Module
22
Icon module 22
Nervous system

Morphogenesis of the spinal cord

The proliferation and differentiation of the neuroepithelial cells of the neural tube leads to a thickening of the side walls to the disadvantage of the roof, of the floor and of the central cavity. Of these, in the region of the spinal cord, only the very narrow central canal remains.
From the 4th week, through local distension of the posterior and anterior portions of the side wall of the a longitudinal groove, the sulcus limitans, forms midway up the side (stage 12).

From the dorsal thickenings, the alar plate, the posterior sensory portion of the spinal cord emerges and from the anterior thickenings, the basal plate, the anterior motor portion.
Moreover, in the thoracolumbar segments (T1 – L3) as well as the sacral segments (S1 – S3), between the alar and basal plates, a protrusion of gray matter appears on the side that is termed a lateral horn. It contains the perikarya of the autonomous nervous system.
The dorsal and ventral portions in the symmetry level are designated as roof and floor plates. Especially at these locations commissure fibers change from one side of the body to the other.
As was already presented, the floor plate plays an essential role in establishing the dorso-ventral polarity as well as in the induction of the motoneurons. A question that has yet to be conclusively answered concerns the origin of the floor plate cells. Certain scientists assume that these neurons could stem from the chorda dorsalis (notochord) and not from neuroectoderm

Fig. 32 - Functional articulation of the
spinal cord in stage 9
media/module22/v5d_developsegmentPOP1.gif

1
Neural tube
2
Central canal
3a
Gray matter (sensory)
3b
Gray matter (motor)
4
White matter

Fig. 33 - Functional articulation of the
spinal cord in stage 13
media/module22/v5d_developsegmentPOP2.gif

5
Roof plate
6
Sensory alar plate
7
Sulcus limitans
8
Motor basal plate
9
Floor plate

Legend
Fig. 32

Primitive neural tube with formation of the peri-ependymal mantle zone from which the future grey matter emerges.

Fig. 33

From the 4th week there is differing growth of the side, anterior and posterior segments of the neural tube. The ventrolateral and dorsolateral thickenings that appear are delimited from one another by a longitudinal groove, the sulcus limitans. On the other hand, the dorsomedian and the ventromedian portions remain thinner and form the roof and floor plates of the neural tube.

Fig. 34 - Development of the spinal cord
segments in stage 19
media/module22/v5d_developsegmentPOP3.gif

10
Posterior root (sensory)
11
Anterior root (motor)

Fig. 35 - Development of the spinal cord
segments in the 10th week
media/module22/v5d_developsegmentPOP4.gif

12
Posterior tract
(white matter)
13
Lateral tract
(white matter)
14
Anterior tract
(white matter)
15
Septum medianum dorsale
16
Posterior horn
(gray matter)
17
Lateral horn
(gray matter)
18
Anterior horn
(gray matter)
19
Fissura mediana ventralis

Legend
Fig. 34

Through the growth of the alar and basal plates, the side walls of the central cavity system approach one another until they fuse. This becomes thereby a narrow, ependyma- coated central canal with a longitudinal sulcus limitans.

Fig. 35

At the level of the thoracolumbar segments and the sacral segments, between the alar and basal plates arises on alls sides a lateral distension of the gray matter, the lateral horn. This contains the perikarya of the neurons of the autonomous (vegetative) nervous system. The white matter can be subdivided into three tracts (posterior, lateral, and anterior) which surround the gray matter and give it its characteristic butterfly shape.