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Rat Retinal Ganglion Cell Topography
Author Response: Rat Retinal Ganglion Cell Topography
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Benjamin E. Reese, Professor Neuroscience Research Institute and Dept. of Psychology, University of California at Santa Barbara
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breese{at}psych.ucsb.edu Benjamin E. Reese
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Using a novel automated counting procedure for sampling the entire ganglion cell population, Danias et al.1 report that the distribution of retinal ganglion cells in the rat’s retina is highly variable between individuals and between the two eyes of the same individual. Specifically, while every sampled retina showed relatively fewer ganglion cells around the peripheral margin, the heightened density in more central locations did not reveal a consistent locus of peak density nor provide evidence for a visual streak. In fact, what is notable about the ten isodensity plots revealing ganglion cell topography in this study is the presence of a broad streak of increased density in every one. While the Wistar rat may display a degree of variability previously undocumented, another possibility is that the plotted orientations for each of these retinas do not approximate their functional orientation in the orbit. Following perfusion fixation, the eye of the rat can adopt a variable position within the orbit, evidenced by the positioning of the population of uncrossed retinal ganglion cells, and so determining proper retinal orientation from some landmark becomes critical.2 Extraocular landmarks such as the caruncle may not permit an accurate determination of the major retinal axes, and so the use of a retinal landmark is necessary. Unfortunately, the rat’s retina does not contain landmarks such as a fovea, area centralis or blood vessel pattern that would permit unambiguous orientation relative to the centrally positioned optic nerve head. One solution, mentioned above, has been to label the uncrossed visual pathway from the target visual nuclei, permitting a delineation of the retinal representation of the vertical meridian.3,4 Using this approach, two different laboratories have reported that the elongated region of heightened cell density within the ganglion cell layer (based on counts of either Nissl-stained neurons or retrogradely-labelled ganglion cells) should be oriented parallel to the horizontal meridian.5,6 Other studies relating the distribution of the population of ipsilaterally-projecting retinal ganglion cells to the total population of cells have all drawn essentially the same conclusion in the rat, regardless of whether the topography of all neurons in the ganglion cell layer was considered or just that of the ganglion cells.7-9 Those studies, each sampling a small number of retinas and typically sampling only a portion of the total population of cells, do not show so impressively the streak-like elongation of ganglion cell topography revealed in the present study. Had the uncrossed visual pathway been so labeled in the present retinas1, the elongated organization of ganglion cell topography would most likely have proven to run perpendicular to the representation of the vertical meridian.
The alternative is to conclude that the isodensity maps in the rat do not favor any particular orientation,1 but direct evidence for this should be provided before we reject the documentation of numerous other investigators. Benjamin E. Reese Neuroscience Research Institute and Department of Psychology
References 1. Danias J, Shen F, Goldblum D, Chen B, Ramos-Esteban J, Podos SM, Mittag T. Cytoarchitecture of the retinal ganglion cells in the rat. Invest Ophthalmol Vis Sci. 2002;43:587-594. 2. Reese BE, Cowey A. The crossed projection from the temporal retina to the dorsal lateral geniculate nucleus in the rat. Neuroscience. 1987;20:951-959. 3. Hughes A. A schematic eye for the rat. Vision Res. 1979;19:569-588. 4. Reese BE, Jeffery GJ. Crossed and uncrossed visual topography in dorsal lateral geniculate nucleus of the pigmented rat. J Neurophysiol. 1983;49:877-885. 5. Jeffery G. The relationship between cell density and the nasotemporal division in the rat retina. Brain Res. 1985;347:354-357. 6. Reese BE, Cowey A. Large retinal ganglion cells in the rat: Their distribution and laterality of projection. Exp Brain Res. 1986;61:375-385. 7. Fukuda Y. A three-group classification of rat retinal ganglion cells: Histological and physiological studies. Brain Res. 1977;119:327-344. 8. Schober W, Gruschka H. Die ganglienzellen der retina der albinoratte: eine qualitative und quantitative studie. Z Mikrosk-Anat Forsch. 1977;91:397-414. 9. Dreher B, Sefton A, Ni SYK, Nisbett G. The morphology, number, distribution and central projections of class I retinal ganglion cells in albino and hooded rats. Brain Behav Evol. 1985;26:10-48. |
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John Danias, Assistant Professor Mt. Sinai School of Medicine
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john.danias{at}mssm.edu John Danias
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We appreciate the comments of Dr. Reese on our paper on the retinal ganglion cell (RGC) cytoarchitecture in Wistar rats. The method described in our paper was developed as a way to count almost all RGCs in the rat retina in an effort to circumvent quantitation problems caused by sampling. Our objective was to apply the method to normal Wistar rats to determine the validity of counting the result and underlying assumptions and to test its limitations. We did not use an internal landmark for orienting the rat retinas, because, as Dr. Reese points out, there is none normally present. We thus had to use external landmarks for orientation such as the carruncle which is always nasal. We enucleated the eyes, after intracardiac fixation perfusion, with the carruncle attached to allow us to orient the retinas reliably. Although some amount of rotation could be caused by the use of this extraretinal orientation landmark it would probably not be more than 30-45 degrees at most. Some of the retinas included in figure 4 show larger deviations from this amount of rotation. Dr. Reese suggests using retrogradely labeled cells from the ipsilateral superior colliculus as an internal orientation landmark. For the purpose of our study it was not possible to use such an approach as we needed to ensure that all RGCs (both ipsilaterally and contralaterally projecting) were retrogradely labeled with the fluorescent tracer applied. Fluorogold has a broad emission spectrum that makes dual-labeling methods difficult. We were as surprised as Dr. Reese to see that the visual streak seemed not to be consistently in the same place in all animal eyes. We have since created isodensity maps of at least 20 more rat retinas which confirm the results reported in the paper. Since our results contradict earlier studies (performed by Dr. Reese and others) that used intra-retinal orientation landmarks we did not make this a major point in our discussion. We accept that our method of orienting the retina might be responsible for our finding of non-consistent orientation of the visual streak. We agree that our finding needs to be corroborated with the more precise methods described by Dr. Reese before it can be fully accepted. John Danias
Mt. Sinai School of Medicine
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