Astrocytes are a type of specialized nervous system cell called glial cells. Glial cells surround neurons (nerve cells), providing mechanical and physical support and electrical insulation. Astrocytes are located only in the brain and the spinal cord, and have star-shaped cell bodies from which they get their name - from the Greek astron = star, and kytos = cell. When organized into tissues they are known as astroglia. These cells not only help maintain the blood-brain barrier by helping endothelial cells, but also help provide nutrients to different parts of the brain and spinal cord, and assist the brain and spinal cord reparations. The processes (extensions) that extend from astrocytes surround synapses (connections) that are made by neurons.
As their name implies, Astrocytes are star shaped cells that are found in the in the human body. These cells have long processes that develop symmetrically from their bodies. The cytoplasm of Astrocytes are made up of glial fibrillary acidic protein. The processes of Astrocytes tighten around the capillaries of the brain and also cause endothelial cells to create taut links. The links that these endothelial cells produce are very important as they are the starting blocks of the blood-brain barrier.
Astrocytes perform many functions in the human body. A majority of these functions take place in and around the areas of the brain. Astrocytes supply neurons with nutrients that help them function correctly. Without these nutrients the neurons would not be able to function efficiently. They also take part in the material makeup of the brain and help with the flow of blood within the brain. Also, it was once thought that Astrocytes helped retain the blood-brain barrier but it was later realized that the cerebral endothelial cells accomplished this function. Astrocytes also deal with repairs in the Nervous System. When nerve cells in the Nervous System become damaged Astrocytes will consume the injured cells. Upon consuming the damaged nerve cell the the Astrocyte will take the place the nerve cell once held and create a glial scar. The main function of the glial scar is to shield the area from any further damage so that the damaged area may heal.
There are many types of Astrocytes and four ways to classify them. These ways are by lineage and antigenic phenotype, anatomical classification, and by location and by transporter receptor classification.
Lineage and Antigenic The classification by lineage and antigenic has been instituted in the early 1980's by Raff et al.
- Type 1: Antigenically Ran2+, GFAP+, FGFR3+, A2B5-, thus resembling the "type 1 astrocyte" of the postnatal day 7 rat optic nerve. These can arise from the tripotential glial restricted precursor cells GRP, but not from the bipotential O2A/OPC oligodendrocyte, type 2 astrocyte precursor, also called Oligodendrocyte progenitor cell cells.
- Type 2: Antigenically A2B5+, GFAP+, FGFR3-, Ran 2-. These cells can develop in vitro from the either tripotential GRP probably via O2A stage or from bipotential O2A cells which some people think may in turn have been derived from the GRP or in vivo when the these progenitor cells are transplanted into lesion sites but probably not in normal development, at least not in the rat optic nerve. Type-2 astrocytes are the major astrocytic component in postnatal optic nerve cultures that are generated by O2A cells grown in the presence of fetal calf serum but are not thought to exist in vivo Fulton et al., 1992.
- Protoplasmic: These astrocytes are found in grey matter and that have an abundance of processes. The feet of these processes surround various synapses. Subventricular zone progenitor cells are also responsible for creating these astrocytes.
- Gömöri-positive astrocytes: These astrocytes are a division of protoplasmic astrocytes. These astrocytes have within them an abundance of cytoplasmic additions. These cytoplasmic additions, or granules, are formed by the remains of dieing mitochondria that are covered with lysosomes. These astrocytes appear in large numbers in the hypothalamus, arcuate nucleus, and the hippocampus. They can also be found in other areas of the brain but they are largely located in those three areas. These astrocytes may also help the hypothalamus in controlling its response to glucose.
- Fibrous astrocytes are found in white matter. They possess numerous processes that have feet that surround the nodes of Ranvier. Radial glia are responsible for generating some fibrous astrocytes.
Transporter Receptor Classification
- GluT type: express glutamate transporters EAAT1/ and EAAT2/ and respond to synaptic release of glutamate by transporter currents.
- GluR type: express glutamate receptors mostly mGluR and AMPA type and respond to synaptic release of glutamate by channel-mediated currents and IP3-dependent Ca2+ transients.
- Type I: Type I astrocytes are astrocytes that have straight interaction with blood capillaries by means of the astrocytique pod. They provide assistance in areas such as neuronal metabolism and glucose delivery.
- Type II: Type II astrocytes are those astrocytes that encircle neurons and the synaptic gap.