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 requi re enlargement of eighth nerve arbors during growth since, at least in fish, eighth nerve neurons are added more slowly than hair cells or n ot at all. This situation provides an intriguing opportunity to study the mechanisms of growth of the neuronal arbors. In this paper, we rep ort the results of studies on the postembryonic growth of eighth nerve dendritic arbors in the saccular epithelium of the cichlid fish Astro notus ocellatus. Arbor sizes and shapes were compared in small and lar ge fish using the axonal tracer cobaltous-lysine. Our data suggest tha t postembryonic eighth nerve arbors enlarge in 2 ways. First, arbors a dd new terminal endings to their distal ends. Second, whole new branch es appear to be added at locations up to hundreds of micrometers proxi mal to the terminal endings. These 2 modes of growth suggest that more than one mechanism may be operative in controlling arbor enlargement.