Retinal ganglion cells in the crucian carp (Carassius carassius). I. Size and number of somata in eyes of different size

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Abstract
Ganglion cell somata were drawn, measured and counted in flatmounted crucian carp and goldfish retinas stained with cresyl violet or methylene blue. Soma diameter histograms suggest that the ganglion cells can be divided into two populations with overlapping soma sizes: a large group of small cells and a small group of large cells, the latter constituting 2.5–5% of all ganglion cells. With increasing distance from the optic disc the mean soma diameter increases while the ganglion cell density decreases. In a peripheral growth zone close to the margin the ganglion cells become smaller again. The total number of ganglion cells in retinas of different size was calculated from the areas of the flat‐mounted preparations and the cell densities in two representative regions. In the crucian carp population used in this work the total number of ganglion cells per retina was found to increase from roughly 140,000 (mean of 8 scattered values) to a full 200,000 between eye diameters 4 and 10 mm, this increase taking place mainly between eye diameters of 4 and 6.5 mm. Thus, due to a drastically decreasing cell density, the total number of ganglion cells increases only by a factor of about 1.5 while the retinal area becomes sixfold. During the same growth period the mean soma diameter increases by a factor of about 1.3 and the soma volume more than doubles. The optic nerve of a small crucian carp was studied by electron microscopy. About equal numbers of unmyelinated and myelinated axons were found. The axons in the optic nerve are, on an average, considerably thicker than the axons on the retinal surface.