24 - Introduction to the Zebrafish

Abstract

Zebrafish develop in a manner typical of teleosts. In freshly laid eggs, yolk and cytoplasm are intermixed, and the egg is surrounded by a transparent chorion, which swells and lifts away from the egg on contact with water. The animal–vegetal axis is already preset during oogenesis, and sperm can enter the egg only at the future animal pole through the micropyle, a specialization in the otherwise sperm-impermeable chorion. After fertilization, cytoplasm streams to the animal pole as it segregates from the yolk. About 30 minutes after fertilization, the cytoplasm forms the blastodisc at the animal pole, and surrounds the vegetal yolk mass as a thin yolk cytoplasmic layer. As in amphibians, zebrafish embryonic cells are readily marked by cell lineage tracers. At the onset of gastrulation, analysis of cellular fates using this technique reveals that the endoderm will derive from the vegetalmost marginal blastomeres. Mesoderm forms from the vegetal third of the blastoderm, whereas ectoderm originates from the animal half of the blastoderm. Neuroectoderm in particular derives from the dorsal section of the animal half. Notochord derives from the dorsal side, where the shield forms, whereas somitic mesoderm, heart and blood, develop from lateral and ventral positions, respectively. The organization of the zebrafish fate map, therefore, is similar to the one of Xenopus.