Early in neurulation, the posterior neural plate, like the mesoderm, undergoes convergence and extension, thus assisting in blastopore closure. This movement is autonomous and does not require the mesoderm to be underneath the neural plate. At later stages, the process of neurulation in Xenopus is much less dramatic than in urodele amphibia, such as Ambystoma mexicanum; the neural folds, in particular, are not as obvious, and they do not form the classic, well-defined "keyhole" shape. That said, the first sign of neurulation in Xenopus is the thickening of the inner layer of dorsal ectoderm, the so-called censorial layer. This is followed, at the midneurula stage, by the formation of a dark pigment line along the dorsal midline of the embryo. This neural groove arises through the formation of bottle-like cells in the neural midline, which, like the bottle cells of the blastopore lip, contract their apices and concentrate the pigment granules in a smaller area. As neurulation proceeds, the neural groove deepens, and the lateral neural folds converge on the dorsal midline, where they eventually fuse to form the neural tube (Fig. 3D).
The most lateral cells of the neural plate do not participate in neural tube closure and are not recruited into the neural tube. These are the future neural crest cells, which in the truck go on to form pigment cells and neural derivatives, such as the dorsal root ganglion, and in the head form a wide variety of structures, such as the cephalic ganglia, the mandibular, hyoid, and branchial arches, and the head mesenchyme.
Finally, during and after neurulation, the mesoderm becomes subdivided into different tissues along the dorso-lventral axis (Fig. 4). The most dorsal mesodermal cell type is the notochord, a rod of vacuolated cells running the length of the embryo. Lateral to the notochord are the cells of the somites, which in Xenopus form predominantly muscle, and lateral and ventral to the somites are the cells of the pronephros. The lateral mesoderm goes on eventually to form structures such as the limbs, and the most ventral mesoderm forms blood.
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