Central-peripheral and rostral-caudal organization of the innervation of the saccule in a cichlid fish.

Saccular eighth nerve arbors were examined in the cichlid fish Astronotus ocellatus to determine if their morphology varies with saccular location. The saccule was divided into four regions: rostral-central, rostral-peripheral, caudal-central, caudal-peripheral. Arbors were filled with cobaltous-lysine. Axon diameter, maximum arbor width, and number of terminal points were taken as quantitative measures. Differences in these measures among the four different saccular regions were evaluated using an analysis of variance. The results indicate two types of organization: central-peripheral and rostral-caudal. The central-peripheral differences involve all three quantitative measures. The central saccule is innervated by arbors with larger axon diameters, larger arbor widths, and more terminal points than the peripheral saccule. The rostral-caudal organization involves only two measures. The rostral saccule is innervated by arbors having larger axon diameters and smaller arbor widths than the caudal saccule. In the oscar, we know of no other parameters that are organized along the rostral-caudal dimension. These findings of a spatial organization of innervation suggest to us that the oscar saccule is not homogeneous in function.