Retinal ganglion cell morphology in the frog, Rana pipiens

Abstract

The morphology of retinal ganglion cells in the frog, Rana pipiens, has been examined in retinal flatmounts following backfilling of axons with horseradish peroxidase (HRP). Size and shape of the cell body and of the dendritic arbor, the dendritic branching pattern, and the depth of dendritic arborization within the inner plexiform layer (IPL) were all used to classify these cells. All of the ganglion cells so visualized can be grouped into one of 7 distinct cell classes. Class 1 contains the largest ganglion cells, with a soma size of 323 ± 5.3 μm² and dendritic fields of 86,819 ± 11,817 μm²; the dendrites branch within strata 1 and 2 of the IPL. The second largest cells are class 2, with somas of 245 ± 19.7 μm² and dendritic fields of 55,983 ± 7,392 μm²; the dendrites also branch within strata 1 and 2 of the IPL. Class 3 cells are the next largest class with somas of 211 ± 11.8 μm² and dendritic fields of 18,186 ± 1,394 μm²; there are three varieties of class 3 cells based on the depth of branching of the dendrites: some cells are bistratified, others are tristratified, while still other cells arborize diffusely within the IPL. Class 4 cells are intermediate in size, with somas of 113 ± 7.4 μm² and dendrites of 4800 ± 759 μm²; the dendrites arborize within strata 4 and 5 of the IPL. Class 5 cells have not been quantititively analyzed because they are heterogeneous in soma and dendritic size. However, class 5 cells all have cell bodies displaced in location into the inner nuclear layer and all have a unique dendritic specialization: they send from 1 to 3 processes into the outer plexiform layer. Class 6 cells are the second smallest cell class with somas of 68.1 ± 5.13 μm² and dendritic fields of 888 ± 182 μm²; the dendrites arborize within strata 3, 4, and 5 of the IP. Class 7 contains the smallest ganglion cells with somas of 62.1 ± 2.86 μm²; and dendritic fields of 831 ± 74.2 μm²; the dendrites arborize within strata 3, 4, and 5 of the IPL. The frequency of each cell class is inversely proportional to the size of the dendritic field. Thus, class 7 cells are the most frequent; class 1 cells are the least frequent. Furthermore, each of these 7 classes of ganglion cells has representative cells located in the inner nuclear layer. The majority of these displaced ganglion cells are located in the central retina, within a horizontal streak just dorsal to the optic nerve head. The various morphological types of ganglion cells we observe may provide the structural basis for the diversity of frog ganglion cell receptive field types.