MORPHOLOGY OF A SMALL-FIELD BISTRATIFIED GANGLION-CELL TYPE IN THE MACAQUE AND HUMAN RETINA

In in-vitro preparations of both macaque and human retina, intracellul ar injections of Neurobiotin and horseradish peroxidase were used to c haracterize the morphology, depth of stratification, and mosaic organi zation of a type of bistratified ganglion cell. This cell type, here c alled the small bistratified cell, has been shown to project to the pa rvocellular layers of the dorsal lateral geniculate nucleus (Rodieck, 1991) and is therefore likely to show color-opponent response properti es. In both human and macaque, the two dendritic tiers of the bistrati fied cell are narrowly stratified close to the inner and outer borders of the inner plexiform layer. The inner tier is larger in diameter an d more densely branched than the outer tier and gives rise to distinct spine-like branchlets bearing large, often lobulated heads. By contra st the smaller, outer tier is sparsely branched and relatively spine-f ree. In human retina, the small bistratified cells range in dendritic field diameter from approximately 50 mum in central retina to approxim ately 400 mum in the far periphery. The human small bistratified cells are about 20% larger in dendritic-field diameter than their counterpa rts in the macaque. However, when the difference in retinal magnificat ion between human and macaque is taken into account, the small bistrat ified cells are similar in size in both species. In macaque, the small bistratified cell has a dendritic-field size that is approximately 10 % larger than that of the magnocellular-projecting parasol ganglion ce ll. Human small bistratified ganglion cells tend to have smaller dendr itic-field diameters than parasol cells. This is because parasol gangl ion cells are larger in human than in macaque retina (Dacey & Petersen , 1992). In macaque retina, intracellular injections of Neurobiotin re vealed heterotypic tracer coupling to a distinct mosaic of amacrine ce lls and probable homotypic coupling to an array of neighboring ganglio n cells around the perimeter of the injected cell's dendritic tree. Th e amacrine cell mosaic had a density of 1700 cells/mm2 in peripheral r etina. Individual amacrines had small, densely branched and bistratifi ed dendritic fields. From the homotypic coupling, it was possible to e stimate for the small bistratified cell a coverage factor of approxima tely 1.8, and a density of approximately 1% of the total ganglion cell s in central retina, increasing to approximately 6-10% in the retinal periphery. The estimated density, dendritic-field size, and depth of s tratification all suggest that the small bistratified ganglion cell ty pe is the morphological counterpart of the common short-wavelength sen sitive or 'blue-ON' physiological type.