Astrocytes are the most abundant glial cells in the CNS. They arise from neuroectoderm and are intimately associated with neural processes, synapses, vasculature, and the pial-glial membrane investing the CNS. Astrocytes in gray matter are called protoplasmic astrocytes, and in white matter they are called fibrous astrocytes.
The somas vary in diameter from a few µm to 10 or more µm. Astrocytes are arrayed in non-overlapping 3D polyhedral domains of 100-200 µm across (up to 400 µm in hominids). Structurally, astrocytic processes interdigitate, forming a syncytium to protect synapses (as close as 1µm to these structures). Astrocytic endfeet associate with vascular endothelial cells and associated smooth muscle cells. Astrocytic processes invest the entire pial membrane from the inside.
Physiologically, astrocytic processes affect ion balance (sequester K+), transport water via aquaporin 4 channels, uptake and recycle glutamate and GABA, provide metabolic support to neurons, and can become reactive after CNS injury and lay down glial scar tissue. Astrocytes also can release growth factors and bioactive molecules (termed gliotransmitters) such as glutamate, ATP, and adenosine. In development, specialized astrocytes, called radial glia, provide a scaffold for orderly neural migrations in the CNS.