The myelinization takes place first in connection with histogenesis and that from the 4th month of fetal development. Myelinization of the peripheral nervous systems precedes that of the CNS. Indeed, this process begins in the root fibers of the spinal nerves. In this the motor roots are myelinized before those of the sensory roots. In contrast, the sensory nerve cells of the CNS obtain their myelin sheaths before the motor neurons
The glia cells that are responsible for myelinization stem from different embryonic predecessors:
- For myelinization in the CNS the oligodendrocytes, which arise out of the ventricular zone of the neural tube, are responsible.
- In the PNS myelinization takes place through Schwann’s cells, which are descendents of the neural crest.
Myelinization in the central nervous system
Oligodendrocytes are responsible for myelinization in the CNS. They stem from the ventricular zone of the neural tube
Each oligodendrocyte produces several offshoots each of which wraps around various neighboring axons. The membrane layers that come to lie on top of each other in spiral wrappings of an axon section, myelinize an internode, i.e., the section of an axon between two neighboring nodes of Ranvier. In the region of the nodes the axons are without their myelin sheath so the astrocyte processes can thrust in between the processes of the olidodendrocytes.
Myelinization in the peripheral nervous system
In the peripheral nervous system (PNS) myelinization of the axons results from the Schwann’s cells. These arise from the neural crest.
Whether myelinized or myelin-poor: the axons of the peripheral nerves are always surrounded by Schwann’s cells. Myelin-poor nerve fibers are characterized by the fact that they sink, individually or severally, into the grooves formed by the cellular membrane of a Schwann’s cell. Thus, in the PNS, there are essentially two forms of axons
- Myelin-poor fibers
Myelin-poor nerve fibers are formed by axons that individually or severally sink into a Schwann’s cell without the wrapping of the corresponding internodes by cellular processes of Schwann’s cells taking place
- Myelinized nerve fibers
A myelinized nerve fiber in the PNS consists of a single axon. Each of its internodes (section between neighboring nodes of Ranvier) becomes wrapped by the cellular process of a Schwann’s cell
During the myelinization in the PNS an axon sinks into a groove of a Schwann’s cell. Through this, an almost complete wrapping takes place, only leaving a narrow gap, the mesaxon, that leads from the axon to the surface. Via the formation of a considerable amount of cell membrane material a wrapping of the axon (up to 100 layers) occurs. At the beginning cytoplasma is still to be found within the cytoplasma process of a Schwann’s cell. Gradually, this is suppressed until the membrane layers stick to one another. Thus a lipoprotein complex is formed that represents the compact myelin sheath.
At birth myelinization is not yet complete. To the contrary, this process reaches its maximum during the first six months of life and continues, when less pronounced, into puberty and beyond. The myelinization of the connective apparatus (tractus corticospinales) continues up to the second year of age.
Myelin is lipoprotein type of substance that both protects the axons as well as isolates them electrically. Through this the stimulus conduction of is considerably accelerated (from 1 m/s in myelin-poor fibers up to 150 m/s in myelinized ones). Only the axons are myelinized, never, though, the dendrites.