Astrocyte

Astrocytes (also known collectively as astroglia) are characteristic star-shaped glial cells in the brain and spinal cord. They perform many functions, including biochemical support of endothelial cells which form the blood-brain barrier, the provision of nutrients to the nervous tissue, and a principal role in the repair and scarring process in the brain.

Description

Astrocytes are a sub-type of the glial cells in the brain and spinal cord. They are also known as astrocytic glial cells, or astroglia. The primary processes of an astrocyte are star-shaped, hence the name; however, the numerous fine processes largely fill the space between the "star beams" to create a spongiform shape. Fine distal processes of the astrocyte envelope, or "ensheath" synaptic connections that are formed between neurons. Proximity and interaction between synaptic elements and astrocytic processes gave rise to a concept of "tripartite synapse" which is gaining acceptance within the Neuroscience community. The degree of synapse ensheathing varies between brain areas and between individual synapse. Importantly, it also changes over time in an activity-dependent manner, allowing for structural plasticity of tripartite synapse.

Astrocytes are classically identified histologically by their expression of the intermediate filament glial fibrillary acidic protein (GFAP), although the level of GFAP expression varies widely between individual astrocytes. Two forms of astrocytes exist in the CNS, fibrous and protoplasmic. Fibrous astrocytes are usually located within white matter, have relatively few organelles, and exhibit long unbranched cellular processes. This type often has "vascular feet", also called endfeet, that physically connect the astrocyte to the outside of capillary wall. Protoplasmic astrocytes, found in grey matter tissue, possess a larger quantity of organelles, and exhibit short and highly branched cellular processes. When in proximity to the pia mater, astrocytes send out their processes to form the pia-glial membrane.

Previously in medical science, the neuronal network was considered the only important one, and astrocytes were looked upon as gap fillers. But recently this view has been reconsidered, and astrocytic networks are now thought to play a number of active roles in the brain, including the secretion and uptake of neurotransmitters, "siphoning" K⁺ ions away from synapses, regulation of synaptogenesis, modulation of synaptic transmission, short- and long-term synaptic plasticity, neuro-vascular coupling, maintenance of the blood-brain barrier and more.

Functions

• Structural: involved in the physical structuring of the brain.
• Metabolic support: they provide neurons with nutrients such as glucose.
• Blood-brain barrier: the astrocyte end-feet encircling endothelial cells were thought to aid in the maintenance of the blood-brain barrier, but recent research indicates that they do not play a substantial role; instead it is the tight junctions and basal lamina of the cerebral endothelial cells that play the most substantial role in maintaining the barrier.Template:Fact
• Transmitter reuptake and release: astrocytes express plasma membrane transporters such as glutamate transporters for several neurotransmitters, including glutamate, ATP and GABA. More recently, astrocytes were shown to release glutamate or ATP in a vesicular, Ca²⁺-dependent manner.{{}}{{}}

• Regulation of ion concentration in the extracellular space: astrocytes express potassium channels at a high density. When neurons are active, they release potassium, increasing the local extracellular concentration. Because astrocytes are highly permeable to potassium, they rapidly clear the excess accumulation in the extracellular space. If this function is interfered with, the extracellular concentration of potassium will rise, leading to neuronal depolarization by the Goldman equation. Abnormal accumulation of extracellular potassium is well known to result in epileptic neuronal activity.Template:Fact
• Modulation of synaptic transmission: in the supraoptic nucleus of the hypothalamus, rapid changes in astrocyte morphology have been shown to affect heterosynaptic transmission between neurons.
• Vasomodulation: astrocytes may serve as intermediaries in neuronal regulation of blood flow.
• Promotion of the myelinating activity of oligodendrocytes: electrical activity in neurons causes them to release ATP, which serves as an important stimulus for myelin to form. Surprisingly, the ATP does not act directly on oligodendrocytes. Instead it causes astrocytes to secrete cytokine leukemia inhibitory factor (LIF), a regulatory protein that promotes the myelinating activity of oligodendrocytes. This suggest that astrocytes have an executive-coordinating role in the brain.
• Nervous system repair: upon injury to nerve cells within the central nervous system, astrocytes become phagocytic to ingest the injured nerve cells. The astrocytes then fill up the space to form a glial scar, repairing the area and replacing the CNS cells that cannot regenerate.Template:Fact

Recent studies have shown that astrocytes play an important function in the regulation of neural stem cells. Research from the Schepens Eye Research Institute at Harvard shows the human brain to abound in neural stem cells, which are kept in a dormant state by chemical signals (ephrin-A2 and ephrin-A3) from the astrocytes. The astrocytes are able to activate the stem cells to transform into working neurons by dampening the release of ephrin-A2 and ephrin-A3.Template:Fact

Furthermore, studies are underway to determine whether astroglia play an instrumental role in depression, based on the link between diabetes and depression. Altered CNS glucose metabolism is seen in both these conditions, and the astroglial cells are the only cells with insulin receptors in the brain.

Calcium waves

Astrocytes are linked by gap junctions, creating an electrically coupled syncytium.

An increase in intracellular calcium concentration can propagate outwards through this syncytium. Mechanisms of calcium wave propagation include diffusion of IP3 through gap junctions and extracellular ATP signalling. Calcium elevations are the primary known axis of activation in astrocytes, and are necessary and sufficient for some types of astrocytic glutamate release.

Classification

There are several different ways to classify astrocytes:

by Lineage and antigenic phenotype

These have been established by classic work by Raff et al in early 1980s on Rat optic nerves.

• Type 1: Antigenically Ran2⁺, GFAP⁺, FGFR3⁺, A2B5^- thus resembling the "type 1 astrocyte" of the postnatal day 7 rat optic nerve. These can arise from the tripotential glial restricted precursor cells (GRP), but not from the bipotential O2A/OPC (oligodendrocyte, type 2 astrocyte precursor, also called Oligodendrocye progenitor cell) cells.
• Type 2: Antigenically A2B5⁺, GFAP⁺, FGFR3^-, Ran 2^-. These cells can develop in vitro from the either tripotential GRP (probably via O2A stage) or from bipotential O2A cells (which some people think may in turn have been derived from the GRP) or in vivo when the these progenitor cells are transplanted into lesion sites (but probably not in normal development, at least not in the rat optic nerve). Type-2 astrocytes are the major astrocytic component in postnatal optic nerve cultures that are generated by O2A cells grown in the presence of fetal calf serum but are not thought to exist in vivo (Fulton et al., 1992).

by Anatomical Classification

• Protoplasmic: found in grey matter and have many branching processes whose end-feet envelop synapses. Some protoplasmic astrocytes are generated by multipotent subventricular zone progenitor cells.
• Fibrous: found in white matter and have long thin unbranched processes whose end-feet envelop nodes of Ranvier. Some fibrous astrocytes are generated by radial glia.

by Transporter/receptor classification

• GluT type: express glutamate transporters (EAAT1/Template:Gene and EAAT2/Template:Gene) and respond to synaptic release of glutamate by transporter currents
• GluR type: express glutamate receptors (mostly mGluR and AMPA type) and respond to synaptic release of glutamate by channel-mediated currents and IP3-dependent Ca2+ transients

Bergmann glia

Bergmann glia, a type of glia also known as radial epithelial cells (as named by Camillo Golgi), are astrocytes in the cerebellum that have their cell bodies in the Purkinje cell layer and processes that extend into the molecular layer, terminating with bulbous endfeet at the pial surface. Bergmann glia express high densities of glutamate transporters that limit diffusion of the neurotransmitter glutamate during its release from synaptic terminals. Besides their role in early development of the cerebellum, Bergmann glia are also required for the pruning or addition of synapses.^{citation needed}

Pathology

Astrocytomas are primary intracranial tumors derived from astrocytes cells of the brain.