Cleavage and Gastrulation in Xenopus laevis Embryos

Abstract The Xenopus laevis egg contains an asymmetric distribution of ribonucleic acids (RNAs) , which are parceled out to different cells during cleavage. These RNAs initiate the formation of the germ layers, ectoderm, mesoderm and endoderm, as well as a set of dorsal axial structures, including the central nervous system. Cleavage is initially rapid and synchronous, but becomes slower and asynchronous at the midblastula transition, when the embryo's genes become active. The different cells signal among each other in a process known as induction, leading to more types of cells. The cells undergo rearrangements in a complicated set of coordinated movements during gastrulation. Signalling continues through gastrulation with a major centre, the Spemann organiser, responsible for generating the dorsal axial structures. These inductions and movements lead to the proper positioning of the forming tissues and organs, and the body plan of the embryo emerges. Key Concepts: The job of the fertilised egg is to generate the body plan. The body plan of a vertebrate embryo consists of three tubes, known as germ layers, and a set of dorsal axial structures. In Xenopus , RNAs localised in the egg initiate formation of both the germ layers and the dorsal axial structures. The Xenopus egg rapidly divides into smaller cells to prepare for cell movements and organ formation. A key signalling centre is established known as the Spemann organiser. Signals from the Spemann organiser cause the formation of the dorsal axial structures including the central nervous system. The complicated cell movements of gastrulation can be followed using fate maps. Several types of cell movements are coordinated during gastrulation to bring about their final arrangement in the embryo. The body plan emerges with the completion of gastrulation.