Astrocyte

Astrocytes, stellate cells or spiders cells (from the Greek; composite of astron, star ' and kytos, cell ') form the majority of glial cells in the central nervous system of mammals, and are therefore also called astroglia. There are star - or spin -branched cells that form the projections border membranes for the brain surface (or pia mater ) and to the blood vessels.

Classification

There are known two types of astrocytes:

• Protoplasm tables glia ( Astrocytus protoplasmaticus - also: spotlights short ) occurs mainly in the gray matter.
• Faserglia ( Astrocytus fibrosus - also: long- emitter) is fibrillenreich and is located primarily in the white matter. In the electron microscope, these cells are characterized by numerous microtubules and intracellular fiber structures.

Cell architecture

Cell body

Astrocytes possess numerous, radially from the cell body (10 to 20 microns in size ) extending cell processes for covering surfaces such as neuronal synapses, Ranviersche Schnürringe or nichtmyelinisierte axons ( discontinuous).

Furthermore, they form in the CNS border structures through dense aggregation of projections and cell bodies from:

• Limiting membrane glialis perivascularis as a layer around blood vessels.
• Limiting membrane glialis superficial forms a cell layer to which the pia mater pia mater connects to the outside.

Cell membrane

The cell membrane contains particle complexes ( 12 nm in size ), which consist among other things of aquaporin 4 (water channel protein). There are also voltage-dependent ion channels and receptors and transporters for neurotransmitters and glucose ( Glut1 ).

Cell contacts

Astrocytes with each other form a tight network by Nexus, consisting of connexin 43, which serves as the electrical coupling of the cells as well as mechanical linkages by Adhärenskontakte ( puncta adhaerentia ).

Cell plasma

The cytoplasm is pale, organellarm and contains as Zytoskelettbestandteile type intermediate filaments glial fibrillary acidic protein GFAP. Glycogen particles represent para plasmatic components

Function

• Astrocytes nutrition to the neurons via contacts to blood vessels
• Astrocytes are significantly involved in the regulation of fluid in the brain and provide for the maintenance of potassium homeostasis. The during conduction of excitation in the nerve cells released potassium ions are absorbed mainly by a high potassium conductance and in part by K and Cl- cotransporter in the glial cells. On the wide-mesh network Nexus the ions can be transferred to other astrocytes, whereby an effective buffer system is formed. Thus, they also regulate the extracellular pH balance in the brain. By ligand binding ( glutamate ) ion displacement can also be affected.

• You are still in direct interaction with neurons. Neurotransmitters such as glutamate, among other things, GABA and glycine are taken up by specific transporters and modified by enzymatic activity in the cytoplasm and mitochondria. The cleavage products can be taken up by neurons and re-packaged in synaptic vesicles in the terminals after their release into the extracellular matrix (ECM). In particular, the concentration of glutamate, which is cytotoxic to neurons due to postsynaptic over-excitation is kept so low.

• Astrocytes after transection of the axons of nerve cells Glianarben that are instrumental in helping to prevent the renewed growth of the axons. This is a major problem for patients with paraplegia.

In astrocytes comes as a marker in front of the intermediate filament GFAP ( glial fibrillary acidic protein, glial fibrillary acidic protein '), which can be used for the detection of CNS tissue in meat products, for example, therefore, what has become particularly with regard to BSE in importance. The formation of the protein is enhanced by pathological changes in the brain tissue.

Pathology

Brain tumors, histologically resemble the tumor cells astrocytes are called astrocytomas. This group but include benign tumors and malignant tumors such as glioblastoma.

Neurogenin -2

Some astrocytes appear to have a kind of stem cell function: In certain areas of the brain they transform into neurons, if needed. This process can also be done artificially in the laboratory: It drops at the blueprint for a protein called neurogenin -2 in cultured astrocytes one, after a short time, the cells show the typical shape of nerve cells, including functional synapses. It can even be used to control which type of nerve cells is formed by other proteins, such DLX2, infiltrates. However, it is currently still unclear whether these laboratory results can be applied also in living organisms.

• Cell type
• Central Nervous System
• Nervous tissue