Oligodendrocytes

Oligodendrocytes are the key cells in myelination of the CNS. They are cells of moderate size with a small number of short, branched processes. They are the predominant type of neuroglia in white matter and are frequently found interposed between myelinated axons. Actual connections between oligodendrocytes and myelin sheaths can be observed. In the gray matter they aggregate closely around neuronal cell bodies, where they are called satellite oligodendrocytes. PNS myelin is formed by Schwann cells. The CNS myelin membranes originate from and are part of the oligodendroglial cell membrane. The oligodendro-cytes form flat cell processes, which are wrapped around the nerve axon in a spiral fashion (Fig. 1.8).

Myelin Unrolled Oligodendrocyte
Fig. 1.8. Diagram showing the axon being rolled in the myelin sheath

Fig. 1.9. Impression of the three-dimensional structure of oligodendrocytes with their plasma membrane extensions as myelin sheaths covering the axons that cross their region

Fig. 1.9. Impression of the three-dimensional structure of oligodendrocytes with their plasma membrane extensions as myelin sheaths covering the axons that cross their region

Oligodendrocytes

With the exception of the outer and lateral loops of the flat cell processes, the cellular cytoplasm disappears from these processes and the remaining cell membranes condense into a compact structure in which each membrane is closely apposed to the adjacent one. If myelin were unrolled from the axon it would be a flat, spade-shaped sheet surrounded by a tube containing cytoplasm.

Although the myelin sheath is an extension of the oligodendroglial cell membrane, the chemical composition of myelin is quite different from that of the oligodendroglial cell membrane. The oligoden-droglial cell membrane is transformed into myelin in processes of modification and differentiation.

On the same axon, adjacent myelin segments belong to different oligodendrocytes. A single oligodendrocyte provides the myelin for many internodal segments of different axons simultaneously. One oligo-dendrocyte can be responsible for the production and maintenance of up to 40 nerve fibers (Fig. 1.9). This has implications for disease conditions and repara-tive processes, as the destruction of even only a few oligodendrocytes can have an extensive demyelinat-ing effect.

Together with the Schwann cells of the PNS, oligo-dendrocytes are unique in their ability to produce vast amounts of a characteristic unit membrane. The ratio between cell body surface membrane and myelin membrane is estimated at 1:620 in the case of oligodendrocytes. The deposition and maintenance of such large expanses of membrane require optimal coordination of the synthesis of its various lipid and protein components and their interaction to ensure production of a stable membrane on the one hand and a well-regulated and controlled breakdown and replacement of spent components needed to support the myelin membrane on the other.

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