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
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
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
THERE IS FURTHER DIFFERENTIATION and 2 new cytoplasmic processes appear on opposite sides of the cell
body. Thus, the bipolar neuroblasts are formed
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
THE CELL IS NOW A MULTIPOLAR NEUROBLAST and with further development becomes the adult nerve cell
or neuron
THE AXONS OFTHE NEURONS in the posterior sensory horn behave differently than those in the anterior
horn
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)
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
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)
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
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
ANOTHER TYPE OF SUPPORTING CELL, possibly of glioblast origin, is the oligodendroglia cell
This cell is mainly found in the marginal layer and forms the myelin sheaths around the ascending and
descending axons in that layer
They may be derived from mesenchyme cells which have penetrated into the central nervous system, thus,
their origin from neuroepithelial cells is in doubt
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
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.
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