Modes of neuronal arbor enlargement in the ear of a postembryonic fish, Astronotus ocellatus.

New hair cells are added during postembryonic life in several species of fishes and birds. The production of new hair cells appears to require enlargement of eighth nerve arbors during growth since, at least in fish, eighth nerve neurons are added more slowly than hair cells or not at all. This situation provides an intriguing opportunity to study the mechanisms of growth of the neuronal arbors. In this paper, we report the results of studies on the postembryonic growth of eighth nerve dendritic arbors in the saccular epithelium of the cichlid fish Astronotus ocellatus. Arbor sizes and shapes were compared in small and large fish using the axonal tracer cobaltous lysine. Our data suggest that postembryonic eighth nerve arbors enlarge in 2 ways. First, arbors add new terminal endings to their distal ends. Second, whole new branches appear to be added at locations up to hundreds of micrometers proximal to the terminal endings. These 2 modes of growth suggest that more than one mechanism may be operative in controlling arbor enlargement.