As the spinal cord develops
neuroblasts of the neural tube’s mantle layer proliferate in 2 zones. In cross
section the mantle layer develops a characteristic “butterfly”-shape of gray
matter. The lateral walls of the tube thicken but leave a shallow, longitudinal
groove called the
sulcus limitans
which separates the developing gray matter into a dorsal
alar plate
and a ventral
basal plate.
These plates signal the future locations of sensory and motor functions,
respectively. The
sulcus limitans
extends the length of the spinal cord and through the mesencephalon.
Alar
plate
- Neuronal cell bodies here form nuclei which constitute the uninterrupted
dorsal gray matter (or columns) that receive and relay input from afferent
(sensory, somatic and visceral) neurons. It extends the length of the cord.
These neurons receive sensory information from axons in the dorsal roots of
spinal nerves.
Basal
plate
- Cell bodies here form the uninterrupted ventral gray matter (or columns) of
efferent neurons that extend the length of the cord. Axons of these neurons
project motor fibers to skeletal muscle and form the ventral roots of the spinal
nerves.
Proliferation of alar and basal plates causes accompanying changes. Bulges of
the plates result in the formation of the longitudinally running
dorsal and
ventral median septi. The lumen of the neural tube becomes reduced to a small
central canal.
The marginal layer increases in mass due to the addition of longitudinally
running intersegmental axons, long ascending axons from the gray matter, long
descending axons from supraspinal levels and incoming dorsal root sensory
fibers. The mass of fibers in the marginal layer subsequently becomes myelinated
(beginning in the 4th month) and is called white matter. The dorsal gray and
ventral gray columns partially segregate the white matter into
posterior,
ventral
and
lateral white funiculi.
