The human brain consists of approximately one trillion cells, of which approximately 10% are considered to be neurons, and the remaining 90% are support cells called glia.1 One type, Astroglia, often called astrocytes, are one of the major types of glial cell.2 The brains of smaller mammalian creatures, naturally, have a smaller number of cells; and the ratio of astrocytes to neurons declines.3
The term astrocyte derives from a combination of a Greek word for star (astron; plural, astra) and the scientific word for cell (cyte, which is in turn derived from the Greek word kytos, meaning vessel). Astrocytes are characteristic of star-shaped glial cells in the brain and spinal cord.
Astrocytes are classically identified using histological analysis; many of these cells express the intermediate filament glial fibrillary acidic protein (GFAP). Several forms of astrocytes exist in the central nervous system including fibrous (in white matter), protoplasmic (in grey matter), and radial astrocytes.
The fibrous glia cell is usually located within white matter, has relatively few organelles, and exhibits long unbranched cellular processes. This type often has "vascular feet" that physically connect the cells to the outside of capillary walls when they are in proximity to them. The protoplasmic glia cells are the most prevalent and are found in grey matter tissue. They possess larger quantities of organelles, and exhibit short and highly branched tertiary processes.
The radial glia cell is disposed in planes perpendicular to the axes of ventricles. One of their processes abuts the pia mater, while the other is deeply buried in gray matter. Radial glia cells are mostly present during development, playing a role in neuron migration. Müller cells of the retina and Bergmann glia cells of the cerebellar cortex represent an exception, being present still during adulthood. When in proximity to the pia mater, all three forms of astrocytes send out processes to form the pia-glial membrane.