Astrocytes - The Star Cells
Everyone is familiar with the night sky; at any time, you can look up and see numerous twinkling lights, maybe even recognize a few constellations. Before the advent of telescopes, stars were merely seen as dots in the sky and only in the last centuries humankind realized that the night sky is a vaster, deeper, and more complex space than we could have ever imagined. Astrocytes, literally meaning the “star-shaped cells”, follow the same analogy.
Scientists knew that glial cells were important for neural growth and survival; and astrocytes, the most numerous and diverse glial cells in the brain did not seem to play a major role. However, new discoveries have put astrocytes in the spotlight of neuroscience.
A different type of communication
Astrocytes are not electrically active in the classic way that neurons are. They were, therefore, long assumed not to play any active roles in neural communication. But recently astrocytes have been recognized as a crucial player in the CNS. Astrocyte processes contact many synapses putting them into a position to interact with neurons and play an active role in modifying synaptic function, by regulating the neurotransmitters in the synaptic cleft.
It was also demonstrated that astrocytes communicate, not through electricity and voltage, but by signaling via calcium levels. The effect is rather subtle: By altering the calcium concentrations around a cell, the astrocytes can influence, but not initiate neuronal activity, thus modulating the communication in neuronal networks.
Distant reach and essential role
In addition to playing complex roles in the brain, scientists underestimated their size and reach. A single astrocyte in the human brain may have connections with as many as 100.000 neurons. Their processes extend to every corner of the brain and spinal cord. They play a number of important roles in the developing, healthy, and damaged brain, serving divers functions like a “Cellular Swiss knife”.
They are involved in the physical structuring of the brain, in the cerebral blood flow, the formation of the blood brain barrier, and in the metabolism and water transport.
They can act as neural stem cells during development, promote the myelinating activity of oligodendrocytes, and are necessary for the formation of mature, functional synapses in the CNS as mentioned above.
Furthermore, astrocytes participate in the response to an injury. The astrocytes’ response is multifaceted: by creating a glial scar, they play a beneficial role and prevent the spread of the damage to neighboring tissue, but contribute to the inhibition of the outgrowth of axons and regeneration. Many studies also suggest that reactive astrocytes are neuroprotective by releasing neurotrophic molecules.
A key player of our brain power?
With their sheer size and complexity astrocytes form a sophisticated and interconnected network across the entire brain.The human brain is significantly more complex than any other animal’s. And yet, although their number increased our neurons aren’t fundamentally any different than those from other species. Astrocytes, on the other hand, become larger, more complex, more diverse and more numerous as species go up the evolutionary chain.
Future research will demonstrate the role of the interaction of astrocytes and neurons in health and disease.
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