These are small star-shaped cells that give off a number of processes (Fig. 1.23). The processes are often flattened into leaf-like laminae that may partly surround neurons and separate them from other neurons. The processes frequently end in expansions in relation to blood vessels or in relation to the surface of the brain. Small swellings called gliosomes are present on the processes of astrocytes. These swellings are rich in mitochondria. Fibrous astrocytes are seen mainly in white matter. Their processes are thin and are asymmetrical. Protoplasmic astrocytes are, on the other hand, seen mainly in grey matter. Their processes are thicker than those of fibrous astrocytes and are symmetrical. Intermediate forms between fibrous and protoplasmic astrocytes are also present. Protoplasmic extensions of astrocytes surround nodes of Ranvier, but the significance of this is not understood.
The processes of astrocytes are united to those of other astrocytes through gap junctions. Astrocytes communicate with one another through calcium channels. Such communication is believed to play a role in regulation of synaptic activity, and in metabolism of neurotransmitters and of neuromodulators.
Astrocytes play a role in maintenance of the blood brain barrier. Substances secreted by end feet of astrocytes probably assist in maintaining a membrane, the glia limitans externa, which covers the exposed surfaces of the brain. They also help to maintain the basal laminae of blood vessels that they come in contact with.
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