Neuroscience Online (i,8,13)

Oligodendroglia

Oligodendrocytes are also located in both gray and white matter. They are the predominant cell type in white matter where they are often located as rows of cells between groups of neuronal processes. These are termed interfascicular oligodendroglia and are involved in the formation and maintenance of the myelin surrounding the neuronal processes nearby. In gray matter, oligodendroglia are usually located near neurons and, therefore, are known as perineuronal satellite cells. Cell bodies of oligodendroglia are often located near capillaries, but they lack the definite perivascular end feet characteristic of astrocytes.

The processes of oligodendrocytes are fewer and more delicate than astrocytes, and the cell body shape is polygonal to spherical. The oligodendrocyte nucleus is smaller than that of the astrocyte, is eccentrically located in the cell body, contains clumps of chromatin and can be stained by alkaline dyes. The cytoplasm of oligodendrocytes tends to be darker than that of astrocytes with silver stains, and does not contain glial fibrils (although they do contain microtubules).

The role of oligodendroglia in the central nervous system, particularly of the interfascicular oligodendrocytes, is the formation and maintenance of myelin. Myelin is the sleeve of membranous material described by Dr. Byrne, that wraps the neuronal axon as shown in Figure 8.15 to facilitate the conduction of the action potential through saltatory conduction. Myelin is composed of concentric layers of membranes compacted against one another with an internal (i.e. against the nerve fiber) and an external collar of cytoplasm. As shown in Figure 8.15, a single oligodendrocyte contributes to the myelination of several adjacent nerve processes. Moreover, more than one oligodendrocyte contributes to the myelination of a single internode of an axon. The lamellae of myelin membranes result from the spiral wrapping of the axon by cytoplasmic processes of interfascicular oligodendroglia. Also, the oligodendrocyte forming a particular myelin internode (i.e. the myelin between two nodes) is seldom seen directly adjacent to the myelin-wrapped process. This is because thin cytoplasmic bridges connect the region of the oligodendrocyte cell body to the external wrap of myelin. It is important to note that the region of the axon exposed at the node of Ranvier is not bare. It may be the site of branching of the axon, the site of synaptic contacts, or it may be covered with various glial processes. The axon in the nodal region usually contains concentrations of organelles, especially mitochondria.

In the Peripheral Nervous System (PNS), Schwann cells are responsible for the myelin formation. These cells myelinate axons differently than the interfascicular oligodendroglia. As shown in Figure 8.16, they migrate around the axon, laying a membrane covering around the axon by squeezing out the cytoplasm of the Schwann cell. Also, every internode of a PNS axon represents a single Schwann cell. In addition unmyelinated axons in the PNS are also enclosed by membranes formed by Schwann cells.