Mudpuppy retinal ganglion cell morphology revealed by an hrp impregnation technique which provides golgi‐like staining

A new technique of retrograde labeling of ganglion cells with horseradish peroxidase (HRP) has been developed, based on orbital injections of HRP combined with a detergent (lysolecithin). When injections are followed by an appropriate survival time, dense staining of a small number of widely scattered cells results in Golgi‐like filling of each neuron. This technique, as well as a variation which causes mass staining of ganglion cell somas, has been used to analyze the morphology of mudpuppy retinal ganglion cells. Morphological analysis has relied on computer reconstruction techniques for display, analysis of dendritic sublamination pattern, and morphometric analysis of the dendrites and soma. Based on morphological criteria, the mudpuppy retina contains a rich variety of ganglion cell types which vary according to soma placement, dendritic field size, polar vs. non‐polar dendritic fields, dendritic branching pattern, and dendritic sublamination. The mudpuppy retina contains both conventional and displaced ganglion cells: the latter constitute about 15% of the total ganglion cell population. Both conventional and displaced ganglion cells show morphological diversity of dendritic sublamination branching pattern; cells from each group have a dendritic branching pattern confined to either distal or proximal divisions of the inner plexiform layer, whereas other cells have dendrites which branch in both sublaminae. Using morphological criteria, two subtypes of ganglion cells were identified, which have a distinctive branching pattern and dendritic tree size. The size and distribution of ganglion cell somas were analyzed from retinas in which mass staining of ganglion cells was present. The total number of ganglion cells was estimated at approximately 14,500 cells per retina. There was a tendency for soma size and density to decrease near the optic disk. The somas of displaced ganglion cells are smaller than their conventional counterparts, at the same retinal eccentricity. The somas of all HRP filled cells swell when compared to those of unstained fixed and freshly dissected retinas. The degree of swelling is proportional to the length of exposure to HRP. Cell swelling was evident for both retrograde labeling and intracellularly injected HRP. This artifact of HRP staining could influence the interpretation of studies in which quantitative differences in soma sizes are based on the use of HRP labeling.