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Purves D, Augustine GJ, Fitzpatrick D, et al., editors. Neuroscience. 2nd edition. Sunderland (MA): Sinauer Associates; 2001.

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Neuroscience. 2nd edition.

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The Initial Formation of the Nervous System: Gastrulation and Neurulation

Well before the patch of cells that will eventually become the brain and spinal cord appears, embryonic polarity and the primitive cell layers required for the subsequent formation of the nervous system are established. Critical to this early framework in all vertebrate embryos is the process of gastrulation. This invagination of the developing embryo (which starts out as a single sheet of cells) produces the three germ layers: the outer layer, or ectoderm; the middle layer, or mesoderm; and the inner layer, or endoderm (Figure 22.1). Gastrulation defines the midline and the anterior-posterior axes of all vertebrate embryos as well.

Figure 22.1. Neurulation in the mammalian embryo.

Figure 22.1

Neurulation in the mammalian embryo. On the left are dorsal views of the embryo at several different stages of early development; each boxed view on the right is a midline cross section through the embryo at the same stage. (A) During late gastrulation (more...)

One key consequence of gastrulation is the formation of the notochord, a distinct cylinder of mesodermal cells that extends along the midline of the embryo from anterior to posterior. The notochord forms from an aggregation of mesoderm that invaginates and extends inward from a surface indentation called the primitive pit, which subsequently elongates to form the primitive streak. As a result of these cell movements during gastrulation, the notochord comes to define the embryonic midline. The ectoderm that lies immediately above the notochord is called the neuroectoderm, and gives rise to the entire nervous system. In addition to specifying the basic topography of the embryo and determining the position of the nervous system, the notochord is required for subsequent neural differentiation (see Figure 22.1). Thus the notochord (along with the primitive pit) sends inductive signals to the overlying ectoderm that cause a subset of neuroectodermal cells to differentiate into neural precursor cells. During this process, called neurulation, the midline ectoderm that contains these cells thickens into a distinct columnar epithelium called the neural plate. The lateral margins of the neural plate then fold inward, eventually transforming the neural plate into a tube. This structure, the neural tube, subsequently gives rise to the brain and spinal cord.

The progenitor cells of the neural tube are known as neural precursor cells. These precursors are dividing stem cells that produce more precursors and, eventually, nondividing neuroblasts that differentiate into neurons. As a result of their proximity to the notochord, the cells at the ventral midline of the neural tube differentiate into a special strip of epithelial-like cells called the floorplate. The position of the floorplate at the ventral midline determines the dorso-ventral polarity of the neural tube and further influences the differentiation of neural precursor cells. Inductive signals from the floorplate lead to the differentiation of cells in the ventral portion of the neural tube that eventually give rise to spinal and hindbrain motor neurons (which are thus closest to the ventral midline). Precursor cells farther away from the ventral midline give rise to sensory neurons within the spinal cord and hindbrain. At the most dorsal limit of the neural tube, a third population of cells emerges in the region where the edges of the folded neural plate join together. Because of their location, this set of precursors is called the neural crest (Figure 22.2). The neural crest cells migrate away from the neural tube along specific pathways that expose them to additional inductive signals that influence their differentiation. As a result, neural crest cells give rise to a variety of progeny, including the neurons and glia of the sensory and visceral motor (autonomic) ganglia, the neurosecretory cells of the adrenal gland, and the neurons of the enteric nervous system. They also contribute to variety of non-neural structures such as pigment cells, cartilage, and bone.

Figure 22.2. The neural crest.

Figure 22.2

The neural crest. Diagram of a cross section through a developing mammalian embryo at a stage similar to that in Figure 22.1C. The neural crest cells follow four distinct migratory paths that lead to differentiation of distinct cell types and structures. (more...)

Image ch22f11

By agreement with the publisher, this book is accessible by the search feature, but cannot be browsed.

Copyright © 2001, Sinauer Associates, Inc.
Bookshelf ID: NBK10993

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