Proliferation zones in adult medaka (Oryzias latipes) brain.

Cell proliferation in the adult mammalian brain is maintained at a low rate, but cell proliferation in the adult fish brain is prominent. To compare the distribution of proliferating cells among fish species, mutants, and under different growing environments, we mapped the zones of cell proliferation in the adult medaka (Oryzias latipes) brain and identified 17 proliferation zones in both male and female brains. These zones were distributed in the telencephalon (4 zones), preoptic area (2 zones), pineal body (1 zone), hypophysis (1 zone), habenular nucleus (1 zone), optic tectum (2 zones), third ventricular zone (1 zone), ventromedial nucleus (1 zone), hypothalamus (1 zone), and cerebellum (3 zones). Of the 17 zones, 16 corresponded to brain regions where cells proliferate in the zebrafish brain, suggesting that the persistence of the generation of new cells, at least in these zones, might be conserved among some fish species. We then compared the distribution of proliferation zones using two body-color mutant medaka, the T5 and Quintet, the latter of which is an albino mutant that completely lacks pigmentation. There was no apparent difference in the distribution pattern among these mutant strains. Finally, we compared these proliferation zones in the brains of isolated- and group-reared fish and detected no significant difference between the two groups. These findings demonstrate that there is persistent cell proliferation in at least these 16 zones of the adult medaka brain, irrespective of sex, body-color, and growth environment, suggesting that proliferation capacity in the 16 zones is maintained robustly in the adult medaka brain.