Astrocyte

Astrocytes have numerous radiating processes packed with glial fibrillary acidic protein (GFAP) intermediate filaments, the immunohistochemical marker. Protoplasmic astrocytes in grey matter have short, highly branched processes that enwrap synapses. Fibrous astrocytes in white matter have long, thin processes. Each astrocyte occupies its own non-overlapping domain, contacting hundreds of thousands of synapses, and astrocytes are coupled into a syncytium by gap junctions (connexin 43). Specialised forms include Müller cells of the retina, Bergmann glia of the cerebellum and pituicytes of the posterior pituitary.

Their perivascular end-feet cover almost the whole capillary surface. They induce and maintain the endothelial tight junctions of the blood–brain barrier and regulate water flux through aquaporin-4. Their processes also form the glia limitans beneath the pia. At synapses, astrocytes take up glutamate (EAAT1/GLAST and EAAT2/GLT-1) and convert it to glutamine (glutamine synthetase), which they return to neurons. This glutamate–glutamine cycle prevents excitotoxicity. Together with the pre- and postsynaptic elements they form the tripartite synapse, releasing "gliotransmitters" such as D-serine and ATP.

Astrocytes also redistribute K⁺ released by active neurons (spatial buffering via Kir4.1 channels), store the brain's only significant glycogen, and supply lactate to neurons (the astrocyte–neuron lactate shuttle). They couple local blood flow to neuronal activity (neurovascular coupling, the basis of the fMRI BOLD signal). After injury they become reactive, proliferating, upregulating GFAP and forming a glial scar (gliosis) that walls off damage but inhibits axon regeneration.