Cellular differentiation

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

Figure 14.1 (A–C) Cellular differentiation in the embryonic spinal cord.

The neuroepithelium contains germinal cells which synthesize DNA before retracting to the innermost, ventricular zone, where they divide. The daughter nuclei move outward, synthesize 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 CNS are derived from basophil cells of the blood.

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

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

Enlargement of the intermediate zone of the basal plate creates the ventral gray horn and the ventral median fissure (Figure 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 spinal cord and brain.

Neural cord

The neural tube reaches caudally only to the level of the second lumbar mesodermal somites. Following closure of the posterior neuropore, the ectoderm and mesoderm at the level of the more caudal lumbar and sacral somites blend to form a neural cord. This ribbon of cells becomes canalized and links up with the neural tube; it forms the lower end of the spinal cord.

Ascent of the spinal cord (Figure 14.2)

The spinal cord occupies the full length of the vertebral canal until the end of the 12th postconceptual week. The sixth to eighth are marked by regression of the caudal end of the neural tube, to become a neuroglial thread, the filum terminale.

Figure 14.2 (A, B) Regression of coccygeal segments of spinal cord creates the filum terminale. (C, D) Ascent of spinal cord. (Note: Recent evidence indicates that, as represented here, the number of embryonic coccygeal vertebrae does not exceed three or four.)

After the 12th week, the vertebral column grows rapidly and drags the cord upward. The tip of the cord is at second or third lumbar level at the time of birth. The adult level (first or second lumbar) is attained 3 weeks later.

In consequence of 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 (Figure 14.3).

Figure 14.3 Segmental and vertebral levels compared. Spinal nerves 1–7 emerge above the corresponding vertebrae; the remaining spinal nerves emerge below.

Neural arches

During the fifth week, the mesenchymal vertebrae surrounding the notochord give rise to neural arches for protection of the spinal cord (Figure 14.4). The arches are initially bifid (split). Later, they fuse in the midline and form the vertebral spines.

Figure 14.4 Normal, bifid stage of neural arch development in an embryo of 8 weeks.

Conditions where the two halves of the neural arches have failed to unite are collectively known as spina bifida (Clinical Panel 14.1).