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.
Layers of 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).
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.
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.
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 migration, nerve processes in the adult brain are often curved rather than straight. The newer cells of the future cerebral cortex migrate from the deepest to the more superficial layers. The small neurons of the incipient cerebellum migrate first to the surface and later to deeper layers, and this process continues for several months after birth.
Neurons vary in size and complexity. For example, the nuclei of one type of small cerebellar cortical cell (granule cell) are only slightly larger than the nucleoli of an adjacent large Purkinje cell. 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).
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.