We studied the morphology and diversity of retinal ganglion cells in the Pacific redfin, Tribolodon brandtii. These cells were retrogradely labeled with horseradish peroxidase and examined in retinal whole mounts. A sample of 203 cells was drawn with a camera lucida. A total of 19 structural parameters were estimated for each cell, and a variety of clustering algorithms were used to classify the cells. The optimal solution was determined by using silhouette analysis. It was based on three variables associated with dendritic field size and dendrite stratification in the retina. Kruskal-Wallis ANOVA-on-ranks with post hoc Mann-Whitney U tests showed significant pairwise between-cluster differences in two or more of the original variables. In total, eight cell types were discovered. The advantages and drawbacks of the methodology adopted are discussed. The present classification is compared with classifications proposed for other teleosts.

The topography and morphology of retinal ganglion cells (RGCs) in the eastern newt were studied. Cells were retrogradely labeled with tetramethylrhodamine-conjugated dextran amines or horseradish peroxidase and examined in retinal wholemounts. Their total number was 18,025 +/- 3,602 (mean +/- SEM). The spatial density of RGCs varied from 2,100 cells/mm(2) in the retinal periphery to 4,500 cells/mm(2) in the dorsotemporal retina. No prominent retinal specializations were found. The spatial resolution estimated from the spatial density of RGCs varied from 1.4 cycles per degree in the periphery to 1.95 cycles per degree in the region of the peak RGC density. A sample of 68 cells was camera lucida drawn and subjected to quantitative analysis. A total of 21 parameters related to RGC morphology and stratification in the retina were estimated. Partitionings obtained by using different clustering algorithms combined with automatic variable weighting and dimensionality reduction techniques were compared, and an effective solution was found by using silhouette analysis. A total of seven clusters were identified and associated with potential cell types. Kruskal-Wallis ANOVA-on-Ranks with post hoc Mann-Whitney U tests showed significant pairwise between-cluster differences in one or more of the clustering variables. The average silhouette values of the clusters were reasonably high, ranging from 0.52 to 0.79. Cells assigned to the same cluster displayed similar morphology and stratification in the retina. The advantages and limitations of the methodology adopted are discussed. The present classification is compared with known morphological and physiological RGC classifications in other salamanders.

A variety of visually evoked responses are recorded in the fish optic tectum using single-cell recording technique. Based on indirect criteria (frequency power spectrum of spikes, spike waveform, receptive field size), they may be divided into two groups: those presumably recorded from axon terminals of retinal ganglion cells projecting to the tectum (precynaptic recording), and those recorded from tectal neurons (postsynaptic recording). In the present study, we used cobalt, a reversible blocker of synaptic transmission, as a more crucial criterion to identify the source of these responses. After cobalt application, some units (such as ON- and OFF-types of direction-selective units, orientation-selective and spontaneously active units) were visually responsive, while others (including ON-OFF direction-selective units with large receptive fields) ceased firing. Discrimination of the units by the use of cobalt has been found to coincide with that by the indirect physiological criteria. Thus, the differences in frequency power spectrum of spikes, spike waveform, and receptive field size may be used for efficient and reliable discrimination between pre- and post-synaptic recordings in the fish tectum.

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Vision plays a crucial role in the life of the vast majority of vertebrate species. The spatial arrangement of retinal ganglion cells has been reported to be related to a species’ visual behavior. There are many studies focusing on the ganglion cell topography in bony fish species. However, there are still large gaps in our knowledge on the subject. We studied the topography of retinal ganglion cells (GCs) in the Japanese smelt Hypomesus nipponensis, a highly visual teleostean fish with a complex life cycle. DAPI labeling was used to visualize cell nuclei in the ganglion cell and inner plexiform layers. The ganglion cell layer was relatively thin (about 6‐8 μm), even in areas of increased cell density (area retinae temporalis), and was normally composed of a single layer of cells. In all retinal regions, rare cells occurred in the inner plexiform layer. Nissl‐stained retinae were used to estimate the proportion of displaced amacrine cells and glia in different retinal regions. In all retinal regions, about 84.5% of cells in the GC layer were found to be ganglion cells. The density of GCs varied across the retina in a regular way. It was minimum (3990 and 2380 cells/mm2 in the smaller and larger fish, respectively) in the dorsal and ventral periphery. It gradually increased centripetally and reached a maximum of 14,275 and 10,960 cells/mm2 (in the smaller and larger fish, respectively) in the temporal retina, where a pronounced area retinae temporalis was detected. The total number of GCs varied from 177 × 103 (smaller fish) to 212 × 103 cells (larger fish). The theoretical anatomical spatial resolution (the anatomical estimate of the upper limit of visual acuity calculated from the density of GCs and eye geometry and expressed in cycles per degree) was minimum in the ventral periphery (smaller fish, 1.46 cpd; larger fish, 1.26 cpd) and maximum in area retinae temporalis (smaller fish, 2.83 cpd; larger fish, 2.75 cpd). The relatively high density of GCs and the presence of area retinae temporalis in the Japanese smelt are consistent with its highly visual behavior. The present findings contribute to our understanding of the factors affecting the topography of retinal ganglion cells and visual acuity in fish.