Table 3 Types of astrocytes

Protoplasmic astrocytes

Uniformly distributed within the grey matter [3]

Bushy appearance, with numerous short, branched, thick processes [50]. The cell body is ovoid or fusiform (see Figure 5)

• Form the blood–brain barrier

Their processes exhibit endfeet enveloping the synapses and the blood vessels [51]. The processes express

• Regulate the blood flow

• Neuronal metabolism

• Receptors for neurotransmitters, cytokines, growth factors

• Implicated in the synapse function

• Transporters

• Fluid, ion, pH and transmitter homeostasis [45]

• Ion channels [7]. In rodents, there is minimal overlapping between the processes of the neighbouring astrocytes [43, 44, 52–54]. In humans, the superposition of the domains occupied by the astrocytes processes is augmented [3]

Fibrous astrocytes

Within the white matter, oriented longitudinally, along the nervous fibers bundles [1]

Star-shaped cells. Posses long, thin and straight processes [45] (see Figure 6)

Their endfeet processes envelop the nodes of Ranvier and the blood vessels [45]

Interlaminar astrocytes

In the molecular 1st layer of the cerebral cortex, next to the pial surface

Spherical cell bodies and processes

Unknown Support the calcium wave propagation in humans [3]

Are found only in humans and primates. Their processes are included in the pial glial membrane, creating a thick network of GFAP fibers [46–49]

Varicose projection astrocytes

In the 5th and the 6th layers of the cerebral cortex

Exhibit 1 to 5 long processes (up to 1 mm in length), characterized by evenly (10 μm) spaced varicosities [3, 46]

Unknown

Were identified only in humans and chimpanzees. They are GFAP⁺ cells [3, 46]

Bergmann glia (epithelial glial cells)

In the Purkinje-cell and the granular layers of the cerebellar cortex

Posses long processes extending towards the molecular layer of the cerebellar cortex, in a fan-like arrangement, exhibiting pial vascular endfeet [23]

Implicated in synapse function: capable to interfere with synaptic transmission by communicating with neurons via the extracellular space, by modulating ion concentrations or transmitter levels in the synaptic cleft [23]

Display receptors with distinct biophysical and pharmacological features allowing them to sense the activity of synapses [23]

Fananas cells

In the molecular layer of the cerebellar cortex

Posses several short side processes with a characteristic feather-like arrangement [23]

Müller cells

In the 6th layer of the visual retina

Supportive cells: they form the inner and the outer limiting membranes

The limiting membranes consist of junctional complexes between the cellular processes of the Müller cells

The outer membrane separates the external segment of the photoreceptor cells from the cell bodies and the outer membrane separates the retina from the vitrous body [23]

They have an intense metabolic activity and contain microfilaments and glycogen within their cytoplasm [23]

Pituicytes

In the neurohypophysis

Irregular in shape with many cytoplasmic processes extending in the proximity of the capillaries and surrounding the Herring bodies [24]

Their cytoplasm contains lipid droplets and pigment granules.

They are immunoreactive for GFAP, vimentin and S100 protein [24]

Inerstitial epiphysial cells

In the epiphysis

Exhibit cytoplasmic processes

Contain numerous filaments within their processes [23]