The human brain is made up of an enormous number of neurons and glial cells. The sources of all these neurons and glia are the cells of the neural tube, described in the previous chapters. Neurogenesis and gliogenesis, the generation of neurons and glia during development, occurs in many different ways in the various regions of the embryo. Part of the complex process of making the brain includes the proper migration of neurons and glia from their site of origin to their final position in the adult brain. Precisely orchestrated cell movements or migrations are an integral part of what is collectively known as histogenesis in the brain. This chapter describes the cellular and molecular principles by which the appropriate numbers of neurons and glia are generated from the neural precursors, and gives an overview of some of the complex cellular migration processes involved in the construction of the brain.
The number of cells generated in the developing nervous system is likely regulated at several levels. In some cases, the production of neurons or glia may be regulated by apparent intrinsic limits to the number of progenitor cell divisions essentially, a "cellular clock." The level of proliferation and ultimately the number of cells generated can also be controlled by extracellular signals, acting as mitogens, promoting progenitor cells to reenter the cell cycle and mitotic inhibitors that induce progenitor cells to exit from the cell cycle. However, it must also be remembered that the number of neurons and glia in the mature nervous system is a function not only of cell proliferation, but also of cell death. There are many examples of neuronal overproduction and subsequent attrition through programmed cell death; this process will be described in Chapter 7.
In the many small invertebrates, like the nematode, the lineages of the cells directly predict their numbers.
Since most of the mitotic divisions are asymmetric, the final number of cells that are produced during embryogenesis depends on the particular pattern of cell divisions and the number of cells that die through programmed cell death. The regulation of these divisions does not appear to depend on interactions with surrounding cells, but rather it is an intrinsic property of the lineage. The lineage of these cells also predicts the particular types of neurons that are generated from a particular precursor, and it appears that the information to define a given type of cell resides largely in factors derived directly from the precursors.
In the Drosophila central nervous system, neuronal number is also highly stereotypic. The neuroblasts of the insect CNS delaminate from the ventral-lateral ectoderm neurogenic region in successive waves (see Chapter 1). In Drosophila, about 25 neuroblasts delaminate in each segment, and they are organized in four columns and six rows. The pattern is basically the same for other insects and other arthropods, but the number of neuroblasts is dependent on the species (Doe and Smouse, 1990). Once the neuroblast segregates from the ectoderm, it then undergoes several asymmetric divisions, giving rise to approximately five smaller ganglion mother cells. Each ganglion mother cell then divides to generate a pair of neurons. These neurons make up the segmental ganglia of the ventral nerve cord and have stereotypic numbers and types of neurons.
In the vertebrate, the situation gets considerably more complex. The neural tube of most vertebrates is initially a single layer thick. As neurogenesis proceeds, the progenitor cells undergo a considerable number of cell divisions to produce a much thicker tube, with several layers. A section through the developing spinal cord is shown in Figure 3.1, and this basic structure is present throughout the developing central nervous
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