What are astrocytes?

Astrocytes are a type of glial cells found in the brain and spinal cord. They can be easily distinguishable from other cells because they have a star shape, have numerous branching processes and they represent the most numerous cell populations in the brain, with an estimation that there are 50:1 astrocytes per neuron. There are three types of astrocytes – fibrous, protoplasmatic and radial.

Fibrous astrocytes are found in the white matter and have long unbranched processes. Protoplasmatic astrocytes are found in the grey matter and have short and branched processes. Radial astrocytes can be found between the grey matter and pia mater, in the vertebrate eye and in the cerebellum. Radial astrocytes in the eye make for 23% of the retina and are called Mueller cells. In the cerebellum, they are called Bergmann cells. Astrocytes are connected via gap junctions, meaning that they form an electrically coupled functional connections and this enables them to communicate with neighboring cells as well as with distant ones.

What astrocytes do?

Previously it was considered that astrocytes do not have a specific role other than to fill in the gaps between neurons. Today it is known that astrocytes are involved in a number of brain processes including maintenance of blood-brain barrier, brain homeostasis, nutrition and scarring processes. But besides these functions, astrocytes are involved in neuronal signaling and are able to influence signal propagation. Astrocytes release specific transmitters called gliotransmitters via calcium ion-dependent mechanism and therefore are able to influence neuronal signaling.[1] This phenomenon is called the tripartite synapse consisting of the presynaptic cell, postsynaptic cell, and the glial cell.