Apoptosis in the cerebellum of adult teleost fish, Apteronotus leptorhynchus.

While involvement of programmed cell death (apoptosis) in embryogenesis is well established, only very little is known about this phenomenon in later stages of development. Based primarily on indirect evidence, it has been proposed that during postembryonic development of fish cell death does not occur. We have re-addressed this issue by examining the gymnotiform fish Apteronotus leptorhynchus. This teleost exhibits a high degree of proliferative activity in the brain during adulthood. Most of these cells are born in the cerebellum, where they differentiate, migrate into specific target regions, and are added to the population of already existing cerebellar cells. By applying morphological criteria and an in situ technique for the detection of DNA fragmentation (a feature characteristic of apoptotic cells), we show here that a large number of cerebellar cells undergo apoptosis. The density of apoptotic cells is significantly higher in the granule cell layers of the subdivisions of the cerebellum than in the corresponding molecular layers. This finding is consistent with previous observations indicating a drastic reduction in areal density of newborn cells within these granule cell layers in a period 4-7 weeks after their generation. In the granule cell layers of two cerebellar subdivisions, the corpus cerebelli and the valvula cerebelli pars medialis, the areal density of apoptotic cells displays a significant negative correlation with body weight, thus pointing to a decrease in the number of apoptotic events with age. The results of our investigation provide clear evidence for the existence of apoptosis during adulthood in fish and underline the significance of this process in the postembryonic development of the brain.