Cat retinal ganglion cells projecting to the superior colliculus as shown by the horseradish peroxidase method.

1. Cat retinal ganglion cells projecting to the superior colliculus (SC) were detected in flat whole-mounts of Nissl-stained retinas by means of unilateral intracollicular injections of horseradish peroxidase (HRP). 2. With the purpose of drawing a comparison between HRP-labeled ganglion cells and the remaining population of unlabeled ganglion cells, the latter were classified according to their perikaryal characteristics into three groups: large, medium and small ganglion cells. Measurements of perikaryal sizes of ganglion cells distributed from the central area to the periphery of the retina showed that large cells ranged from 20 to 44 mum, medium cells from 15 to 30 mum and small cells from 8 to 17 mum. 3. Peroxidase-labeled retinal ganglion cells (HRP-cells) occurred among representatives of each of the three groups into which retinal ganglion cells had been classified. Small HRP-cells clearly constituted the majority of the HRP-cell population. When the injection site was restricted to the superior colliculus, medium HRP-cells as large as 19 mum were only occasionally observed, the 15 to 18 mum range being predominant over the medium cell population. 4. Medium HRP-cells larger than 20 mum were a common finding only when the injection site involved the pretectal nuclei and were never observed when the injection site was restricted to superior colliculus. 5. Possibility of a correspondence between the large, medium and small HRP-cells of the present study with, respectively, the physiological types Y-, X- and W- of cat retinal ganglion cells is discussed.