A VAST ARRAY OF NEURONS AND GLIAL CELLS is produced during development of the vertebrate nervous system. Different types of neurons develop in discrete anatomical positions, acquire varied morphological forms, and establish connections with specific populations of target cells. Their diversity is far greater than that of cells in any other organ of the body. The retina, for example, has dozens of types of interneurons, and the spinal cord has more than a hundred types of motor neurons. At present, the true number of neuronal types in the mammalian central nervous system remains unknown, but it is surely more than a thousand. The number of glial types is even less clear; unexpected heterogeneity is being discovered in what was thought, until recently, to be rather homogeneous classes of astrocytes and oligodendrocytes.
The diversity of neuronal types underlies the impressive computational properties of the mammalian nervous system. Yet, as we describe in this chapter and those that follow, the developmental principles that drive the differentiation of the nervous system are begged and borrowed from those used to direct the development in other tissues. In one sense, the development of the nervous system merely represents an elaborate example of the basic challenge that pervades all of developmental biology: how to convert a single cell, the fertilized egg, into the highly differentiated cell types that characterize the mature organism. Only at later stages, as the neurons form complex circuits and experience modifies their connections, do principles of neural development diverge from those in other organs.
Early developmental principles are conserved not only among tissues but also across species and phyla. Indeed, much of what we know about the cellular and molecular bases of neural development in vertebrates comes from genetic studies of so-called simple organisms, most notably the fruit fly Drosophila melanogaster and the worm Caenorhabditis elegans. Nevertheless, because a main goal of studying neural development is to explain how the assembly of the nervous system underlies both human behavior and brain disorders, our description of the rules and principles of nervous system development focus primarily on vertebrate organisms.
The Neural Tube Arises From the Ectoderm
The vertebrate embryo arises from the fertilized egg. Cell divisions initially form a ball of cells, called the morula, which then hollows out to form the blastula. Next, infoldings and growth generate the gastrula, a structure with polarity (dorsal-ventral and anterior-posterior) and three layers of cells—the endoderm, mesoderm, and ectoderm (Figure 45–1A).
The neural plate folds to form the neural tube. (Scanning electron micrographs of chick neural tube reproduced, with permission, from G. Schoenwolf.)
A. Following fertilization of the egg by sperm, cell divisions give rise successively to the morula, blastula, and gastrula. Three germ cell layers—the ectoderm, mesoderm, and endoderm—form during gastrulation.
B. A strip of ectoderm becomes the neural plate, the precursor of the central and ...