Radial astrocytes extend a single or group of long, thick, longitudinal processes that extend from the ventricle toward the surface of the brain. Radial astrocyte processes span the entire white matter and abut on the pia mater, the outermost surface of the brain. Because of their location in the ventricle and bipolar morphology, some of these cells are sometimes considered ependymal cells. There are three major subtypes of radial astrocytes. First, Radial glial cells are primarily observed during development. They extend processes from the ventricle throughout the neural tissue. The main function of these cells is to act as "cables" on which CNS progenitor cells can migrate to their final destination. Radial glial cells are transient since they are scarce in the adult brain. It is thought that some of these cells can later become other astrocyte subtypes. Second, Bergman glial cells, also called Golgi epithelial cells, are the radial astrocytes of the cerebellum. The cell bodies are located between the Purkinje neurons and send processes radially through the molecular layer of the cerebellar cortex. Endfeet of Bergman glia processes terminate at the surface of the cerebellum and form a continuous layer known as the glial limitans. Figure 1 demonstrates the immunohis-tochemical staining for GFAP on human fetal brain. Third, tanycytes are radial astrocytes found mainly in adult brain, the nuclei of which are located within or just below the ependymal, or innermost, lining of the ventricle. These cells extend processes radially through the white matter.
Fibrous astrocytes are located in the white matter of the brain, characterized by long, unbranched processes that radiate in all directions from the cell body but rarely reach the pia mater. Fibrous astrocytes can be found in the cortex and cerebellum, and they express high levels of GFAP in their processes. The endfeet of these processes terminate on blood vessels and synapses as well as on the cell bodies and processes of neurons.
Protoplasmic astrocytes are mainly found in the gray matter of the brain. These astrocytes possess numerous short, thin, ramified processes that radiate in all directions from the cell body. These processes express little GFAP and are better observed by Golgi
impregnation or dye injection. The endfeet terminate on blood vessels, synapses, and neuronal cell bodies, and the processes form the subpial glia limitans.
Astrocytes have varying morphologies in culture that may not fully reflect the range of morphologies found in the brain. In vitro, astrocytes can be separated from other neural cell types, thereby generating 95-99% astrocyte-enriched cultures. Figure 2 represents the immunofluorescent staining of cultured human astro-cytes. In culture, most astrocytes derived from fetal brain appear fibroblast-like and immunostain with GFAP. These in vitro cultures maintain most of the functional properties of astrocytes in vivo. Cultured astrocytes are sometimes divided into two subtypes, type 1 and type 2, based on phenotypic characteristics as identified in rodent tissue cultures. Type 1 astrocytes stain with GFAP, whereas type 2 astrocytes costain for GFAP and A2B5, a sialoganglioside. The majority of astrocytes in culture are type 1. The type 2 astrocytes found in culture have not been observed in human brain tissue in vivo and may be an artifactual property of culturing.
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