Neurulation

The entire nervous system originates from the neural plate, an ectodermal thickening in the floor of the amniotic sac (Figure 1.1). During the third week after fertilization, the plate forms paired neural folds, which unite to create the neural tube and neural canal. Union of the folds commences in the future neck region of the embryo and proceeds rostrally and caudally from there. The open ends of the tube, the neuropores, are closed off before the end of the fourth week. The process of formation of the neural tube from the ectoderm is known as neurulation.

Figure 1.1 Cross-section (A) is from a 3-somite (20-day) embryo. Cross-sections (B) and (C) are from an 8-somite (22-day) embryo.

Cells at the edge of each neural fold escape from the line of union and form the neural crest alongside the tube. Cell types derived from the neural crest include spinal and autonomic ganglion cells and the Schwann cells of peripheral nerves.

Spinal nerves

The dorsal part of the neural tube is called the alar plate; the ventral part is the basal plate (Figure 1.2). Neurons developing in the alar plate are predominantly sensory in function and receive dorsal nerve roots growing in from the spinal ganglia. Neurons in the basal plate are predominantly motor and give rise to ventral nerve roots. At appropriate levels of the spinal cord, the ventral roots also contain autonomic fibers. The dorsal and ventral roots unite to form the spinal nerves, which emerge from the vertebral canal in the interval between the neural arches being formed by the mesenchymal vertebrae.

Figure 1.2 Neural tube, spinal nerve, and mesenchymal vertebra of an embryo at 6 weeks.

The cells of the spinal (dorsal root) ganglia are initially bipolar. They become unipolar by the coalescence of their two processes at one side of the parent cells.

Brain parts

Late in the fourth week, the rostral part of the neural tube undergoes flexion at the level of the future midbrain (Figure 1.3A). This region is the mesencephalon; slight constrictions mark its junction with the prosencephalon (future forebrain) and rhombencephalon (future hindbrain).

Figure 1.3 (A and B) Brain vesicles, seen from the right side. Asterisks indicate the site of initial development of the cerebellum.

The alar plate of the prosencephalon expands on each side (Figure 1.3A) to form the telencephalon (cerebral hemispheres). The basal plate remains in place here as the diencephalon. Finally, an optic outgrowth from the diencephalon is the forerunner of the retina and optic nerve.

The diencephalon, mesencephalon, and rhombencephalon constitute the embryonic brainstem.