Morphogenetic differentiation of the brain of the cichlid fish, Oreochromis mossambicus

In the present study, the morphogenetic brain differentiation of the cichlid fish, Oreochromis mossambicus, is described using conventional histological methods. Cross sections through the developing brain of fish larvae aged 2, 8, 10, 12 and 20 days post hatch, and of adults, were investigated in order to characterize individual brain regions and nuclei, especially parts of the diencephalon, brainstem and cerebellum during critical phases of early larval development. The above age stages conform to the following developmental (larval) stages: 2 days = stage 7, 8 days = stage 10, 10 days = stage 11, 12 days = stage 12 and 20 days = stage 15. An increasing number of nuclei and a volumetric increase in size of individual brain parts in older fish larvae and in adult fish were registered. In the optic tectum, an extreme lateral extension was found in contrast to an insignificant increase in stratification at this developmental time. In one part of the brainstem, the Area octavolateralis (AOL) of 2-day-old larvae, only a few undifferentiated perikarya were found and in the AOL of 20-day-old animals up to 5 nuclei, versus 7 AOL-nuclei in adult fish brain, could be discriminated. Variations in the position of the Mauthner cell versus the surrounding and corresponding brain nuclei were shown during the ontogenetic development. The results were presented by cartoon portrayals of the relationships between characteristic nuclei and by drawings of specific brain cross sections in different aged larval stages.