From Larval Brain to Adult Brain and Summary of Telencephalic Ontogenesis

Abstract

This chapter describes the development of the medaka brain after hatching and summarizes the telencephalic ontogenesis in the medaka. We refer to the fish from stages 39 to 42 as larvae, and those at stage 43 as juveniles. The stage 44 fish before adults (stage 45) are called immature fish or young fish. In the brain ontogenesis the ventral regions of caudal brain (tegmentum) are formed earlier and remain relatively unchanged, whereas the dorsal regions including those of diencephalon and telencephalon develop slowly and change their morphologies in the post-hatching period. In the larval diencephalon, the inferior lobe enlarges extensively, and several nuclei differentiate in the medial hypothalamus. In the larval telencephalon, the pallium elongates rostrally, and the olfactory bulb becomes positioned rostroventral to the telencephalic hemisphere. The juvenile telencephalon continues to grow, especially in the caudalmost regions, forming new telencephalic structures. In the juvenile, telencephalic efferent fibers develop to form a part of the lateral forebrain bundle. This chapter and Chap. 10 show that the entire process of telencephalic ontogenesis is composed of six different phases; (1) formation of telencephalic primordium at stage 23/24, (2) emergence of subdivisions in the primordium at stage 26, (3) neuronal migration, eversion, and formation of spatially restricted proliferation zones at stages 28–30, (4) telencephalic growth and formation of cell-masses at stages 31–39, (5) rostral elongation of the pallium after hatching (stages 40–42), (6) telencephalic growth and differentiation of novel cell-masses at later stages (from stage 43 onward).