Early Development of the Cerebellum in Teleost Fishes: A Study Based on Gene Expression Patterns and Histology in the Medaka Embryo

Abstract The cerebellar structures of teleosts are markedly different from those of other vertebrates. The cerebellum continues rostrally into the midbrain ventricle, forming the valvula cerebelli, only in ray-finned fishes among vertebrates. To analyze the ontogenetic processes that underlie this morphological difference, we examined the early development of the cerebellar regions, including the isthmus (mid/hindbrain boundary, MHB), of the medaka (Oryzias latipes), by histology and in-situ hybridization using two gene (wnt1 and fgf8) probes. Isthmic wnt1 was expressed stably in the caudalmost mesencephalic region in the neural tube at all developmental stages examined, defining molecularly the caudal limit of the mesencephalon. The wnt1-positive mesencephalic cells became located rostrally to the isthmic constriction at Iwamatsu's stages 25–26. Isthmic fgf8 expression changed dynamically and became restricted to the rostralmost metencephalic region at stage 24. The rostralmost part (prospective valvula ...