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 an d 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 5-1 0 mum in diameter. Between 2 and 6 mm eccentricity, midget cells showe d a steep, 1 0-fold increase in dendritic field size, followed by a mo re shallow, three- to fourfold increase in the retinal periphery, atta ining a maximum diameter of approximately 225 mum. Despite large local variation in dendritic field size, midget cells formed one morphologi cally 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 pre sumably correspond to ON- and OFF-center cells respectively. The mosai c organization of the midget cells was examined by intracellularly fil ling neighboring cells in small patches of retina. For both the inner and outer midget populations, adjacent dendritic trees apposed one ano ther 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 th e inner midget cells. An outer:inner cell density ratio of 1.7:1 was f ound in the retinal periphery. An estimate of total midget cell densit y suggested that the proportion of midget cells increases f rom about 45% of total ganglion cell density in the retinal periphery to about 9 5% in the central retina. Nyquist frequencies calculated from midget c ell spacing closely match a recent measure of human achromatic spatial acuity (Anderson et al., 1991), from approximately 6-degrees to 55-de grees eccentricity. Outside the central retina, midget cell dendrites arborized in clusters within the overall dendritic field. With increas ing eccentricity, the dendritic clusters increased in number and remai ned small (approximately 10-20 mum diameter) relative to the size of t he dendritic field. Because neighboring midget cell dendritic trees do not overlap, the mosaic as a whole showed a pattern of clusters and h oles. We hypothesize that midget cell dendritic trees may contact indi vidual axon terminals of some midget bipolar cells and avoid contactin g others, providing a basis for the formation of cone-specific connect ions in the IPL.