Astrocytes belong to the glial cells of the central nervous system and perform important functions in the brain. They not only act as support cells for neurons, but also actively participate in information exchange. Important pathological processes in the brain have an impact on astrocyte activity.
What are astrocytes?
Astrocytes are star-shaped cells in the central nervous system and are the largest component of glial cells. Until recently, glial cells were considered purely support cells for holding neurons together in the nervous system. This is where the syllable “glia” is derived from, meaning glue. Astrocytes look star-shaped, or spider-shaped, because they have radiating extensions. Astrocyte is the derivation from the Greek term star-shaped cell or stellate cell. Here, however, there must be no confusion with the true star cells, which in turn have nothing to do with astrocytes. The true star cells are neuronal cells (nerve cells) and are located in the cortex and cerebellum. In addition to neurons, the brain consists of over 50 percent astrocytes. Unlike neurons (nerve cells), they did not appear to perform any functions other than support functions. However, the view of glial cells and specifically astrocytes has changed radically in recent years. For example, according to the latest findings, astrocytes are not only glue or cement for neurons, but also play a prominent role in communication processes through close interactions with neurons.
Anatomy and structure
Astrocytes in the brain are star- or spider-shaped branched cells. Their projections form boundary membranes to the brain surface and blood vessels. There are two types of astrocytes in the brain. Protoplasmic glia, also called astrocytus protoplasmaticus or short rays, are components of the gray matter. In contrast, the fibrous glia (also called astrocytus fibrosus or long rays), which are found in the white matter, are rich in fibrils. They also contain many microtubules. The astrocytes of the brain have radially extending cell processes that cover synapses, Ranvier’s cord rings, and axons of neuronal surfaces. Furthermore, the processes also form boundary structures in the central nervous system by aggregation. Their cell membrane has receptors for neurotransmitters and voltage-gated ion channels. Among themselves, they form a tight network through gap junctions. It serves for the electrical coupling of the cells. In other parts of the central nervous system, astrocytes may also have a different structure. For example, the retina of the eye contains the elongated or rod-shaped Müller glial cells, which are also astrocytes.
Function and tasks
Astrocytes perform a variety of functions. They have long been known to have supporting functions in the CNS. In addition, they provide nutrition to neurons through their contacts with blood vessels by means of their projections. Furthermore, they maintain the potassium balance in the brain. In this process, the potassium ions released during excitation transmission are taken up by the astrocytes and distributed throughout the entire network. This forms an effective buffer system that also regulates the pH balance in the brain. Binding of glutamate to receptors in the membrane further influences the ionic shift. There is direct interaction between astrocytes and neurons via neurotransmitters. Electrical stimuli from the neurons are thus also partially transmitted to the astrocytes. Within the astrocytes, signal transmission takes place in the immediate vicinity of the corresponding neurons. Via a feedback mechanism, the astrocytes then have a modulating effect on the signal transmission between the neurons. Thus, there is a constant exchange of information between the neurons and the glial cells. Thus, the astrocytes act much like consultants to generate an appropriate response. Another role of astrocytes is to establish and maintain the blood-brain barrier by forming the membrana limitans glialis perivascularis. Severing of neuron axons causes astrocytes to form glial scars that inhibit axon regrowth. For patients with paraplegia, this is a problem. According to recent studies, it has also been found that some astrocytes in the hippocampus can serve as stem cells for neurons.
Diseases
Astrocytes play a major role in the context of neurological diseases, epilepsy, Alzheimer’s disease, or inflammation in nervous tissue. It has been shown that inflammatory processes in nervous tissue result in changes in the metabolism of astrocytes that ensure their survival in the network. Thus, they have the ability to stop the process of cell death in traumatic events such as brain injury or stroke. However, not much is yet known about the complex relationships involved. However, research suggests that astrocytes also play a major role in pathological processes within the nervous system. For example, it was found that in patients with Alzheimer’s disease, astrocytes are stimulated by the increased production of ATP. They become hyperactive and absorb more calcium. Regular calcium waves are formed. It is not yet clear whether the hyperactivity of astrocytes is a positive defensive reaction or whether it is a negative consequence of the disease process that makes the situation worse. Astrocytes can acquire pathological significance through increased cell proliferation. Thus, they can be the starting point for benign or even malignant brain tumors. These tumors are generally referred to as astrocytomas. In most cases, astrocytomas are benign, but they are often very space-occupying. In some circumstances, they can develop into glioblastomas, which are the most common malignant brain tumors in adults.
