Retrograde transport of horseradish peroxidase (HRP) was used to characterise the soma and dendritic arborization of retinal ganglion cells in adult Xenopus laevis toad. HRP was administered to the cut end of the optic nerve and the morphological characteristics of HRP-filled ganglion cells were analysed in retinal wholemount preparations using computer assisted morphometry. Ganglion cells were classified according to their soma size, dendritic branching pattern, dendritic field and the number of shaft dendrites. Ganglion cells were divided into 3 major classes on the basis of soma sizes and extent of dendritic field: large (soma size, mean 258.04 micron 2 +/- 52.03 SD; dendritic field size 0.104 mm2 +/- 0.23), medium size (126.7 micron 2 +/- 37.01; 0.041 mm2 +/- 0.013) and small (87.3 micron 2 +/- 22.69; 0.0061 mm2 +/- 0.0035). A more detailed analysis allowed 12 morphologically distinct subgroups to be identified (Types I-XII). Quantitative studies showed that large cells comprise about 1%, medium size about 8-9% and the small cells over 90% of total ganglion cell population. The number of large and medium size ganglion cells corresponded well with the number of myelinated optic fibres and the number of small neurons with the number of unmyelinated optic fibres in the optic nerve. Large ganglion cells were correlated with Class 4 and 5, medium size ganglion cells with Class 3 and small ganglion cells with Class 1 and 2 functionally characterized ganglion cells in the frog retina (Maturana et al. 1960). The retinal distribution of large ganglion cells appear to suggest certain similarities to mammalian alpha type ganglion cells.

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The number and distribution of neurons in the retinal ganglion cell layer were studied from the metamorphic climax to adulthood in the toad Bufo marinus. Retinal wholemounts stained with cresyl violet showed that total neuron numbers increased from 55,000 at metamorphic climax to about 950,000 in adult animals. During the same time the entire retinal area increased 46-fold from an average 3.4 mm2 to 157 mm2. The morphological character of the neurons and their density across the retina changed during development. In metamorphosing animals, the neurons of the ganglion cell layer had a uniform appearance and their density increased slightly from the centre to the dorsal ciliary margin. After metamorphosis a high neuron density area, the visual streak, evolved in the retinal centre, resulting in the formation of a 6 to 1 density gradient from the visual streak out to the dorsal and ventral retinal poles in adult animals. Optic fibre numbers in juvenile and adult optic nerves were estimated to be 330,000 and 745,000, respectively, corresponding to similar ganglion cell numbers. One optic nerve was sectioned in a few animals and 4 weeks later the number of intact neurons--assumed to be displaced amacrine cells (DA)--was estimated. They amounted to 80,000 in juvenile and 189,000 in adult animals or about 20% of the total neuron population of the retinal ganglion cell layer, the remaining 80% being GC. A 1.7 to 1 density gradient of DA from the visual streak out to the dorsal and ventral retinal periphery was established.(ABSTRACT TRUNCATED AT 250 WORDS)

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Serotonin-synthesizing and serotonin-accumulating neurons were studied in the retinas of Xenopus laevis and Bufo marinus. All previously identified cell types exhibiting serotonin-like immunoreactivity (SLI) were labeled by intravitreal injection of 5,7-dihydroxytryptamine (5,7-DHT). They included two amacrine cell types (large and small) in both species, and one bipolar cell type in Xenopus. Incubation of retinas in culture medium in the ambient light reduced SLI in amacrine cells and enhanced the labeling in bipolar cells. After incubation, some photoreceptor cell bodies and large numbers of outer segments also displayed SLI in both species. Incubation with the serotonin-uptake inhibitor, fluoxetine, reduced immunolabeling in bipolar cells and outer segments to the level in the untreated retinas. Both large SLI and 5,7-DHT-accumulating amacrine cells in Xenopus and Bufo were labeled with an antibody raised against phenylalanine hydroxylase (PH), which binds to tryptophan 5-hydroxylase, one of the synthesizing enzymes for serotonin. Small SLI and 5,7-DHT-accumulating amacrine cells in both species represented two populations, one with and the other without PH-like immunoreactivity (PH-LI). The anti-PH antibody failed to label any SLI or 5,7-DHT-accumulating bipolar cells in Xenopus. These observations indicate that all large and some small SLI amacrine cells in the retinas of Xenopus and Bufo synthesize serotonin, while other small SLI amacrine, bipolar and photoreceptor cell bodies, and outer segments only accumulate serotonin.