We analyzed postembryonic neurogenesis in zebrafish brain using the thymidine analogue 5-bromo-2′-deoxyuridine (BrdU). At 1 and 3 weeks after fertilization, BrdU-labeled cells were detected at the brain midline, especially in the telencephalon, optic tectum, hypothalamus, and cerebellum. Cell replication ceased in the telencephalon and hypothalamus by 6 weeks after fertilization, 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 medial and lateral margin of the tectum and the dorsal region of the cerebellum. These areas of replication showed common morphological features, with small, tightly packed spherical cells which were stained very densely by toluidine 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 remained unchanged. These results indicate that there are two different neurogenic systems in teleost fish. One system, found in the telencephalon and the hypothalamus, stops replication at an early stage of development; dividing cells in these areas are repressed or removed. The other system, found in the tectum and cerebellum, retains its replicative activity even in adult fish; the number of dividing cells in these areas seems to be maintained during development of the brain.