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These cells have rounded or pear-shaped bodies with relatively few processes (olig = scanty). These cells provide myelin sheaths to nerve fibres that lie within the brain and spinal cord. Their relationship to nerve fibres is basically similar to that of Schwann cells to peripheral nerve fibres. However, in contrast to a Schwann cell that ensheaths only one axon, an oligodendrocyte may enclose several axons. Oligodendrocytes are classified into several types depending on the number of neurons they provide sheaths to. As a rule oligodendrocytes present in relation to large diameter axons provide sheaths to fewer axons than those related to axons of small diameter. The plasma membranes of oligodendrocytes comes into contact with axolemma at nodes of Ranvier.
The composition and structure of myelin sheaths formed by oligodendrocytes show differences from those formed by Schwann cells. The two are different in protein content and can be distinguished by immunocytochemical methods. As damage to neurons within the central nervous system is not followed by regeneration, oligodendrocytes have no role to play in this respect. Also note that in multiple sclerosis myelin formed by oligodendrocytes undergoes degeneration, but that derived from Schwann cells is spared.
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