The Anatomy of Astrocytes

Astrocytes are the most abundant type of glial cell in the brain and are diverse in their ability to offer several protective functions to the central nervous system. The arrangement of astrocytes is related to the chemical and physical interactions with other brain cells.

Astrocytes contribute to the production and maintenance of the blood-brain barrier. The blood-brain barrier is a semi-permeable membrane that prevents certain components of the blood from passing out of circulation into the extracellular fluid of the central nervous system.

Astrocytes, also known as astroglia or astroglial cells, were discovered by scientist Karl Bergmann and were originally called Bergmann glia due to their classification as a protective and insulating cell. These cells were later named astrocytes for their star-like shape.

Anatomy

Astrocytes are star-shaped cells found in the brain. Similarly to other neuronal cells, astrocytes are comprised of synapses, or cell ends that allow for chemical and electrical communication between cells. Astrocytes also consist of dendrites that allow for communications from other cells to be transmitted within the cell body of the astrocyte.

There are two major types of astrocytes, called protoplasmic or fibrous, that are classified based on their structural differences and locations. Protoplasmic astrocytes are found throughout the grey matter in the brain, which is where the majority of neurons are located. The grey matter in the brain controls functions such as muscle control and sensory perception, including speech, taste, sense of smell, memory, judgment, and more. Fibrous astrocytes are found throughout the white matter in the brain, which is where insulated nerve fibers are present. 

While there are no anatomical variations specific to astrocytes, diseased astrocytes have a large impact on the presence and development of neurodegenerative conditions. It is the general consensus among researchers that astrocytes contribute to neurodegenerative diseases when they lose their ability to balance and detoxify brain cells.

Function

Astrocytes have a variety of functions within the brain and central nervous system as a whole. Astrocytes provide nutrients to neurons while maintaining the balance between cellular ions such as sodium chloride, potassium, and phosphate. These ions play an integral role in the functioning of each neuron by providing cells with electrolytes and energy. Astrocytes also assist in repairing damage within the brain and central nervous system through a process called astrogliosis. Astrocytes compose and maintain the structure of the blood-brain barrier.

Astrocytes also improve communications between brain cells and regulate the metabolic processes within the brain. Metabolic processes within the brain include the removal of waste products and the processing of substances used for cellular energy.

While astrocytes play an integral role in repairing damage to the brain, their structure and function also lend them to a less helpful purpose. Astrocytes may play a role in the progression of certain brain tumors. Their mechanisms may cause them to protect tumor cells from chemotherapy by adding additional calcium to strengthen the tumor cell. This function is the subject of much research, as it proves harmful to individuals with various types of cancer.

Associated Conditions

In addition to aiding in the progression of brain tumors under certain circumstances, astrocytes have also been identified as contributing or serving as the primary cause of neurodegenerative disorders of the central nervous system. These conditions include dementia and Alzheimer’s disease. This dysfunction of astrocytes is called an astrocytopathy.

Another type of astrocytopathy is a condition called autoimmune glial fibrillary acidic protein (GFAP) astrocytopathy. This condition causes an individual's immune system to attach protein fibers that are part of the glial cell network, resulting in a variety of symptoms, including high fever, migraines, brain swelling (also known as encephalopathy), repetitive and involuntary movements, and vision changes.

Recent studies also show that astrocyte dysfunction plays a role in the development of neurodevelopmental conditions such as Rett syndrome, fragile X, and other severe intellectual disabilities. Astrocyte dysfunction in these instances is often due to specific variations in cell patterns that lead astrocytes to serve different functions during early development. 

Rehabilitation

There is much research being done at this time to identify the reason why astrocytopathies develop and progress to the point that they do. Treatment of new cases of GFAP astrocytopathy consists of high doses of corticosteroids and immunoglobulin, which is a formula of proteins that boost the immune system. In this instance, immunoglobulin is provided intravenously, or via infusion through the arm.

Most individuals affected by GFAP astrocytopathy will also receive a plasma exchange in which a part of their blood called plasma is replaced with substitute plasma in order to clean the blood. Long-term maintenance of GFAP astrocytopathies consists of continued oral steroids and other medications that suppress the immune system, called immunosuppressants. These immunosuppressants assist in lessening the body's immune response and addressing the astrocytopathy from the source.