Topographic and Morphologic Analyses of Retinal Ganglion Cell Loss in Old DBA/2NNia Mice

PURPOSE. To evaluate the relationship between retinal ganglion cell (RGC) size, density distribution, and survival in senescent DBA2/NNia mice that develop pigmentary glaucoma. To evaluate the validity of nearest neighbor distance (NND), a measure of focal density for surviving RGCs in the retina, as a method to quantify RGC loss in mice. METHODS. Fifteen-month-old DBA2/NNia mice were labeled retrogradely with fluorogold. Retinas were flat mounted and imaged in their entirety using an epifluorescence microscope with a motorized stage. Digital maps of the retinal wholemounts were constructed to automatically count and establish spatial coordinates for RGCs over the entire retina. RGC size and NND were determined from these maps. RESULTS. RGC counts in the group of 15-month-old DBA/2NNia animals ranged from 22,330 to 92,157 cells per retina. Mean RGC cell size per retina ranged from 22.35 to 35.64 m 2 and correlated linearly with total RGC counts. NND distribution histograms were compared for retinas with variable degrees of RGC loss. The distribution of NNDs in each retina was skewed toward larger distance values in more affected retinas. In partially damaged retinas, areas with severe pathology coincided with areas of maximal loss of large RGCs, and areas of preserved RGCs correlated with larger cell sizes. CONCLUSIONS. Damaged retinas have a smaller mean cell size, indicating preferential loss of larger RGCs or size reduction of surviving cells. NND analysis of the RGC population in a retina is a useful measure of glaucomatous RGC loss. The skewed NND distribution of surviving RGCs and the finding that RGC loss correlates with a shift/amplitude change in the mode of the histogram and its tail suggests two different patterns of RGC loss possibly attributable to different pathologic processes in glaucomatous DBA/2 mice. (Invest Ophthalmol Vis Sci. 2006;47:1968 -1974) DOI:10.1167/iovs.05-0955 I nduced glaucoma models in the rat have been the subject of increasing research interest in recent years 6 -10 These models exhibit a range of pathology, most likely because of the diverse characteristics and the severity and duration of the surgical insult used to induce IOP elevation. More recently, murine models of glaucomatous optic neuropathy have also become of interest to some investigators. 11-16 Highly inbred mouse strains have a more restricted biologic variability that, together with spontaneous or geneinduced development of high IOP, Automated counting methods to quantify RGC loss in the glaucomatous retina have certain advantages over manual counting based on sampling. From th