Astroglia

Initially, epithelial cells on the surface of the embryo proliferate focally and form the neural groove. Continued multiplication of these cells and their elongation from a flat to a cuboidal or columnar shape results in the formation of the neural tube by a process called neurulation. During this process the sequestered neuroepithelium forms the wall of a centrally located canal or cavity, which contains enclosed amniotic fluid (Fig. 1 A). At this stage (e.g., in rat fetus), the surface ectoderm and the neural wall have the same basic appearance. Both structures consist of a single layer of cells that are joined to each other near their luminal ends by junctional complexes. Their opposite ends rest upon a basement membrane that separates them from the mesenchymal tissue. Even at this early stage, the mesenchyme contains blood vessels that later will invade the wall of the neural tube.

The simple shape of the early neural tube is soon modified locally along the neuraxis. Investigators studied changes in cell morphology, arrangement, and number to learn what roles different types of developing cells have in creating these regional differences. These modifications appear early and reflect the future cellular organization of the CNS.