VISUAL SYSTEM: STATE OF THE ART 



63 



AXON 



Figure 15 Flat mounted, methylene blue 

 stained giant ganglion cell from the retina of 

 Mustelus canis. Concentric circles were used 

 by Stell and Witkovsky to map and quantify 

 the organization of the dendritic arbors of 

 giant ganglion cells. (Redrawn from Stell and 

 Witkovsky (1973a). Reproduced by kind per- 

 mission of the authors and the Journal of 

 Comparative Neurology © 1973 Wistar Press.) 



average dendritic field diameter is very nearly equal to the average distance 

 between displaced GGC's, and thus this cell group forms a gridwork of 

 overlapping fields. They concluded that each retinal area of overlap is pro- 

 vided with dendrites of seven to eight cells. While different dendritic patterns 

 between different classes of GGC's were detected, dendritic density in all 

 cases fell exponentially from the center of the cell to a value of 1% at the 

 periphery. 



For every ganglion cell counted, six times as many small (9 X 12 /mi) 

 ellipsoidal cells were observed in the ganglion cell layer. Stell and Witkovsky 

 suggested that these are glial cells— perhaps sheath cells associated with the 

 myelinated optic nerve axons. Still, they could be ganglion cells with 

 unmyelinated fibers. One piece of evidence against this view is that in the 

 area centralis of Mustelus the density of ganglion cells increases dramatically 

 while the number of "glial" cells remains the same. Franz (1931) estimated 

 the increase at from 800 to 2500 cells/mm 2 . 



Shibkova (1971) reported on cytoarchitecture and histochemistry of 

 retinal ganglion cells in Squalus and Raja. Many of her findings were con- 

 firmed in Stell and Witkovsky's (19736) independent study on Mustelus. In 

 addition she found that the glycogen-filled small and medium ganglion cells 

 are histochemically less active than GGC's; this suggested that the main 



