Morphology of astrocytes
Morphology of astrocytes is highly heterogeneous. Some astrocytes do have a star-like appearance, with several primary (also called stem) processes originating from the soma, although many more morphological profiles exist. An archetypal morphological feature of astrocytes is their expression of intermediate filaments, which form the cytoskeleton. The main types of astroglial intermediate filament proteins are glial fibrillary acidic protein (GFAP) and vimentin; expression of GFAP is commonly used as a specific marker for the identification of astrocytes. It works well in cultured astrocytes, but in situ the levels of GFAP expression vary quite considerably: for example, GFAP is expressed by virtually every Bergmann glial cell in the cerebellum whereas only about 15–20 percent of astrocytes in the cortex of mature animals express GFAP.
Protoplasmic astrocytes are present in gray matter. They have many fine processes (approx ~50 µm long), which are extremely elaborate and complex. The processes of protoplasmic astrocytes contact blood vessels, forming so called ‘perivascular’ endfeet, and form multiple contacts with neurones. Some protoplasmic astrocytes also send processes to the pial surface, where they form ‘subpial’ endfeet. Protoplasmic astrocyte density in the cortex varies between 10 000 and 30 000 per mm3; the surface area of their processes (in rodents) may reach up to 60000 - 80 000 µm2, and cover most of neuronal membranes within their reach.
Fibrous astrocytes are present in white matter. Their processes are long (up to 300 µm), though much less elaborate as compared to protoplasmic astroglia. The processes of fibrous astrocytes establish several perivascular or subpial endfeet. Fibrous astrocyte processes also send numerous extensions (‘perinodal’ processes) that contact axons at nodes of Ranvier. The density of fibrous astrocytes is ~200 000 cell per mm3.
The second big group of astroglial cells are the radial glia, which are bipolar cells each with an ovoid cell body and elongated processes. Radial glia usually have two main processes, one of them forming endfeet at the ventricular wall and the other at the pial surface. Radial glia are a common feature of the developing brain, as they are the first cells to develop from neural progenitors; from very early embryonic stages radial glia also form a scaffold, which assist in neuronal migration. After maturation, radial glia disappear from many brain regions and transform into stellate astrocytes, although radial glial cells remain in the retina (Müller glia) and in lower vertebrates such as turtle.
The retina contains specialized radial glia called Müller cells, which make extensive contacts with retinal neurones. The majority of Müller glial cells have a characteristic morphology, extending longitudinal processes along the line of rods and cones. In humans, Müller glial cells occupy up to 20 percent of the overall volume of the retina, and the density of these cells approaches 25 000 per mm2 of retinal surface area. Each Müller cell forms contacts with a clearly defined group of neurons organized in a columnar fashion; a single Müller cell supports ~16 neurones in human retina, and up to 30 in rodents.
The cerebellum contains specialized semi-radial glia called Bergmann glia. They have relatively small cell bodies (~15 µm in diameter) and 3 - 6 processes that extend from the Purkinje cell layer to the pial surface. Early in development these cells have contacts to the ventricular surface and are true radial glial cells, but with the development of the granular layer, they acquire the classical morphology of Bergmann glial cells. Usually several (~8 in rodents) Bergmann glial cells surround a single Purkinje neuron and their processes form an ensheathment of the Purkinje cell dendrites. The processes of Bergmann glial cells are extremely elaborated, and they form close contacts with synapses formed by parallel fibres on Purkinje neuron dendrites; each Bergmann glial cell provides coverage for up to 8000 of such synapses.
Other regions of the CNS contain many different populations of astroglial cells. Velate astrocytes are found in the cerebellum, where they form a sheath surrounding granule neurones; each velate astrocyte enwraps a single granule neurone. A similar type of astrocyte is also present in the olfactory bulb. Interlaminar astrocytes are specific to the cerebral cortex of higher primates. Their characteristic peculiarity is a very long single process (up to 1 mm) that extends from the soma located within the supragranular layer to cortical layer IV. Tanycytes are specialized astrocytes found in the periventricular organs, the hypophysis and the raphe part of the spinal cord. Astroglial cells in the neuro-hypophysis are known as pituicytes; the processes of these cells surround neuro-secretory axons and axonal endings under resting conditions. Perivascular and marginal astrocytes are localized very