A total of 125 RGCs from 16 retinas with clear axonal and dendritic structures visualized by the CSLO were analyzed. On average, 11 RGCs were analyzed in each mouse, and eight RGCs were analyzed in each eye. Cluster analysis identified six groups of RGCs (
Fig. 1C) with reference to eight morphologic and anatomic parameters (cell body size, axonal diameter, dendritic field size, ending branch number, total dendritic branch length, symmetry, branching complexity [Sholl analysis], and distance from the optic disc; see the Methods section and
Fig. 2). Group 4 RGCs generally had a large dendritic field (148,678 ± 36,853 μm
2, mean ± SD) and soma size (3,180 ± 940 μm
2), long total dendritic branch length (4,164 ± 870 μm), and a location distant from the optic disc (2,020 ± 426 μm) compared with other cell groups. Although there is no consensus on the morphologic criteria for RGC classification, most classification systems have been based on measurement of dendritic field and soma size.
12 –17 With a large soma size and dendrites covering a wide area, group 4 RGCs have close resemblance to α-cells originally described by Boycott and Wässle
12 in cats. For RGCs in groups 5 and 6, the dendritic field (37,285 ± 15,562 and 36,380 ± 11,615 μm
2, respectively) and the number of ending dendritic branches (17.3 ± 3.9 and 16.7 ± 3.4) were the smallest, the total dendritic branch length was the shortest (1,608 ± 532 and 1,617 ± 336 μm), and the dendritic branching was the least complex. Group 6 RGCs had smaller soma size (1,033 ± 293 μm
2) and were located closer to the optic disc (731.1 ± 364.9 μm). These morphologic characteristics are similar to those reported in γ-cells.
12 The dendrites of β-cells ramify frequently and cover a relatively smaller area.
12 These features were found in group 1 RGCs in which they had a medium-sized dendritic field (89,988 ± 26,457 μm
2), yet also had an increased number of ending branches (28.6 ± 4.7) and highly complex branching. The morphologic measurements were similar in groups 2 and 3 RGCs, except the axon diameter, dendritic field, and cell body size were generally larger for RGCs in group 3. Most RGCs selected in the analysis belonged to group 5 (33.6%) whereas group 4 RGCs were the least in number (4.0%;
Fig. 1D).