Nerve Roots

The Notochord

By day 17 of embryonic development a small aggregate of cells come together to form a thin rostral/caudal strip known as the notochord . This chord of cells induces the process of neurulation , in which a flattened region of the embryo known as the neural plate rolls up to form the neural tube . The notochord regresses in the adult except for a small portion contributing to the nucleus pulposus of the intervertebral disk.

Cellular Differentiation

The neural tube of the embryo consists of a pseudostratified epithelium surrounding the neural canal ( Fig. 14.1A ). Dorsal to the sulcus limitans the epithelium forms the alar plate ; ventral to the sulcus it forms the basal plate .

The neuroepithelium contains germinal cells that synthesise DNA before retracting to the innermost ventricular zone , where they divide. The daughter nuclei move outward, synthesise fresh DNA, then retreat and divide again. After several such cycles, postmitotic cells round up in the intermediate zone . Some of the postmitotic cells are immature neurons; the rest are glioblasts , which after further division become astrocytes or oligodendrocytes. Some of the glioblasts form an ependymal lining for the neural canal.

The microglial cells of the central nervous system (CNS) are derived from basophil cells of the blood.

Enlargement of the intermediate zone of the alar plate creates the dorsal horn of grey matter. The dorsal horn receives central processes of dorsal root ganglion cells ( Fig. 14.1B ). As explained in Chapter 1 , the ganglion cells are derived from the neural crest.

Partial occlusion of the neural canal by the developing dorsal grey horn gives rise to the dorsal median septum and to the definitive central canal of the cord ( Fig. 14.1C ).

Enlargement of the intermediate zone of the basal plate creates the ventral grey horn and the ventral median fissure ( Fig. 14.1C ). Axons emerge from the ventral horn and form the ventral nerve roots.

In the outermost marginal zone of the cord, axons run to and from the spinal cord and brain.

Ascent of the Spinal Cord ( Fig. 14.2 )

The spinal cord occupies the full length of the vertebral canal until the end of the twelfth postconceptual week. The sixth to eighth weeks are marked by the regression of the caudal end of the neural tube, to become a neuroglial thread, the filum terminale .

After the 12th week, the vertebral column grows rapidly and drags the spinal cord upward. The tip of the spinal cord is at the third lumbar (L3) at birth. The adult level (L1 or L2) is attained 2 months postnatally.

As a consequence of the greater ascent of the lower part of the cord compared to the upper part, the spinal nerve roots show an increasing disparity between their segmental levels of attachment to the cord and the corresponding vertebral levels ( Fig. 14.3 ).