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Siegel GJ, Agranoff BW, Albers RW, et al., editors. Basic Neurochemistry: Molecular, Cellular and Medical Aspects. 6th edition. Philadelphia: Lippincott-Raven; 1999.

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Basic Neurochemistry: Molecular, Cellular and Medical Aspects. 6th edition.

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General Development of the Nervous System

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Correspondence to Jean de Vellis, Neurochemistry Group, 68225 MRRC, University of California Los Angeles Medical School, 760 Westwood Plaza, Los Angeles, California 90024.

During animal development, a single fertilized egg cell divides and differentiates to produce all of the cells and tissues of the mature organism. Genetic information contained in the fertilized egg provides all of the instructions to produce these tissues. The initial divisions of a fertilized egg produce a blastula. The blastula undergoes gastrulation to produce the three primary germ layers of the developing embryo. During gastrulation, cells migrate inward between the upper and lower layers of the blastula through the primitive streak, resulting in a trilaminar “sandwich.” The initial point of inward migration is established by the organizer, which in Xenopus is the dorsal lip of the blastopore and in birds and mammals is the node. The resulting gastrula has three layers, the ectoderm, the endoderm and the mesoderm. The nervous system develops from the ectoderm following an inductive signal from the mesoderm. The initial mesodermal cells condense to form the notochord, which elongates under the primitive streak along the anterior—posterior axis of the developing embryo. Signals are released from the notochord which induce the ectoderm to thicken into neural ectoderm in the area immediately overlying the notochord (Fig. 27-2). This neural ectoderm is now committed to develop into neural tissue, as can be demonstrated by transplantation experiments in which neural ectoderm surgically placed into other areas of the developing embryo produces auxiliary neural tissue. In addition, neural ectoderm arising in different places along the anterior-posterior axis of the developing embryo is predestined to develop into specific brain regions. Three proteins secreted by the organizer can induce production of anterior neural plate markers in undifferentiated ectodermal cells. These three proteins, noggin, follistatin and chordin, may establish anterior neural domains by inactivating two neural inhibitors, bone morphogenetic protein 2 (BMP2) and BMP4. The establishment of posterior domains within the neural plate may stem from signaling by basic fibroblast growth factor (bFGF) or by retinoic acid. Neither of these molecules is by itself sufficient to generate regional posterior identity, but both may mediate signals established by some other mechanism. An additional mechanism used to regionalize the neural plate may be the expression of head-specific genes. Deletion of several of these genes, notably Lim-1 and Otx-I, deletes all head structures, suggesting a role for these genes in establishing the most anterior portion of a developing animal. Cerberus, a secreted protein expressed in the gastrulating endomesoderm, also has forebrain-inducing activity.

Figure 27-2. Four stages in the development of the neural tube.

Figure 27-2

Four stages in the development of the neural tube. A: After gastrulation, the mesodermally derived notochord induces the overlying ectoderm to form the neural plate. The most lateral portions of the neural plate will give rise to the neural crest (dark (more...)

Once the neural plate has been induced by the underlying mesoderm, it begins to differentiate into the neural tube, the primary rudiment of the developing CNS [7]. The center of the neural plate develops a groove in the direction of the anterior—posterior axis, and the edges of the plate rise up, forming the neural folds (Fig. 27-2). The neural folds fuse together along the dorsal edge of the animal, and the neural tube separates from the overlying dorsal ectoderm. During neural tube formation, the adjacent mesoderm is segmented into blocks of tissue called somites. The somites provide precursor cells for axial and appendicular skeletal elements and attached musculature. During neural tube closure, a population of cells separates from the neural folds and migrates into the periphery (Fig. 27-2). These cells are the neural crest (NC), which provides a progenitor population for the developing PNS. The NC migrates away from the neural tube along defined pathways through the anterior half of each adjacent somite and along a dorsal pathway under the ectoderm. NC cells which migrate through the somites populate the dorsal root ganglia as neurons and glia, contribute Schwann cells to the peripheral nerves and provide neurons and glia for the sympathetic ganglia. NC cells which migrate along the dorsal pathway develop into melanocytes.

The neural tube is a pseudostratified epithelium with cells extending between the apical and basal surfaces of the epithelial wall. The neuroepithelium initially contains only one undifferentiated population of stem cells. With time, these cells give rise to the two main lineages, neuronal and glial stem cells, and subsequently many different sublineages. After the final mitotic division, neurons migrate away from the ventricular surface of the neural tube to form the mantle zone. This outward migration is guided by radial glial fibers, which provide a scaffold extending from the ventricular zone to the surface of the developing brain. The accumulation of postmitotic neuronal cells results in the progressive thickening of the neural tube to produce expansions of the CNS, brain and spinal cord.

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

Copyright © 1999, American Society for Neurochemistry.
Bookshelf ID: NBK28253

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