Cellular Aspects of Neural Development

Layers of the Neural Tube Maps of the World Within the Brain

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 2–1 B). 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 & 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 Maps of the World Within the Brain

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). Maps of the World Within the Brain Some neurons project from the cerebral cortex to the lower spinal cord, a distance of less than 2 ft in infants or 4 ft or more in adults; others have very short processes, reaching, for example, only from cell to cell in the cerebral cortex. These small neurons, with short axons that terminate locally, are called interneurons.

Extending from the nerve cell body are usually a number of processes called the axon and dendrites. Most neurons give rise to a single axon (which branches along its course) and to many dendrites (which also divide and subdivide, like the branches of a tree). The receptive part of the neuron is the dendrite, or dendritic zone (see Dendrites section). The conducting (propagating or transmitting) part is the axon, which may have one or more collateral branches. The downstream end of the axon is called the synaptic terminal, or arborization. The neuron's cell body is called the soma, or perikaryon.

Cell Bodies

The cell body is the metabolic and genetic center of a neuron (see Fig 2–3). Although its size varies greatly in different neuron types, the cell body makes up only a small part of the neuron's total volume.

The cell body and dendrites constitute the receptive pole of the neuron. Synapses from other cells or glial processes tend to cover the surface of a cell body (Fig 2–4).