Astrocytes

The central nervous system (CNS) is characterized by an immense structural complexity with intimate associations of its constituent cell types: neurons and different types of glial cells, mainly oligodendrocytes and astrocytes. A century ago (1893 ) described what are now called protoplasmic astrocytes found in grey matter and fibrous astrocytes found in white matter. As their name implies, astrocytes are generally process-bearing cells. A characteristic of all astrocytes, but particularly those in grey matter, is the enormous surface area owing to the extensive branching of the cellular processes; this process formation results in astrocytes being structurally highly complex. The cellular processes of fibrous astrocytes are generally fingerlike, whereas those of the protoplasmic astrocytes are variable in shape and closely follow the contours of adjacent cells. The protoplasmic astrocytic processes often end as thin sheets that surround synapses (see Narlieva, 1988 ). The protoplasmic astrocyte can be divided into a number of distinct morphological forms (Palay and Chan-Palay, 1974 ; Kosaka and Hama, 1986 ). In addition, the cellular processes of astrocytes impinge on small blood vessels as perivascular endfeet and astrocytic foot processes also terminate on the pia mater forming the glia limitans (Peters et al., 1976 ).