Postembryonic development of the cerebellum in gymnotiform fish.

In contrast to adult mammals, adult teleost fish regularly generate new neurons and glial cells in many brain regions. A previous quantitative mapping of the proliferation zones in the brain of adult Apteronotus leptorhynchus (Teleostei, Gymnotiformes) has shown that 75% of all mitotically active cells are situated in the cerebellum (Zupanc and Horschke [1995] J. Comp. Neurol. 353:213-233). By employing the thymidine analogue 5-bromo-2'-deoxyuridine, we have, in the present study, investigated the postembryonic development of this brain region in detail. In the corpus cerebelli and the valvula cerebelli, the vast majority of newborn cells originate in the respective molecular layers. Within the first few days of their life, these cells migrate toward specific target areas, namely, the respective granule cell layers. In the caudal part of the cerebellum, the granule cell layer of the eminentia granularis pars medialis displays the highest mitotic activity. From there, the cells migrate through the adjacent molecular layer to the granule cell layer of the eminentia granularis pars posterior. Combination of retrograde-tracing techniques with immunohistochemistry for 5-bromo-2'-deoxyuridine showed that at least a portion of the newly generated cells develop into granule neurons. Many of the newly generated cells survive for long periods of time. A large fraction of these cells is added to the population of already existing cells, thus resulting in a permanent growth of the target areas and their associated structures.