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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:

  1. 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.

  2. 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.

  3. 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.).
Fig. 36 - Histogenesis of the spinal cord

Membrana limitans interna
Membrana limitans externa
Neuroblast in migration
Radial glia

Fig. 36

Neuroblasts arise out of the ventricular zone. Following their migration, neuroblasts form the intermediate zone (mantle layer) that develops into gray matter. The axons of the neurons pass through the marginal zone, whereby the white matter arises. The pia mater overlays this from the outside.

Cave: the ependyma cells differentiate only after the formation of neuroblasts in the ventricular zone is completed in stage 19 (46th day).

NB: the gray matter of the spinal cord and brain stem lie centrally (thus surrounding the central cavity system). However, in the pallium of the telencephalon and in the cerebellum the gray matter can also be encountered on the surface (peripherally) in the form of cerebral cortex!

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)
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The interneurons.

Fig. 37 - Thoraco-lumbar spinal cord
at around the 10th week

  1. Posterior horn
  2. Viscerosensory part of the lateral horn
  3. Visceromotor part of the lateral horn
  4. Anterior horn
  5. Anterior tract
  6. Lateral tract
  7. Posterior tract

Fig. 37

In the thoracolumbar and the sacral spinal cord segments, between the dorsal horn and the ventral horn, an accumulation of gray matter develops that is called the lateral horn. This contains the perikarya of the vegetative nervous system. The white matter is subdivided into three tracts (posterior, lateral and anterior tracts) that surround the butterfly-shaped gray matter.

Fig. 38 - Functional Organization of the spinal cord

Somatosensory neuron
Viscerosensory neuron
Visceromotor neuron
Somatomotor neuron
Posterior horn
Lateral horn
Anterior horn
Sensory dorsal root of the spinal nerves
Spinal ganglion
Mixed spinal nerve
Motor ventral root of the spinal nerves
Pseudounipolar neuron in the spinal ganglion
Somatosensory region (interneurons)
Viscerosensory region (interneurons)
Visceromotor region (motoneurons)
Somatomotor region (motoneurons)

Fig. 38

From a functional point of view the spinal cord can be divided into  afferent (dorsal) and an efferent (ventral) part. The interneurons receive the information from the sensory spinal ganglion cells and give the information further in integrated form to the motoneurons. All neurons, whose perikarya lie in the gray matter of the spinal cord, are multipolar neurons.

A cross-section through the spinal cord reveals the butterfly-shaped profile of the gray matter. The dorsal offshoots form the posterior horns, the ventral offshoots the anterior horns. In the thoracolumbar and sacral segments, the lateral horns are added. These differing regions of the gray matter contain the perikarya of neurons, each of which has a specific task.