136. Early Nervous System Development: The Neural Tube and Neural Crest

  1. Formation of the neural tube consists of 3 successive stages of development
    1. THE NEURAL PLATE results from thickening and differentiation of ectoderm overlying the notochord. The ectoderm of the plate is called neuroectoderm and will give rise to the central nervous system, consisting of the brain and spinal cord
      1. The plate is wider at its cephalic end than at its caudal end and is formed, in humans, before the appearance of the first somite or about day 18 of development
      2. The ectoderm remains relatively thin at the periphery
      3. The plate is first seen cranial to the primitive knot and dorsal to the notochordal process. As the notochordal process elongates, the plate broadens and eventually extends cranially to the oropharyngeal membrane
      4. The notochordal process and paraxial mesoderm act as primary inductors for the differentiation of the embryonic ectoderm into the neural plate
    2. THE NEURAL GROOVE AND NEURAL FOLDS: the lateral edges of the neural plate elevate to form the neural folds. The neural groove develops as a result of invagination of the neural plate along its central axis, between the neural folds (about day 18)
    3. The neural tube: the neural folds move together, fuse by the end of week 3, and convert the plate into a tube (also called the neurula stage)
      1. The ectoderm thus is separated from the nervous tissue to form the surface ectoderm. Between the 2, mesenchymal cells gradually infiltrate
  2. The 3 stages (plate, groove and folds, and tube) all coexist at the same time in different regions of the embryo
    1. CLOSURE OF THE NEURAL GROOVE, at about day 21, in humans, takes place near the fourth somite (future cervical region), which is in the middle portion of the embryo, and progresses toward both its cephalic and caudal ends
  3. Two orifices at the ends of the neural tube persist for a relatively short period of time and are called the anterior and posterior neuropores. They temporarily form open connections between the neural tube lumen and the amniotic cavity
    1. THE ANTERIOR NEUROPORE indicates the area of the lamina terminalis and closes about day 25 or the 18-to-20-somite stage
  4. B. THE POSTERIOR NEUROPORE closes at about day 28 or the 25-somite stage
  5. The neural crests: when the neural folds fuse and, before the neural groove closes completely, some neuroectodermal cells detach themselves from the region where the neural tube borders on the ectoderm (lateral edges of the neural plate). These groups of cells are not incorporated into the neural tube, but form a neural crest over the tube
    1. THE CRESTS are distinct from both the neural tube as well as the overlying ectoderm after the neural groove is closed. They form longitudinal tracts which extend from the middle brain regions of the neural tube to its caudal end
    2. THE NEURAL CRESTS soon fragment into segments that give rise to the primordia of the ganglia. The segmentation corresponds to that of the somites, with each ganglionic primordium corresponding to a muscle primordium. The ganglia later give rise to general sensory innervation, and the corresponding level of the spinal cord gives motor innervation to the muscles
      1. Derivatives of the neural crests
        1. Ectomesenchyme is the result of mesenchymal differentiation of neuroblastic cells and is therefore of ectodermal origin. It gives rise to the dermis of the head, the meninges, the skeleton, and perhaps the musculature of the branchial arches
        2. Cells of the spinal ganglia, ganglia of the sympathetic and parasympathetic systems, and adrenal medulla
        3. Schwann cells that form the myelin sheaths of peripheral nerves
        4. Pigment cells, particularly of the skin

early nervous system development:  the neural tube and neural crest: image #1