Retinal structure in the smooth dogfish, Mustelus canis: General description and light microscopy of giant ganglion cells

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Abstract
Giant ganglion cells (GGC) were demonstrated in retinas of adult dogfish by Golgi impregnation, reduced silver and vital methylene blue staining. All GGC have large, flattened perikarya, and dendrites which radiate in a single horizontal plane of the inner synaptic (plexiform) layer. They are identifiable as ganglion cells by their axons, which after a tortuous initial course enter the nerve fiber layer. We divide the GGC into three varieties: (1) ordinary, with perikarya in the layer of ganglion cells and nerve fibers, and with dendrites radiating in the inner (proximal) one‐fourth of the inner synaptic layer; (2) displaced or Dogiel's cells, with perikarya in the amacrine cell layer and dendrites radiating in the outer (distal) one‐fourth of the inner synaptic layer; and (3) intermediate, with both perikarya and dendritic trees at intermediate levels in the inner synaptic layer.Only the ordinary and displaced GGC in the ventral portion of retinas stained with methylene blue were studied in detail. Cells of both types are arranged in irregular patterns with perikarya about 0.8–1.0 mm apart. Dendrites of the ordinary GGC spread within an elliptical field having a major axis of about 1.7 mm (2.0 mm max) and a minor axis of about 1.5 mm (1.7 mm max). The major axis is vertical in the eye. Dendrites of the displaced GGC spread within a circular field having a diameter of about 1.9–2.0 mm (2.2 mm max). Quantitative comparison of the dendritic trees of ordinary and displaced GGC shows that they differ also in details of dendritic morphology, the dendrites of the ordinary GGC being on the average more numerous and highly branched but shorter than those of the displaced GGC. Ordinary and displaced GGC, therefore, comprise distinct populations which must be presumed to differ functionally.For both ordinary and displaced GGC, the dendritic density and, therefore, the total available postsynaptic surface per unit retinal area decline exponentially with distance from the center to the edge of the dendritic tree. In contrast, the sensitivity to light is constant throughout the receptive field centers of comparable diameter which have been analysed in parallel electrophysiological studies. While the diameters of the largest ganglion cell dendritic fields and receptive field centers are similar, therefore, more detailed correlations of structure and function cannot be made at present.