Figure 4.

Morphological comparisons of M6 cells to other ipRGC types based on traced micrographs of dye filled cells. (a, b) Tracings of M6 cells drawn from Cdh3-GFP mice (a) or from Opn4^cre/+;Z/EG^+/− mice (b). c: Tracings of representative M1-M5 cells from Opn4-Cre mice. In a-c, dendrites in the ON sublayer are black, those in the OFF sublayer are red, and those that return to the ON sublayer after diving into the OFF sublayer are blue. Scale bar in (c) equals 50 μm and also applies to (a,b). (d-e) Scatterplots comparing morphological parameters of M1, M2, M4 and M5 ipRGCs from the Opn4^cre/+;Z/EG^+/− mouse line (grey and black symbols) compared to M6 cells from Cdh3-GFP mouse line (red). Total branch points are related to dendritic field diameter (d) or total dendritic length (e) for M1 (n = 17), M2 (n = 20), M4 (n = 27) and M5 (n = 44) cells. Overall, M6 cells have greater total dendritic length than M1 and most M2 cells, and many more branch points than M1, M2, or M4 cells. (f) Sholl analysis comparing dendritic branching patterns for M1 (n = 16), M2 (n = 17), M4 (n = 18) and M5 (n = 44) cells. M6 cells have many more dendritic branches than M1 or M2 cells. Though they have only slightly more dendrites total than M4 cells, these are compressed into a much more compact dendritic arbor. M1, M2, M4 and M5 data were replotted from Estevez et al., 2012 and Stabio et al., 2018. (g) Bar plots showing the variability among M6 cells from Cdh3-GFP (red) in the percentage of total dendritic length found within the OFF sublamina of the IPL; M5 cells (grey), shown for comparison, invariably lack any OFF arbor. Note that the M5 population plotted here consists of 8 monostratified cells from Cdh3-GFP line and 44 M5 cells from the Opn4^cre/+;Z/EG^+/− line replotted from Stabio et al., 2018; all lack an off arbor.