Neural Tube Formation

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AbstractThis chapter describes medaka development in the pre-phylotypic period, i.e., from the fertilized eggs to the embryos that have the neural tubes accompanied by fully developed ventricles and five brain vesicles. In the teleost eggs, cell divisions occur only in a small area (blastodisc) in the animal pole, and the early embryo (blastoderm) is formed in the blastodisc. The blastoderm becomes the morula, blastula, gastrula, and then neurula, as the development proceeds. A cell movement, termed the epiboly, begins in the early gastrula, and the blastoderm forms a sheet-like structure expanding over the yolk. Internalization of cells occurs in the pre-mid gastrula to form germinal layers. The neural plate, a slightly thickened neural ectoderm, is formed in the mid- and late-gastrulae. After the late-gastrula stage, cells in the neural ectoderm form five non-overlapping cell groups, establishing distinct tissue compartments in the CNS primordium. By the primary neurulation, the neural plate develops into the neural keel and subsequently into the neural rod, which is homologous to the neural tubes in many other vertebrates. Development of the ventricular system in the neural rod transforms the neural rod into the neural tube. The ventricular system is formed from stages 21 to 28, and five brain vesicles are distinctly visible at stage 24. The medaka embryonic brain now enters the phylotypic period (stages 21–28). Finally, four axes are set up in the neural tube.KeywordsGastrulaNeural keelNeural plateNeural tubeNeural rodNeurocoelNeurulaNeurulationPhylotypic stageVentricular system