141. The Spinal Cord: Normal Development

  1. Neuroepithelial, mantle, and marginal layers
    1. THE WALL OF A RECENTLY CLOSED NEURAL TUBE consists of only one cell type, the neuroepithelial cells, which extend over the entire thickness of the wall and form a thick pseudostratified epithelium
      1. The cells are connected to each other by terminal bars at the lumen
      2. During interphase, when DNA synthesis takes place, the cells are wedge-shaped, with the broader portion containing the nucleus in the outer zone of the wall and a slender cytoplasmic portion extending toward the lumen
      3. Just after DNA synthesis, the nucleus begins to move toward the lumen, while the cell contracts toward the terminal bars
      4. During metaphase, the cells are round and in broad contact with the lumen, squeezing the thin cytoplasmic processes of neighboring nondividing cells
    2. DURING THE NEURAL GROOVE STAGE and just after tube closure, the neuroepithelial cells divide rapidly, resulting in the production of more cells, and we now refer to the thickened epithelium in the recently closed neural tube as the neuroepithelial layer or neuroepithelium
    3. ONCE THE TUBE IS CLOSED, the neuroepithelial cells give rise to another cell type characterized by a round nucleus with pale cytoplasm and a dark-staining nucleus, the primitive nerve cells or neuroblasts
      1. The neuroblasts form a zone that surrounds the neuroepithelial layer, called the mantle layer (the future gray matter of the spinal cord)
      2. The outermost layer of the cord contains the nerve fibers emerging from the neuroblasts in the mantle layer and is called the marginal layer
        1. As a result of myelination of the nerve fibers, the marginal layer takes on a "white" appearance and is called the white matter of the cord
  2. Basal, alar, roof and floor plates
    1. WITH THE CONTINUAL ADDITION OF NEUROBLASTS to the mantle layer, each side of the neural tube shows a ventral and dorsal thickening
      1. The ventral thickenings, the basal or motor plates, contain the anterior motor horn cells and form the motor areas of the spinal cord
      2. The dorsal thickenings, the alar or sensory plates, form the sensory areas of the cord
      3. A longitudinal groove, the sulcus limitans, is found bilaterally on the inner surface of the tub It marks the boundary between the anterior motor and posterior sensory areas of the cord and ends in the region of the mamillary recess in the ventral portion of the diencephalon
        1. One should not expect, therefore, any truly motor nerves emerging from the brain rostral to the mamillary recess
        2. Since all neural tissue rostral to this point is an extension of the alar plate, its function is regarded as sensory or associational
    2. THE THIN DORSAL PORTION AND VENTRAL MIDLINE PARTS of the tube are the roof and floor plates, respectively. They contain no neuroblasts and serve primarily as pathways for nerve fibers crossing from one side of the cord to the other
    3. THE BASAL PLATES bulge ventrally on each side of the midline as a result of the continuous enlargement of the neuroblasts, creating a deep longitudinal groove called the ventral fissure, which later will contain the anterior spinal artery
    4. THE ALAR PLATES expand predominantly in a medial direction, compressing the dorsal portion of the lumen of the neural tube
      1. The posterior median septum is formed where the 2 alar plates fuse in the midline
    5. ACCUMULATION OF NEURONS between alar and basal plates causes the formation of the intermediate horn, which contains motor neurons of the autonomic nervous system
    6. THE SPINAL CORD acquires its definitive form: motor horns anteriorly, sensory horns posteriorly, intermediate horns laterally, and a small lumen, the central canal

the spinal cord: normal development: image #1