Postembryonic neurogenesis in zebrafish (Danio rerio) brain: Presence of two different systems

We analyzed postembryonic neurogenesis in zebrafish brain using the thymidi ne analogue 5bromo-e'-deoxyuridine (BrdU). At 1 and 3 weeks after fertiliza tion, BrdU-labeled cells were detected at the brain midline, especially in the telencephalon, optic tectum, hypothalamus, and cerebellum. Cell replica tion ceased in the telencephalon and hypothalamus by 6 weeks after fertiliz ation, but continued in the optic tectum and cerebellum even in adult fish. Although the area of replication was widely distributed in the tectum and cerebellum of young fish, it seemed to gradually become restricted to the m edial and lateral margin of the tectum and the dorsal region of the cerebel lum. These areas of replication showed common morphological features, with small, tightly packed spherical cells which were stained very densely by to luidine blue. In the optic tectum and cerebellum, the areas which were BrdU-positive (or densely stained by toluidine blue) gradually decreased in size relative to the growing brain, but the total volume of these areas in each individual r emained unchanged. These results indicate that there are two different neur ogenic systems in teleost fish. One system, found in the telencephalon and the hypothalamus, stops replication at an early stage of development; divid ing cells in these areas are repressed or removed. The other system, found in the tectum and cerebellum, retains its replicative activity even in adul t fish; the number of dividing cells in these areas seems to be maintained during development of the brain.