1 - Embryology

Abstract

The nervous system is one of the first to develop during human embryogenesis. At 18 days post-fertilisation, the notochord (a cellular rod in the embryo that defines the primordial axis and gives it some rigidity) induces ectoderm thickening, creating the neural plate. By the end of week 3, the neural plate forms paired neural folds that unite to create the neural tube and canal. This process, called neurulation, is completed by the end of week 4. Cells at the edge of each neural fold separate to form the neural crest, which is destined to become spinal and autonomic ganglia, melanocytes, and Schwann cells. The dorsal part of the neural tube is called the alar plate and the ventral part the basal plate. Late in week 4, the rostral neural tube flexes at the mesencephalon (midbrain), while the prosencephalon (forebrain) and rhombencephalon (hindbrain) develop concomitantly. By the end of week 6, twelve pairs of cranial nerves are present. The prosencephalon, mesencephalon, and rhombencephalon give rise to the cerebral hemispheres, midbrain, and hindbrain (pons, medulla oblongata, and cerebellum), respectively. Neural tube expansion creates the ventricular system. Each of the two lateral ventricles communicates with the third ventricle via the interventricular foramen of Monroe, whereby the third and fourth ventricles communicate via the cerebral aqueduct of Sylvius. The fourth ventricle is contiguous with the central canal of the spinal cord. These structures contain cerebrospinal fluid, which is formed by choroid plexus. By week 28, the central sulcus, lateral sulcus, and calcarine sulcus appear.