Glial cells in the brain. Light micrograph of a section through glial cells (dark) in the grey matter of the brain. Due to their star shape these glial cells are called astrocytes. Astrocytes are classified into fibrous types (seen here) and protoplasmic types. Fibrous astrocytes are not neurons (nerve cells) but serve as supporting cells in brain tissue with a range of functions. The astrocytes shown here have cytoplasmic extensions that are closely associated with blood vessels. Their function is to form a seal around the vessels to selectively restrict the entry of substances into the brain thus contributing to the blood-brain barrier. The total population of glial cells in the brain outnumbers neurons by a factor of ten and it is these cells that6