Astrocyte
Astrocytes (from the Greek άστρον ástron 'star' and κύτος kýtos 'cell'), also stellate cells or spider cells , make up the majority of glial cells in the central nervous system of mammals. Their entirety is therefore also referred to as astroglia . They are star or spider-shaped branched cells, the extensions of which form boundary membranes to the brain surface (or to the pia mater ) and to the blood vessels .
Classification
Two types of astrocytes are known:
- Protoplasmic glia consists of cytoplasmic 'short rays' (singular: Astrocytus protoplasmaticus). This glia is found primarily in gray matter .
- Fiber glia contain 'long rays ', named for their long fibrils. (Sigular: Astrocytus fibrosus). This glia mainly characterizes the white matter . In the electron microscope , these cells can be characterized by numerous microtubules and intracellular fiber structures.
Cell architecture
Cell body
Astrocytes have numerous cell processes that run radially from the cell body (10 to 20 μm in size) to cover neuronal surfaces such as synapses , Ranvier constrictor rings or non-myelinated axons (discontinuous).
Furthermore, they form border structures in the CNS through dense agglomeration of processes and cell bodies:
- Membrana limitans glialis perivascularis as a layer around blood vessels .
- Membrana limitans glialis superficialis forms a layer of cells to which the soft meninges pia mater connects to the outside.
Cytoplasm
The cytoplasm appears light in the electron microscope, poor in organelles . It contains intermediate filaments of the acid glial fiber protein GFAP as cytoskeletal components . Glycogen particles represent paraplasmic components.
Cell membrane
The cell membrane contains particle complexes (12 nm in size) which, among other things, consist of aquaporin 4 (water channel protein ). There are also voltage-dependent ion channels as well as receptors and transporters for neurotransmitters and glucose (Glut1).
Cell contacts
Astrocytes form a tight network with one another. The prerequisite for this are gap junctions ( adherence contacts ). They consist of Connexin 43 and are used for mechanical linking and electrical coupling.
Functions
- Astrocytes feed the neurons through contacts with blood vessels .
- Astrocytes play a key role in the regulation of fluids in the brain and ensure that the potassium balance is maintained. The potassium ions released in nerve cells during the conduction of excitation are mainly taken up by the glial cells due to their high potassium conductivity and partly also by K + and Cl - cotransporters. The ions can be transferred to other astrocytes via the wide-meshed nexus network, creating an efficient buffer system. In doing so, they also regulate the extracellular pH balance in the brain. The ion shift can also be influenced by ligand binding ( glutamate ).
- You are still in direct interaction with neurons. Neurotransmitters such as glutamate, GABA and glycine are absorbed by specific transporters and modified by enzymatic activity in the cytoplasm and mitochondria . After they have been released into the extracellular matrix (ECM), the cleavage products can be taken up by neurons and packed again in synaptic vesicles in their presynaptic terminals . Above all, the glutamate concentration, which has a cytotoxic effect on neurons due to postsynaptic overexcitation, is kept low.
- The endothelial blood-brain barrier is induced and maintained by the membrana limitans glialis perivascularis .
- After the axons have been severed by nerve cells , astrocytes form glian scars , which play a key role in preventing the axons from growing again. This is a key problem for patients with paraplegia .
- Astrocytes are the main element of the microcircuit for waste disposal in the brain and spinal cord (CNS), the glymphatic system discovered in 2012 . Liquor that reaches all areas of the CNS via the Virchow-Robin space around the arteries is absorbed directly from the Virchow-Robin space via the astrocytes, distributed in the intercellular space and at the end - taking with it waste materials - along the outer walls the veins flushed out of the CNS again.
Possible further functions
- 2006: Airbag thesis: The soft cells in the brain could perform a kind of " airbag function" and protect the neurons in the event of a strong shock to the brain.
- 2010: Respiratory thesis: Astrocytes are the sensor that monitors the carbon dioxide content of the blood for the supply of the brain and controls breathing accordingly .
- 2011: Pampersthese: extensions of astrocytes surround the synapses with a diaper-like cover (" pampers ") and prevent the signal-transmitting messenger substance from acting in an undesired place.
None of these theses has so far (as of 2017) been supported or even proven by research results.
pathology
Brain tumors whose tumor cells arise from astrocytes are called astrocytomas . This group includes benign, but also malignant tumors such as glioblastoma .
Neurogenin-2
Some astrocytes apparently have a kind of stem cell function : In certain areas of the brain, they transform into neurons when needed. This process can also be carried out artificially in the laboratory: if the blueprint for a protein called Neurogenin-2 is inserted into cultivated astrocytes, the cells show the typical shape of nerve cells, including functional synapses, after a short time . You can even control what kind of nerve cells should form by introducing other proteins, such as DLX2 . However, it is currently unclear whether this laboratory technique can also be used with living organisms.
Detection of brain and spinal cord in meat products
In astrocytes, the intermediate filament GFAP ( glial fibrillary acidic protein , acidic glial fiber protein) occurs as a marker , which can thus be used to detect central nervous tissue, for example in meat products, which has gained in importance, especially with regard to BSE . The formation of the protein is increased by pathological changes in the brain tissue.
literature
- CJ Garwood, LE Ratcliffe, JE Simpson, PR Heath, PG Ince, SB Wharton: Review: Astrocytes in Alzheimer's disease and other age-associated dementias: a supporting player with a central role. In: Neuropathology and Applied Neurobiology. 43, 2017, p. 281, doi : 10.1111 / nan.12338 .
- BS Khakh, MV Sofroniew: Diversity of astrocyte functions and phenotypes in neural circuits. In: Nature Neuroscience . Volume 18, number 7, July 2015, pp. 942-952, doi : 10.1038 / nn.4043 , PMID 26108722 , PMC 5258184 (free full text) (review).
Individual evidence
- ↑ (Willibald) Pschyrembel: Clinical Dictionary. 255th edition. Walter de Gruyter, Berlin 1986. ISBN 3-11-007916-X .
- ↑ Brian A MacVicar, Eric A. Newman: Astrocyte regulation of blood flow in the brain. In: Cold Spring Harb Perspect Biol 7, 5, 2015: a020388. PDF.
- ^ NA Jessen, AS Munk, I. Lundgaard, M. Nedergaard: The Glymphatic System: A Beginner's Guide. In: Neurochemical research. Volume 40, number 12, December 2015, pp. 2583-2599, doi: 10.1007 / s11064-015-1581-6 , PMID 25947369 , PMC 4636982 (free full text) (review).
- ↑ D. Raper, A. Louveau, J. Kipnis: How Do Meningeal Lymphatic Vessels Drain the CNS? In: Trends in neurosciences. Volume 39, number 9, September 2016, pp. 581-586, doi: 10.1016 / j.tins.2016.07.001 , PMID 27460561 , PMC 5002390 (free full text) (review).
- ↑ UKB University Hospital BONN / Medical Faculty - 435-07.11.06: Study sheds new light on the role of glial cells. In: www.ukb.uni-bonn.de. Retrieved September 28, 2015 .
- ↑ Command to change air - Bild der Wissenschaft. In: www.wissenschaft.de. Retrieved September 28, 2015 .
- ^ Pampers for the brain - University of Bonn. In: www3.uni-bonn.de. Retrieved September 28, 2015 .
- ↑ Hui Zong, Luis F Parada, Suzanne J Baker: Cell of origin for malignant gliomas and its implication in therapeutic development. In: Cold Spring Harb Perspect Biol 7, 5, 2015: a020610. PDF.