VISUAL SYSTEM: STATE OF THE ART 61 



retina and are now known as Landolt's clubs. Landolt's clubs (Figure 13) of 

 Mustelus vary in external form from filamentous to bulbous. Landolt's clubs 

 in other organisms resemble the photoreceptor inner segment, since both 

 possess ciliary basal bodies and are packed with mitochondria. However, the 

 function of Landolt's clubs is presently unknown (Stell 1972a). 



Bipolar cell bodies are arranged in three to four levels in the internal 

 nuclear layer and their principal dendrites extend vertically between horizon- 

 tal cells and then branch horizontally before contacting the photoreceptor 

 terminals. Bipolar axons course toward the thick (40 jum) internal synaptic 

 layer and there terminate on ganglion and/or amacrine cells in an apparently 

 preferential manner. Thus, the inner synaptic layer is divided into three 

 sublayers that appear to segregate the specific bipolar types with specific 

 ganglion cell types. 



Amacrine Cells— Amacrine cells are retinal neurons of the inner 

 nuclear layer found in all vertebrate classes. The characteristic feature, as 

 their name suggests, is that they lack an axon. This is also true of elasmo- 

 branch horizontal cells. Little is known of amacrine cells in elasmobranchs. 

 Stell and Witkovsky (19736) briefly described those of Mustelus (Figure 14) 

 as small fusiform cells with incredibly long, bifurcate processes spanning up 

 to 5 mm! Amacrine cells apparently receive input from bipolars and spread 

 information laterally. However, Witkovsky (1971) reported the presence of 

 myelinated fibers of unknown origin entering the retina of Mustelus at the 

 optic disk and running to the internal nuclear layer, there synapsing with 

 bipolar and probably amacrine cells. Witkovsky believed these to be cen- 

 trifugal fibers primarily because of their morphological similarity to efferent 

 fibers in the pigeon retina. The origin of the pigeon efferents was demon- 



1mm 



primary process 

 perikaryon 



Figure 14 Flat mounted, methylene blue stained 

 amacrine cells of the large variety from the retina of 

 Mustelus canis. As their name suggests, these neurons 

 lack an axon but possess exceedingly long processes. If 

 the apparent bilateral symmetry of these cells is real, 

 the horizontal span of amacrines may exceed 5 mm. 

 (Modified from Stell and Witkovsky (1973a). Repro- 

 duced by kind permission of the authors and the 

 Journal of Comparative Neurology © 1973 Wistar 

 Press.) 



