Proliferation zones in the brain of adult fish Austrolebias (Cyprinodontiform: Rivulidae): a comparative study

In contrast with mammals, adult fish brains exhibit an enormous potential to produce new cells. Proliferation zones, however, have been described in only a few species, hindering comparisons among genuses and orders. Here we analyzed brain cell proliferation in annual teleostean fishes Austrolebias (Cyprinodontiform: Rivulidae). Immunocytochemistry against 5-bromo-2'-deoxyuridine (BrdU) was quantitated and mapped 24 h after injection in three species with different phylogenetic positions or habitats. All species had similar brain anatomy and total volume, but olfactory bulbs, torus longitudinalis and cerebellum were of different sizes in different species. Cell proliferation was found throughout the brain. Three-D reconstructions provided evidence for contiguity along the rostro-caudal axis and concentration in the vicinity of the ventricles. Brain regions analyzed exhibited high mitotic activity, and the torus longitudinalis had the highest volume-normalized proliferation index. A. affinis exhibited the highest normalized proliferation indexes in visual regions but the lowest in olfactory bulb. A. reicherti showed an inverse pattern, suggesting that these species have a different hierarchy of sensorial modalities that could be related to phylogeny or habitat. Double immunostaining against BrdU and cell-type specific markers was performed to determine the fate of proliferating cells. A widespread gliogenesis was evidenced. Few cells positive for both BrdU and the neuronal marker HuC/D were found in the brain of the three species, demonstrating neurogenesis in the adult Austrolebias brain. Summarizing, adult members of the three species showed similar brain anatomy and cell proliferation patterns. Among species, volume-normalized proliferation indexes varied in regions involved in different sensory modalities. To our knowledge, this is the first report showing proliferating cells with neuronal markers as earlier as 24 h after BrdU injection.