Sheath and Satellite Cell Formation

Oligodendrocytes in the CNS and Schwann cells in the PNS form myelin sheaths by similar processes. In an action similar to the continuous wrapping of a bolt of cloth, the oligodendroglial cell membrane becomes wrapped around the axon many times. As the wrapping occurs, the oligodendroglial cytoplasm retracts or is extruded so that the two layers of the cell’s plasma membrane, which originally were separated by cytoplasm, come together and fuse. Except for small islands of cytoplasm, which may be trapped between the fused membranes, the fusion is complete. The cell membrane of the myelinating oligodendrocyte, like cell membranes elsewhere, is composed of alternate layers of lipid and protein molecules. Thus myelin is made up of numerous fused layers of lipoprotein membrane.

Myelination is closely associated with the development of the functional capacity of neurons. Unmyelinated neurons have a low conduction velocity and show fatigue earlier, whereas myelinated neurons fire rapidly and have a long period of activity before fatigue occurs. Neurons that ultimately are capable of rapid transmission of impulses become fully functional at about the time their axons become completely insulated with a myelin sheath. In general, the motor neurons of cranial nerves become myelinated before their sensory counterparts. The sensory neurons of the trigeminal nerve and the cochlear division of the vestibulocochlear nerve begin to acquire myelin only in the fifth and sixth months of development. The optic nerve neurons begin to be sheathed at birth, and myelination is completed by the end of the second week after birth.

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