Chapter 143. The Spinal Cord: Neural Crest Cells and Myelination - Review of Medical Embryology Book

DURING INVAGINATION OF THE NEURAL PLATE, a distinct group of cells appears along each edge of the neural groove, are ectodermal in origin, and are called the neural crest

The cells temporarily form an intermediate zone between the tube and surface ectoderm
The zone extends from the mesencephalon to the level of the caudal somite, and in time divides into 2 parts, each of which migrates to the dorsolateral aspect of the neural tube

Here the cells of the neural crest form a series of bipolar cell clusters that give rise to the sensory or dorsal root ganglia of the spinal and cranial nerves (V, VII, IX, and X), and multipolar cells of the future sympathetic ganglion cells

In the spinal cord, they either end in the dorsal horn or ascend through the marginal layer to one of the higher brain centers
Collectively, these processes are called the dorsal sensory root of the spinal nerve

The other process grows peripherally and forms fibers of the ventral motor root and participates in the formation of the trunk of the spinal nerve

IN ADDITION TO FORMING THE SENSORY GANGLIA, the cells of the neural crest differentiate into sympathetic neuroblasts, Schwann cells, pigment cells, odontoblasts, meninges, and cartilage cells of the branchial arches

Removal of neural crest cells of the trigeminal region results in facial abnormalities, including clefts of the primary palate

Myelination follows histogenesis and tends to occur later and persist longer as the systems are phylogenetically more recent

MYELINATION OF THE PERIPHERAL NERVE is brought about by the neurilemma cells or cells of Schwann, beginning in month 4 of fetal life*

These cells originate from the neural crest, migrate peripherally, and wrap around the axons to form the neurilemma sheath
Axons, varying in number from 1 to 20, can be enwrapped by one neurilemma cell

AT MONTH 4 OF FETAL LIFE, the nerve fibers gradually obtain a whitish appearance as a result of myelin deposition between the axon and the neurilemma

BOTH THE NEURILEMMA AND THE MYELIN SHEATH of the peripheral nerve fibers are formed by the cells of Schwann
THE MYELIN SHEATH OF NERVE FIBERS within the spinal cord is of different origin, being formed by the oligodendroglia cells
ALTHOUGH MYELINATION OF NERVE FIBERS IN THE CORD generally begins at about month 4 of fetal life, some motor fibers that descend from higher brain centers to the cord do not become myelinated until the first year of postnatal life

The tracts in the nervous system apparently become myelinated at about the time they begin to function

It appears in the vestibulospinal tract in month 6, and in the rubrospinal tract in month 7

Some motor fibers coming from the upper cerebral centers, the pyramidal tract, for example, are myelinated during the 2 years after birth
The slowness of this process and the correlation between myelination and the final development of functional ability partly explain the long duration of psychomotor development in the child

*Myelination in the interior of the CNS is furnished by the neuroglial cells (oligodendrites) derived from the neural tube.