Glial Cells: Structure, Function & Diseases

Glial cells are located in the nervous system and are structurally and functionally distinct from neurons. According to recent findings, they play a significant role in information processing in the brain as well as throughout the nervous system. Many neurological diseases are due to pathological changes in glial cells.

What are glial cells?

Glial cells, along with neurons, are involved in building the nervous system. They embody many different cell types that are structurally and functionally distinct from one another. Rudolf Virchow, the discoverer of glial cells, viewed them as a kind of glue to hold the nerve cells together in the nervous tissue. Hence, he gave them the name glial cells, the root word “glia” being derived from the Greek word “gliokytoi” meaning glue. Until the recent past, their importance for the function of the nervous system was underestimated. According to recent research findings, however, glial cells are very actively involved in information processing. Humans have about ten times more glial cells than neurons. It has even been found that the ratio of glial cells to nerve cells is decisive for the speed of nerve stimulus transmission and thus also for thought processes. The more glial cells present, the faster the information processing.

Anatomy and structure

Glial cells can be roughly divided into three functionally and structurally distinct cell types. The main part in the brain is formed by the so-called astrocytes. Thus, about 80 percent of the brain is composed of astrocytes. These cells have a star-shaped structure and are located preferably at the contact points (synapses) of the nerve cells. Another group of glial cells are the oligodendrocytes. They surround the axons (nerve processes) that connect the individual nerve cells (neurons). Astrocytes and oligodendrocytes are also called macroglial cells. In addition to macroglial cells, there are also microglial cells. They are present everywhere in the brain. While macroglial cells originate in the ectodermal cotyledon (outer layer of the embryoblast), microglial cells originate in the mesoderm. In the peripheral nervous system, the so-called Schwann cells play a role. Schwann cells are also of ectodermal origin and perform functions similar to those of oligodendrocytes in the brain. Here, too, they surround the axons and supply them. In addition, there are some special forms. For example, the so-called Müller supporting cells are the astrocytes of the retina. Furthermore, there are pituicytes, which are the glial cells of the posterior lobe of the pituitary gland. The HHL is composed of 25-30 percent pituicytes. Their function is not yet fully understood.

Function and tasks

Overall, glial cells perform multiple functions. Astrocytes or astroglia represent the majority of glial cells present in the nervous system. They participate significantly in fluid regulation in the brain. In this process, they also ensure the maintenance of the potassium balance. The potassium ions released during stimulus transmission are taken up by the astrocytes, whereby they simultaneously regulate the extracellular pH balance in the brain. Astrocytes have a special significance in participating in cerebral information processing. Their vesicles contain the neurotransmitter glutamate, which when released leads to the activation of neighboring neurons. In this way, astrocytes ensure that signals travel long distances in the body and are simultaneously processed further for other neurons. They thus differentiate the meaning of individual pieces of information. In addition to moderating the information, they also determine where it should be forwarded. Thus, they are responsible for the permanent building and rebuilding of the information network in the brain. Without astrocytes, the transmission of information would be very laborious. Only through the complex cooperation of astrocytes and neurons is the learning process and thus the formation of intelligence possible. Oligodendrocytes, in turn, form the myelin around the nerve cords. The more certain information strands are developed, the thicker the nerve strands become and the more myelin is needed. The third type of glial cells, microglial cells, react similarly to the macrophages of the immune system to pathogens, toxins and dead endogenous cells in the brain. Since antibodies cannot enter the brain through the blood-brain barrier, this task is performed by the microglial cells.Microglial cells are divided into resting and active cells. The resting cells monitor the processes in their environment. When disturbed by injury or infection, they become freely mobile, migrate like amoebae to the appropriate site, and begin their defense and clean-up function. Overall, it is becoming increasingly clear that glial cells not only have support functions, but are significantly responsible for the performance of the brain and nervous system.

Diseases

In this context, there is also a growing recognition of the importance of glial cells in health. In many neurological diseases, striking changes are observed within glial cells. For example, schizophrenia often breaks out in adolescence, when not all axons are yet coated with myelin. Very few oligodendrocytes, which are responsible for myelin formation, are detected in the corresponding patients. It is also possible that some of the genes important for myelin formation are altered. In multiple sclerosis, the myelin sheath is often destroyed. As a result, the exposed nerve processes can no longer transmit signals and the cut neurons die. Hereditary leukodystrophy is a progressive destruction of the white matter of the nervous system. In this process, the myelin surrounding the nerves is degraded. The result is a massive impairment of the nerves. Affected individuals suffer from motor and other neurological disorders. Finally, some brain tumors take their starting point in the uncontrolled growth of glial cells.