Quantitative studies of retinal ganglion cells in a turtle, Pseudemys scripta elegans: II. Size spectrum of ganglion cells and its regional variation.

Recent evidence suggests that ganglion cell size and its regional variation may be an important feature of vertebrate retinas. Accordingly, we have examined Nissl-stained, whole-mounted Pseudemys scripta retinas to determine the soma size spectrum of ganglion cells at different retinal loci. Cell size histograms reveal that at any given point on the retina, a majority of ganglion cells are small (6-10 microns), and in peripheral samples there is some evidence for a second, larger size class (12-15 microns). Comparison of samples along the dorsoventral and nasotemporal axes suggests that there are two major trends in soma size variation. Along the dorsoventral axis, ganglion cell diameter increases sharply from the visual streak (6-7 microns, cf. Peterson and Ulinski, '79) to the dorsal and ventral periphery (9-10 microns). These changes reflect a tendency toward increased size for the entire distribution as well as a relative decrease in the frequency of small ganglion cells. This soma size variation is significantly correlated with changes in ganglion cell density. Along the nasotemporal axis, temporal ganglion cells are significantly larger than those at more nasal retinal loci. This difference reflects an overall increase in the size of ganglion cells in temporal retina and a small but significant increase in the percentage of neurons larger than 15 microns.