The mosaic of midget ganglion cells in the human retina.

To study their detailed morphology, ganglion cells of the human retina were stained by intracellular tracer injection, in an in vitro, whole-mount preparation. This report focuses on the dendritic morphology and mosaic organization of the major, presumed color-opponent, ganglion cell class, the midget cells. Midget cells in the central retina were recognized by their extremely small dendritic trees, approximately 5-10 microns in diameter. Between 2 and 6 mm eccentricity, midget cells showed a steep, 10-fold increase in dendritic field size, followed by a more shallow, three- to fourfold increase in the retinal periphery, attaining a maximum diameter of approximately 225 microns. Despite large local variation in dendritic field size, midget cells formed one morphologically distinctive class at all retinal eccentricities. Two midget cell types were distinguished by their dendritic stratification in either the inner or outer portion of the inner plexiform layer (IPL), and presumably correspond to ON- and OFF-center cells respectively. The mosaic organization of the midget cells was examined by intracellularly filling neighboring cells in small patches of retina. For both the inner and outer midget populations, adjacent dendritic trees apposed one another but did not overlap, establishing a coverage of no greater than 1. The two mosaics differed in spatial scale, however: the outer midget cells showed smaller dendritic fields and higher cell density than the inner midget cells. An outer:inner cell density ratio of 1.7:1 was found in the retinal periphery. An estimate of total midget cell density suggested that the proportion of midget cells increases from about 45% of total ganglion cell density in the retinal periphery to about 95% in the central retina. Nyquist frequencies calculated from midget cell spacing closely match a recent measure of human achromatic spatial acuity (Anderson et al., 1991), from approximately 6 degrees to 55 degrees eccentricity. Outside the central retina, midget cell dendrites arborized in clusters within the overall dendritic field. With increasing eccentricity, the dendritic clusters increased in number and remained small (approximately 10-20 microns diameter) relative to the size of the dendritic field. Because neighboring midget cell dendritic trees do not overlap, the mosaic as a whole showed a pattern of clusters and holes. We hypothesize that midget cell dendritic trees may contact individual axon terminals of some midget bipolar cells and avoid contacting others, providing a basis for the formation of cone-specific connections in the IPL.