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| Table 1. Neuroglial cells | |
| Name | Description |
| Gliocytes of the central nervous system | |
| Radial Neuroglia | The first glial cells to appear in embryonic development. The nucleus is in the epithelium lining the neurocoele, and a long cytoplasmic process extends to the outside surface of the neural tube. Radial neuroglial cells persist into adult life in submammalian vertebrates. In mammals, they change into ependymocytes and astrocytes. |
| Ependymocytes | These form a columnar epithelium (the ependyma) that lines the neurocoele in the adult. Most ependymocytes have cilia. The basal surface of each cell has cytoplasmic processes that anchor it to the underlying nervous tissue. Ependymal astrocytes are cells with much-branched processes, in submammalian vertebrates. An ependymocyte with a single, long basal process is a tanycyte. These occur in the third ventricle in mammals. Special ependymal cells form the choroidal epithelium on the surfaces of the choroid plexuses, which produce the cerebrospinal fluid that fills the neurocoele. |
| Astrocytes | The "star cells" have numerous processes. They are present everywhere in the brain and spinal cord. The cytoplasm contains glycogen granules and intermediate filaments (8-9 nm diameter) made of glial fibrillary acidic protein (GFAP).
Fibrous astrocytes contain large quantities of this protein; they occur in tracts of nerve fibers and near the surface of the brain. Many of the cytoplasmic processes abut as end-feet on the basal laminae of capillary blood vessels. Astrocytic end-feet also form the thin external glial limiting membrane at the outside surface of the brain and spinal cord and around the larger blood vessels. There is a similar internal glial limiting membrane, subjacent to the ependyma. Protoplasmic astrocytes (also called velate astrocytes) have veil-like processes, which mingle with the tangled axons and dendrites of neurons. They contain less GFAP than fibrous astrocytes, and most of their processes are too thin to be resolved individually by light microscopy. Pituicytes, the glial cells of the neurohypophysis are atypical astrocytes. Olfactory ensheathing cells invest primary olfactory axons on the surface of the olfactory bulb. They, like the primary olfactory neurons, are derived from the olfactory placode of the embryo. They have structural and chemical features in common with astrocytes and peripheral gliocytes. Reactive astrocytes are seen at sites of injury: normal astrocytes enlarge and grow many new processes containing GFAP. |
| Oligodendrocytes | The name means "cells with few branches." They have smaller, denser nuclei than astrocytes. The cytoplasm contains rough endoplasmic reticulum, polyribosomes and microtubules (25 nm), but no glycogen or GFAP. Oligodendrocytes are seen in rows between the bundles of nerve fibers: interfascicular oligodendrocytes. These cells form myelin in the central nervous system. Satellite oligodendrocytes occur, together with astrocytes, around the cell-bodies of large neurons. |
| Resting microglial cells | The rarest normal glial cells resemble oligodendrocytes, but are more irregularly shaped and have elongated rather than round nuclei. They are of mesodermal origin, being derived from blood cells (monocytes).
Reactive microglia In injured or diseased central nervous tissue, these mesodermally derived phagocytic cells participate in inflammatory reactions. Some are activated resting microglial cells. More are monocytes (a type of white blood cell), that have migrated through the walls of small vessels into the abnormal nervous tissue. Pericytes, cells adjacent to some capillary blood vessels in the normal brain, also are derived from monocytes. |
| Gliocytes of the peripheral nervous system | |
| Neurolemmocytes (Schwann cells) | These tube-shaped cells with elongated nuclei intimately ensheath all axons in all parts of the peripheral nervous system, including nerve roots and peripheral nerves. Each axon is suspended in the cytoplasm of its Schwann cell by a double layer of surface membrane, the mesaxon.
The myelin sheaths of peripheral nerve fibers are formed by Schwann cells. A myelinated axon is exposed to extracellular fluid at regular intervals along its length, where there are short gaps between adjacent neurolemmocytes. The gaps are called nodes (of Ranvier). One Schwann cell ensheaths either one myelinated axon or several unmyelinated axons. The surface of an unmyelinated axon is in contact with extracellular fluid along its whole length, through the cleft between the layers of its mesaxon. This cleft is closed off by the formation of a myelin sheath. On the outside surface of each Schwann cell, there is a basal lamina. |
| Ganglionic gliocytes (more often called satellite cells) | In sensory and autonomic ganglia, these cells intimately surround the neuronal somata. Ganglia also contain Schwann cells (around axons). The neuroglial cells of the enteric nervous system, are also derived from the neural crest, but they have structural and chemical features in common with both astrocytes and peripheral gliocytes. No special name has been given to the enteric glial cells. |
| Invertebrate neuroglia | |
| Invertebrate animals also have gliocytes. They are similar to astrocytes, and they intimately invest the neurons. The somata of the very large neurons of some invertebrates are deeply invaginated by glial cytoplasmic processes, which form a trophosporigium ("feeding sponge"). No invertebrate has myelin, but in arthropods several layers of glial cytoplasm often surround an axon. | |
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Last updated: January 2003