Retinal neurons of the California ground squirrel, Spermophilus beecheyi: a Golgi study

Although the optic nerve fibers of the cone-dominant ground squirrel retina have been well studied physiologically, the morphological details of the retinal neurons have not. To that end, retinal neurons of the California ground squirrel have been studied in Golgi-impregnated wholemounts. Two types of horizontal cell have been identified: H1 has an axon and axon terminal, whereas H2 is axonless. The dendritic field of H1 cells enlarges in a nonuniform manner with increasing displacement from the central retina. The smallest examples lie centrally in the visual streak, and the largest occur in the superior periphery. Eight types of bipolar cell are distinguished by morphological differences in dendritic branching pattern and field size in the outer plexiform layer, cell body size, and layering within the inner nuclear layer and by the morphology and stratification of axon terminals in the inner plexiform layer. A large bistratified bipolar cell (B8) is introduced here; the other 7 types closely resemble those in the retinas of other sciurid species described by R.W. West (1976, J. Comp. Neurol. 168:355-378; 1978, Vision Res. 18:129-136). The B1 type is proposed as a blue cone bipolar cell. Amacrine cells are classified into 27 cell types. Six of these occur as mirror-image pairs across the inner plexiform layer, the soma of one of each pair being "displaced" to the ganglion cell layer. The best described of these pairs is the very elaborate starburst amacrine cell, A5, which stains regularly in these wholemounted retinas. Changes in dendritic field size of both A5 subtypes with retinal location are quantified. The morphology of three amacrine cell types identified in Spermophilus beecheyi suggests that their possible counterparts in S. mexicanus (West, 1976) were, as displaced amacrine cells, misidentified as ganglion cells. Amacrine cell types that may play roles in the rod pathway, the blue cone pathway, and ganglion cell directional selectivity are discussed. No type of interplexiform cell was observed. Ganglion cells are classified into 19 cell types, 9 of which probably correspond to the ganglion cells described by West (1976) in the Mexican ground squirrel. The bistratified G11 cell is proposed as an ON-OFF directionally selective type.