Oligodendrocytes are a specialized type of glial cell in the central nervous system. Their primary function is to produce and maintain the myelin sheath, a fatty layer that insulates the axons of neurons. This insulation allows for the rapid transmission of electrical signals, and the health of these cells directly impacts nerve impulse speed.
Morphological Characteristics
When viewed through a microscope, a mature oligodendrocyte has a distinct appearance. The cell body, or soma, is small and contains a round, dense nucleus that stains darkly with histological dyes. This nucleus is enveloped by a minimal amount of cytoplasm, contributing to its compact look and sometimes creating a “fried egg” appearance.
From the cell body, numerous delicate cytoplasmic processes extend to wrap around the axons of nearby neurons, forming segments of the myelin sheath. An individual oligodendrocyte can myelinate multiple axons simultaneously, with some having up to 50 processes. These fine extensions are often challenging to see with standard light microscopy.
The shape and complexity of oligodendrocytes can vary depending on their location and developmental stage. For instance, those in the white matter may have a different arrangement of processes compared to those in the gray matter. This structural diversity reflects their adaptation to the needs of the local neuronal circuits they support.
Visualization Techniques and Staining
Scientists employ specialized staining techniques to visualize oligodendrocytes. Standard stains like Hematoxylin and Eosin (H&E) are useful for identifying the cell’s nucleus but provide limited information about the myelin sheaths.
To highlight the myelin, special stains such as Luxol Fast Blue (LFB) are used. LFB has a high affinity for the lipoproteins that constitute myelin, staining the sheaths a vibrant blue or green. This allows researchers to infer the health of oligodendrocyte populations by observing the integrity of myelination.
Immunohistochemistry (IHC) is a technique used for highly specific identification. This method uses antibodies that bind to unique proteins found in oligodendrocytes. Proteins such as Olig2 and SOX10 are used to label cells of the oligodendrocyte lineage, while markers for Myelin Basic Protein (MBP) or Proteolipid Protein (PLP) identify mature, myelin-producing cells.
Electron microscopy (EM) offers the highest level of detail, revealing the cell’s ultrastructure. Under an electron microscope, the electron-dense cytoplasm is visible, as are the concentric layers of the myelin sheath around an axon. This technique allows for detailed analysis of the myelin’s compact structure.
Distinguishing Oligodendrocytes from Other CNS Cells
In the central nervous system, it is necessary to differentiate oligodendrocytes from other cells. Astrocytes, for example, have a larger, more irregularly shaped nucleus that is less dense than that of an oligodendrocyte. Astrocytes also exhibit a star-like shape with numerous branching processes and can be identified by a protein marker called GFAP.
Microglia, the immune cells of the CNS, are distinguished by their small, elongated, or rod-shaped nuclei. Their cell bodies are typically smaller than those of oligodendrocytes, and they possess their own unique protein marker, Iba1, for positive identification in tissue samples.
The distinction can be made between oligodendrocytes and neurons. Neurons are generally much larger cells with a large, pale-staining nucleus and a prominent nucleolus. These features, combined with their characteristic shape, make them easily distinguishable from the smaller, more compact oligodendrocytes.
Appearance in Pathological Conditions
The appearance of oligodendrocytes and the myelin they produce changes in pathological conditions, such as demyelinating diseases like multiple sclerosis (MS). In an active MS lesion, microscopic examination reveals a significant loss of myelin. When stained with Luxol Fast Blue, these areas appear pale, in stark contrast to the vibrant blue of healthy tissue.
Within these lesions, there is a reduction in mature oligodendrocyte cell bodies, as these cells are often destroyed by the inflammatory process. The area is infiltrated by immune cells, which contribute to the ongoing damage to both myelin and the oligodendrocytes. This attack on the cells can lead to the disruption of nerve signals, causing the neurological symptoms associated with MS.
In some cases, particularly in the early stages of a lesion or during remission, there may be signs of the body attempting to repair the damage. This is visible through the presence of oligodendrocyte precursor cells (OPCs). These immature cells migrate to the site of injury to differentiate into mature oligodendrocytes and remyelinate the denuded axons, though this repair process is often incomplete.