaged by confocal microscopy in retinal whole mounts. A total of 739 RGCs in 22 mice were imaged, an average of 64 YFP-positive RGCs enumerated per retina in 44 eyes. RGCs were assessed by two methods: neurite outgrowth (parameters measuring soma and dendrite quantitatively) and Sholl analysis (number of processes crossing successive concentric rings centered on cell body).
Results: :
The mean somal fluorescence was at the highest level recorded in 23-48% of all RGCs, with the remainder of cells expressing lower somal fluorescence. Glaucoma caused no detectable decrease in the proportion of RGCs at the peak fluorescence level (p >0.3, Chi square) at 1, 3, 6 weeks. Experimental glaucoma caused a median axon loss of 10%, 17% and 23% of RGCs at 1, 3, & 6 weeks (n=7, 9, and 6 mice, respectively). There was no change in cell soma area or number of major initial dendritic processes. However, 1 week after glaucoma, there was a significant decline in mean dendritic complexity at and distal to 250 microns from the soma (p=0.025). Dendritic complexity of the 1 week glaucoma group was not matched by change in total outgrowth, process length or dendritic branches. Remaining RGCs in glaucomatous retinas had no significant differences from control values in outgrowth or complexity at 3 and 6 weeks.
Conclusions: :
Mouse RGCs showed no consistent change in morphology from 1 to 6 weeks after IOP elevation, despite death of a minority of RGCs. This differs from our analysis of the same strain analyzed 1-9 days after optic nerve crush in which dendritic outgrowth and complexity were immediately and significantly affected.
Keywords: ganglion cells • transgenics/knock-outs • imaging/image analysis: non-clinical