Histogenesis of the spinal cord
As was already presented, the proliferation and differentiation of the neuroepithelial cells goes out from three concentric cell layers. From the inmost to the outer one, these are:
- The ventricular zone, out of which the neuroblasts, the glioblasts and – after they have been formed – the ependyma cells that arise as a coating of the cavity system.
- The intermediate zone or mantle layer out of which the gray matter develops. It contains the cellular bodies of the nerve cells that have migrated out of the ventricular zone.
- The marginal zone out of which the white matter arises. The neuronal axons pass through it. Connections from the spinal cord to the brain are termed ascending ones while those going in the opposite direction from the brain to the spinal cord are said to be descending pathways. In each half of the spinal cord three tracts can be distinguished: the posterior, the lateral and the posterior ones. Through it pass fiber bundles that, as a rule, are named according to their origin and target (tractus corticospinalis, vestibulospinalis, spinothalamicus, etc.).
Due to the continuous accumulation of neuroblasts in the intermediate zone there arise:
- The ventral thickening or basal plate that contains the motoneurons. There axons leave the spinal cord via the ventral roots of the spinal nerves. They are the first axons that emerge from the spinal cord.
- The dorsal thickening or alar plate that contains the interneurons or association cells. These create the connection between the central processes of the spinal ganglion cells (that arise from the neural crest) and the motoneurons.
- The sulcus limitans at the level of which the viscerosensory and visceromotor lateral horns develop in the thoracolumbar medulla (sympathetic) as well as in the sacral medulla (parasympathetic)