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