Large retinal ganglion cells in the channel catfish (Ictalurus punctatus): Three types with distinct dendritic stratification patterns form similar but independent mosaics

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Abstract
Retinal ganglion cells in the channel catfish (Ictalurus punctatus) were retrogradely labelled, and those with the largest somata and thickest primary dendrites were categorized by their levels of dendritic stratification. Three types were found, each forming a mosaic making up ∼ 1% of the ganglion cell population. Using a system based on established sublaminar terminology, we call these the alpha‐a (αa), alpha‐b (αb), and alpha‐c (αc) ganglion cell mosaics. Cells of the αa mosaic had large, sparsely branched trees in sublamina a at 10–30% of the depth of the inner plexiform layer (IPL), sclerad to those of all other large ganglion cells. Some αa somata were displaced into the IPL or inner nuclear layer (INL) but belonged to the same mosaic as their orthotopic counterparts. Cells of the αa mosaic had dendrites that branched a little more and arborized in sublamina b at 50–60% of the IPL depth. Many also sent fine branches into sublamina a, and some were fully bistratified in a and b. The αc cells arborized in the most vitread sublamina, sublamina c, at 80–95% of the IPL depth. The soma areas of the three types in the largest retina studied ranged between 139 μm2 and 670 μm2 with significant differences in the order αa > αc ≥ αb. Analyses based on nearest‐neighbour distance (NND) and on spatial auto‐ and cross‐correlograms showed that each mosaic was statistically regular and independent of the others. Mosaic spacings were similar for each type, giving mean NNDs of 242–279 μm in the largest retina and 153–159 μm in a smaller one. Correspondences between these mosaics, previously defined large ganglion cell types in catfish, and other mosaic‐forming large ganglion cells in fish and frogs are discussed along with their implications for neuronal classification, function, development, and evolution. © 1995 Wiley‐Liss Inc.