Oligodendrocytes

The oligodendrocytes are neuroglia support cells that produce and maintain the myelin sheath of neuron axons in the central nervous system. Being slightly smaller than astrocytes, they are round cells with thin outgrowth processes. They surround the nerve cell body and its axon. These highly specialized cells are characterized by a highly active protein and nuclein metabolism. They are also responsible for the transport of matter to the neuron. The network of oligodendrocytes is called 'oligodendroglia', which is part of the neuroglia (the central nervous system support tissue). Function: production and maintenance of axon myelin. The nature and function of these cells were discovered and studied by Spanish pioneer histologist Pio del Rio Ortega.

Oligodendrocytes are found throughout the central nervous system (brain and spinal cord). They provide a supportive role to the brain's neurons by covering their axons with a layer of fatty, insulating material called myelin. Not only does this myelin sheath protect the axon but it also speeds up the transmission of electro-chemical nerve impulses. Oligodendrocytes arise from oligodendrocyte precursor cells (or OPCs), which undergo a process of maturation that takes around three or four days. They can be produced and regenerated throughout the life of an individual. It is a process which continues throughout adulthood.

There are two types of oligodendroglia: 1) perineuronal satellite oligodendrocytes, which are closely associated with neuron bodies and dendrites in the cerebral cortex (grey matter); 2) interfascicular oligodendrocytes, which are involved in myelination of nerve cell axons in the white matter. The large number processes of individual oligodendrocytes form the myelinated internodes for as many as 80 axons. Myelination of neuron axons begins right before birth. However, most pathways in the human brain are not fully myelinated until two years after birth. Oligodendrocytes also take part in the re-myelination which sometimes takes place after loss of myelin sheath due to an acute or chronic inflammation caused by metabolic diseases, autoimmune medical conditions (multiple sclerosis) and alcoholism.

Structure

Viewed through an electron microscope, oligodendrocytes have a round body, with a round or oval nucleus, which is eccentrically located, thus leaving a large mass of cytoplasm at one pole of the cell. However, in interfascicullar oligodendrocytes, the neucleus can be observed at the center of cytoplasm. Their cytoplasm is rich in mitochondria, which play a key role in the production of myelin. This is the reason for oligodendrocytes having a highly active metabolism, producing large amounts of ATP. The nuclear content exhibits slight clumping of chromatin which appears as a rim. The cytoplasm is also rich in ribosomes, either free or associated with the endoplasmic reticulum. The Golgi apparatus is clearly seen and well developed. Oligodendrocytes neither have glycogen granules nor bundles of specific intermediate filaments, such as glial fibrillary acidic protein, which are characteristic of astrocytes.

Oligodendrocytes of cerebral white matter with intricate processes.

Oligodendrocytes type I and II arranged to show the variation in the orientation of their long thin processes.