Study guidelines
This chapter aims to give you sufficient insight into embryologic development to account for the arrangement of structures in the mature nervous system. If not already familiar with adult brain anatomy, we suggest you read this chapter again following study of Chapters 2 and 3 .
For descriptive purposes, the embryo is in the prone (face-down) position, whereby the terms ventral and dorsal correspond to the adult anterior and posterior and rostral and caudal correspond to superior and inferior .
Spinal cord
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. The open cranial and caudal ends of the neural 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 .
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, melanocytes, and the Schwann cells of peripheral nerves.
Spinal nerves
The dorsal part of the neural tube is called the alar plate , and the ventral part is called 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, and those 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 fibres. 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.
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
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).
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.
The brainstem buckles as development proceeds. As a result, the mesencephalon is carried to the summit of the brain. The rhombencephalon folds on itself, causing the alar plates to flare and creating the rhomboid (diamond-shaped) fourth ventricle of the brain. The rostral part of the rhombencephalon gives rise to the pons and cerebellum. The caudal part gives rise to the medulla oblongata ( Figure 1.4 ).
