APOPTOSIS IN THE CEREBELLUM OF ADULT TELEOST FISH, APTERONOTUS-LEPTORHYNCHUS

While involvement of programmed cell death (apoptosis) in embryogenesi s 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 ce ll 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 differenti ate, migrate into specific target regions, and are added to the popula tion of already existing cerebellar cells. By applying morphological c riteria and an in situ technique for the detection of DNA fragmentatio n (a feature characteristic of apoptotic cells), we show here that a l arge number of cerebellar cells undergo apoptosis. The density of apop totic cells is significantly higher in the granule cell layers of the various subdivisions of the cerebellum than in the corresponding molec ular layers. This finding is consistent with previous observations ind icating a drastic reduction in areal density of newborn cells within t hese 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 bo dy weight, thus pointing to a decrease in the number of apoptotic even ts with age. The results of our investigation provide clear evidence f or the existence of apoptosis during adulthood in fish and underline t he significance of this process in the postembryonic development of th e brain.