142. The Spinal Cord: Differentiation of Nerve and Glial Cells

  1. Nerve cells: the neuroblasts or primitive nerve cells arise exclusively by division of the neuroepithelial cells. Once the neuroblasts are formed, they lose their ability to divide
    1. THE NEUROBLASTS OF THE ANTERIOR HORN are formed first, and only when most of these have migrated to the mantle layer does formation of nerve cells for the alar plate begin
    2. THE NEUROBLASTS INITIALLY have a central process extending to the lumen, the transient dendrite, but it disappears when the cells migrate to the mantle zone. The neuroblasts are temporarily round and are called apolar neuroblasts
    3. THERE IS FURTHER DIFFERENTIATION and 2 new cytoplasmic processes appear on opposite sides of the cell body. Thus, the bipolar neuroblasts are formed
    4. THE PROCESS AT ONE END ELONGATES rapidly to form the primitive axon, whereas that at the other end develops a number of cytoplasmic arborizations which are known as the primitive dendrites
    5. THE CELL IS NOW A MULTIPOLAR NEUROBLAST and with further development becomes the adult nerve cell or neuron
    6. THE AXONS OFTHE NEURONS in the posterior sensory horn behave differently than those in the anterior horn
      1. Those in the posterior horn penetrate the marginal layer of the cord and then ascend or descend to a higher or lower level (association neurons)
        1. The dendrites of the posterior horn grow peripherally to form the sensory part of the mixed nerv Their axons grow toward the posterior aspect of the tube to form the sensory roots. Their cell bodies lie in the dorsal root ganglia
      2. Those in the anterior horn break through the marginal zone and are seen on the ventral aspect of the cord where they form the anterior motor root of the spinal nerve (they conduct motor impulses from the cord to the muscles)
  2. Glia cells: the majority of primitive supporting cells are called glioblasts or spongioblasts and are formed by the neuroepithelial cells after the production of neuroblasts has ceased
    1. THE GLIOBLASTS migrate from the neuroepithelial layer to the mantle layer (some even to the marginal layer) where they differentiate into the fibrillar and protoplasmic astrocytes
    2. ANOTHER TYPE OF SUPPORTING CELL, possibly of glioblast origin, is the oligodendroglia cell
      1. This cell is mainly found in the marginal layer and forms the myelin sheaths around the ascending and descending axons in that layer
      2. They may be derived from mesenchyme cells which have penetrated into the central nervous system, thus, their origin from neuroepithelial cells is in doubt
    3. IN THE SECOND HALF OF DEVELOPMENT, a third type of supporting cell, the microglia cell, appears in the central nervous system. It is believed that its origin is from mesoderm which surrounds the neural tube
      1. They are macrophagic cells of the histiocyte system, with a neural localization, brought into the central nervous system by blood vessels. There is also some evidence, however, that the microglia could be ectomesenchymal.
  3. Fibers crossing over from both sides of the spinal cord form the so-called commissures, which connect the right and left portions of the gray matter

the spinal cord: differentiation of nerve and glial cells: image #1