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Chapter 2: Development and Cellular Constituents of the Nervous System

NEURAL DEVELOPMENT

Early in the development of the nervous system, a hollow tube of ectodermal neural tissue forms at the embryo’s dorsal midline. The cellular elements of the tube appear undifferentiated at first, but they later develop into various types of neurons and supporting glial cells.

The Neural Tube

The embryonic neural tube has three layers (Fig 2–1): the ventricular zone, later called the ependyma, around the lumen (central canal) of the tube; the intermediate zone, which is formed by the dividing cells of the ventricular zone (including the earliest radial glial cell type) and stretches between the ventricular surface and the outer (pial) layer; and the external marginal zone, which is formed later by processes of the nerve cells in the intermediate zone (Fig 2–1B).

FIGURE 2–1

Two stages in the development of the neural tube (only half of each cross section is shown). A: Early stage with large central canal. B: Later stage with smaller central canal.

Two illustrations depict cross-sections of stages of development of the neural tube. The first illustration, A, shows one half of the cross-section of early stage of development of the neural tube. In this stage, the central canal is large and the embryonic neural tube comprises of three layers; these are- ventricular zone, or ependyma, that surrounds the lumen or the central canal of the tube; the intermediate zone, formed by dividing cells of the ventricular zone including the earliest radial glial cell type, and the outermost pial layer. The second illustration, B, depicts one half of the cross-section of later stage of neural tube. In this stage, the neural canal is small, and an external marginal zone is seen, that is formed by processes of the nerve cells in the intermediate zone.
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The intermediate zone, or mantle layer, increases in cellularity and becomes gray matter. The nerve cell processes in the marginal zone, as well as other cell processes, become white matter when myelinated.

Cell Differentiation and Migration

The largest neurons, which are mostly motor neurons, differentiate first. Sensory and small neurons, and most of the glial cells, appear later, up to the time of birth. Newly formed neurons may migrate extensively through regions of previously formed neurons. When glial cells appear, they can act as a framework that guides growing neurons to the correct target areas. Because the axonal process of a neuron may begin growing toward its target during cell migration, nerve processes in the adult brain are often curved rather than straight.

NEURONS

Neurons vary in size and complexity. Motor neurons are usually larger than sensory neurons. Nerve cells with long processes (eg, dorsal root ganglion cells) are larger than those with short processes (Figs 2–2 and 2–3).

FIGURE 2–2

Schematic illustration of nerve cell types. A: Central nervous system cells: (1) motor neuron projecting to striated muscle, (2) special sensory neuron, and (3) general sensory neuron from skin. B: Autonomic cells to smooth muscle. Notice how the position of the cell body with respect to the axon varies.

Four schematic illustrations depict the various types of nerve cells. The first set of illustrations, A, shows three types of central nervous system cells; the first is a motor neuron projecting to striated muscle. It comprises of a rounded cell body with thread-like extensions arising from the cell body, known as dendrites. From the cell body arises an elongated fiber or axon, covered by myelin sheath, that terminates by innervating the effector organ or the muscles. The next diagram shows a special sensory neuron; it has a smaller cell body, as compared to the motor neuron, from the upper part of the cell body arise dendrites, projecting in the upward direction. From the cell body, also arises an elongated axon, with thread-like branching extensions at the terminal end. The third diagram shows a general sensory neuron from the skin. This is a pseudounipolar neuron, with axons projecting in two directions from the cell body. One axon carries impulse from the skin and the other carried the impulse to the central nervous system. The cell body has a nucleus with a nucleolus, and the axons are covered by the myelin sheath. The second illustration, B, shows an autonomic cell innervating smooth muscle cells. It comprises of pre-ganglionic and post-ganglionic cells. The preganglionic neuron originates in the brainstem or the spinal cord, and is connected to postganglionic neurons that lie outside the central nervous system, and innervate smooth muscle cells, as shown in the diagram.
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FIGURE 2–3

Schematic drawing of a motor neuron. Note that the neuron gives rise to many dendrites, of to one axon. The myelin sheath which covers the axon is produced by oligodendrocytes in the central nervous system and by Schwann cells in the peripheral nervous system. Note the three motor end-plates, which transmit the nerve impulses to striated ...

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