Glial cells, also known as neuroglia, are an essential part of the nerve tissue. Also, there are different types of glia, and this is determined by their location and function: astrocytes, oligodendrocytes, microglia, and ependymal cells, located in the Central Nervous System; and Schwann cells, Satellite cells, and Müller cells, found in the Peripheral Nervous System.
In this article, we answer the questions: What are glial cells? And, what is their function in the nervous tissue? Besides, we describe the different types of glial cells and tell you about some pathologies related to the dysfunction of these, such as glioblastoma.
The word 'glia' comes from the Greek word 'γλία' and this means 'tie' or 'union.' And thus, this term is used for cells that serve to unite or 'glue' together neurons to nerve tissue. This is why these cells are also called 'neuroglia,' 'glial cells,' or even 'non-neural cells in nerve tissue.' Some call them 'non-neural cells in nerve tissue' since their function isn't necessarily to generate or receive nerve impulses (the neurons take care of this themselves).
Instead, glial cells act as a kind of necessary support or base, so that nerve cells are able to stimulate each other. On the other hand, glial cells also aid in neuronal metabolism; particularly thanks to the significant amount of glycogen, lipids, and carbohydrates that they contain. All of these nutrients and more, help the neurons to form synapses.
In turn, nerve tissue is a group of neurons with connections, terminals, receptors, and glial cells. Currently, we know that there are ten times more glia than neurons, which make up almost half of all nerve tissue (Bustamante, 2007).
The primary function of the nerve tissue is to activate communication between neurons and, with this, the complex activity of the entire nervous system. Said activity takes place through the nervous system's two primary parts on an anatomic level: the Central Nervous System (in charge of receiving and organizing nerve impulses) and the Peripheral Nervous System (where the motor nerves are found). Each part of the nervous system has different types of glial cells that we'll take a closer look at below.
Neuroglia in the Central Nervous System are also known as 'central glial cells' since they traverse the brain and spinal cord (two parts of the Central Nervous System). There are two main types of glia in the CNS: astrocytes and oligodendrocytes. Besides, there are microglia and ependymal cells in this system.
Astrocytes, also known as astroglia, are related to both neurons and other parts of the body. Their primary function is to serve as a sort of wrapper for the synapses located at the border area between the CNS and the rest of the body. Besides this, the astrocytes participate in hematoencephalic barrier activity (that separates the extracellular liquid from the blood, allowing nutrients to flow through to the brain).
This is also why astrocytes play a fundamental role in transporting and metabolizing both neurotransmitters and different nutrients, hormones, vitamins, and oxygen to the nerve tissue. At the same time, there are two different subcategories of astrocytes: fibrillary astrocytes, found in the myelin of the brain's white matter, and then, the protoplasmic astrocytes, located in the gray matter, surrounding the neurons and their dendrites. This type is fundamental when it comes to healing brain injuries.
Oligodendrocytes are mainly found in the brain's white matter, forming the myelin of the thick axons (nerve fibers). Unlike astrocytes, oligodendrocytes have smaller, more rounded nuclei. Another difference is that their role is more nutrient transport focused since oligodendrocytes work as intermediaries to facilitate neuron metabolism.
So, the two main functions of the oligodendrocytes are, on the one hand, to facilitate the removal of neuronal waste, and on the other hand, to generate the substance that protects the thick axons (the myelin sheath).
Microglia, also known as microglial cells, stand out from the previous types mainly due to their structure and properties. Besides, the origins of this glial cell are different: microglia seems to come from cells in the mononuclear phagocyte system called 'macrophages,' carrying out functions from this system within the CNS.
So, what are these functions? The mononuclear phagocyte system facilitates phagocytosis. The word 'phagocytize' means 'to attract cell particles to destroy and digest them' (WordReference, 2018). In this regard, the microglia get rid of dead tissue and defend the cell from invading microorganisms. In fact, microglial cells fall into the 'pleomorphic' cell category since they are able to turn into macrophages to assist in phagocytosis.
Unlike the other cells that we've mentioned, ependymal cells are cylindrical and appear in the spinal cord. Also, in the case of ependymal cells, their exact function in the CNS isn't entirely clear. What we do know is that they ease the passage of cerebrospinal fluid and the transport of some nutrients along with it.
Glial cells or neuroglia in the Peripheral Nervous System are called 'peripheral glial cells,' located in the nervous system's nerve endings and nerves, which are the main components of the Peripheral Nervous System. Since this system is 'peripheral,' that means that its located just outside of the CNS. In this system, there are three main cell types: satellite cells, and Schwann, and Müller cells. Although these aren't true glial cells, they do have a similar function.
Satellite cells are explicitly found in mature skeletal muscle fibers in the peripheral nervous system. They were discovered in 1961, and they've been fundamental in understanding the body's adaptive response to physical exercise (Grau, Guerra y López, 2007).
These cells permit muscle regeneration as well as the proliferation or interruption of muscle fiber activity. At the same time, they form different groups: 'inactive satellite cells' and 'active satellite cells.' The last type plays an important role in the muscular regeneration process since it helps to create new muscle fibers or muscle nuclei.
The function of Schwann cells is similar to that of oligodendrocytes since they generate the myelin sheath that covers the thick fast conducting axons (this time in the Peripheral Nervous System).
The myelin sheath is interrupted at intervals called 'nodes of Ranvier,' which are small grooves connected to the astrocyte extensions, which is how nerve impulses travel (as if they were jumping from groove to groove). Therefore, Schwann cells work both to protect the axons and to permit the flow of electrical signals in the PNS.
Müller cells provide nutrition and physical support to the neurons. Besides, these glial cells contribute water and ionic homeostasis (specifically to the retina), facilitate glutamate transport and protect neurons from oxidative stress, by means of the segregation of neuroprotective substances (Hauck y Ueffing, 2009).
They also act as a kind of optic nerve since they bring light to the retina and the photoreceptors. Since they are an indispensable source of neuroprotective protein specific to the retina, these cells are vital to healthy visual system function. In the same vein, Müller cell dysfunction has been linked to different types of blindness since its activity is related to retinal ganglion cells.
Glioblastoma multiforme (GBM) isa type of malign tumor that originates from the glial cells, which means that it affects the nervous system; specifically the CNS. Because of this, it's known as a 'neuroepithelial' tumor (from the skin of this tissue) or a glial 'neoplasm.' Glioblastoma multiforme mostly affects older adults (50-70 years old), being more common in men than in women. Morphologically speaking, this tumor presents itself in the form of a large mass almost always located in the white matter.
This is one of the most common tumors of its kind, and it reproduces rapidly, which also affects some astrocytes. Another type of neuroepithelial tumor is the anaplastic astrocytoma, which forms in astrocytes. This kind occurs much less frequently than glioblastoma multiforme, although sometimes anaplastic astrocytomas can emerge from these. This type of tumor is generally found on either the frontal or atemporal lobes.
Surgery can be done to remove the tumor, as well as other complementary treatments such as chemotherapy, radiotherapy, and pharmaceuticals (temozolomide and/or bevacizumab) (Vega Molina, 2016).
Watch this video to see more details on how glial cells function:
Bustamante, E. (2007). El sistema nervioso. Desde las neuronas hasta el cerebro humano. Editorial Universidad de Antioquia: Colombia.
Hauck, S.M. y Ueffing, M. (2009). Factores neurotróficos derivados de las células de Müller. En el camino hacia la terapia neuroprotectora en la retina. Archivos de la Sociedad Española de Oftalmología, 84(9): 423:426.
Market, J.M., T. DeVita, V., Hellman, S., y Rosenberg, S. (Eds.). Glioblastoma Multiforme. Jones and Bartlett Publishers: Sudbury, Massachusetts.
Vega Molina, A. (2016). Caracterización clínica e imagenológica de pacientes con glioblastoma o astrocitoma anaplásico atendidos en el Instituto Nacional de Cancerología durante el periodo enero 2007 - diciembre 2013. Universidad Nacional de Colombia, Facultad de Medicina: Bogotá Colombia.