Glial cell is a collective term for cells in nerve tissue that can be structurally and functionally differentiated from nerve cells (neurons). The discoverer of the glial cells was Rudolf Virchow in the middle of the 19th century . He suspected a supporting and holding function and therefore gave the cells the name glial cells, derived from the Greek word glia for "glue".
At the end of the 19th century, Santiago Ramón y Cajal , Pío del Río Hortega and Camillo Golgi managed to classify them in even more detail by means of different silver impregnation ( Golgi color ). Almost all glial cells originate (like nerve cells) from the ectodermal germ layer , more precisely from the neuroectoderm (neuroglia); only the microglia (mesoglia) are of mesodermal origin.
According to current knowledge, glial cells not only form a supporting structure for nerve cells , but also provide electrical insulation through their covering . Furthermore, glial cells are significantly involved in the transport of substances and fluid exchange as well as in maintaining homeostasis in the brain. In addition, they also participate in the process of information processing, storage and forwarding.
About half of the cells in the human brain are glial cells, similar to other primates . Glial cells are usually smaller than nerve cells, but in contrast to these, their average cell mass in nerve tissue varies only slightly in different mammalian species. In their brain structures, the ratio of glia to neurons in terms of number and volume mainly depends on the average neuron size.
Glial cell types
A distinction is made between different types. In the central nervous system are found:
- Neuroglia, further divided into:
- Radial glia
- Microglia (Mesoglia, Hortega cells residentielle macrophages)
- Müller cells in the retina
The following are found in the peripheral nervous system :
- Schwann cells (peripheral glia)
- Mantle cells (satellite cells , amphicytes)
- Teloglia of the motor end plate
In addition, the following cells are assigned to the glial cells:
- Supporting cells of the sensory epithelium
- Pituitary cells are glial cells that can only be found in the neurohypophysis (they influence the transport, storage and release of hormones in the nerve fibers)
The astroglia ( astrocytes ) make up the majority of the glial cells in the central nervous system of mammals . They are star-shaped branched cells, the extensions of which form boundary membranes to the brain surface (or pia mater ) and to the blood vessels .
Two types of astrocytes are known:
- Fiber glia ( Astrocytus fibrosus - also: long rays ), rich in fibrils, especially in the white matter . Characterized by numerous microtubules and intracellular fiber structures in the electron microscope
- Protoplasmic glia ( Astrocytus protoplasmaticus - also: short rays) mainly in the gray matter
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. In doing so, they also regulate the extracellular pH balance in the brain.
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.
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 .
In astrocytes, the intermediate filament GFAP ( glial fibrillary acidic protein , "acidic glial fiber protein") occurs as a marker , which is used to detect central nervous tissue, e.g. B. can be used in meat products, which has become particularly important with regard to BSE . The formation of the protein is increased by pathological changes in the brain tissue.
A special form of astrocytes is the radial glia . B. pull through the molecular layer in approximately parallel and end in end feet at the pia mater . They play an important role as lead structures in the early brain development of vertebrates (vertebrate animals). In the mature (mammalian) brain, they are only present in the cerebellum (Bergmannglia) and in the retina (Müllerglia).
Oligodendroglia ( oligodendrocytes ) form the myelin , the electrical insulation of the axons of the nerve cells in the brain or central nervous system . They thus correspond to the Schwann cells in the peripheral nervous system . However, they differ in principle because an oligodendrocyte can wrap axon sections of several nerve cells, while a myelin-forming Schwann cell always only wraps one neuronal axon. The evolution of oligodendrocytes is regarded as a prerequisite for the development of the cerebrum in chordates .
Astroglia and oligodendroglia are also summarized by some authors under macroglia in order to distinguish them from:
Microglial cells (also known as Hortega cells or mesoglia ) make up about 20% of all glial cells. They are the only cell type of the parenchyma of the central nervous system (CNS) that is neither a neuronal nor a vascular cell, rather the microglial cells represent the resident (resident) inflammatory cells of the CNS. A special feature of the microglial cells is that they are both CNS glial cells and a unique type of mononuclear phagocyte . They not only act as scavenger cells for the immune defense in the CNS, but also ensure the correct number of neuronal progenitor cells during the development of the CNS.
Since antibodies can not cross the blood-brain barrier , microglial cells represent the main form of active immune defense in the CNS. Their task is to identify and eliminate potential pathogenic substances. In this way, they primarily protect the non-regenerable neurons of the CNS from irreversible damage. By mediating inflammatory immune responses, they also support the nerve cells in regeneration after an injury. They have a similar function to macrophages in other tissues, since they phagocytose away cell debris from dead nerve cells and oligodendrocytes . During embryonic development, microglia arise from precursor cells in the yolk sac and not, like the rest of the cells of the nervous system, from the neural crest and the neural tube , i.e. the ectoderm . To the antigen-presenting cells belong, a molecular activation is required for their function. For example, activation after head trauma , in diseases such as multiple sclerosis or in hereditary leukodystrophies has been observed. The territorial behavior of the microglia is noticeable: there is always a certain distance between two cells.
Also schizophrenia have significantly more patient-activated microglia in the brain than healthy.
- Dormant microglia have heterochromatin- rich cell nuclei and electron-dense cytoplasm . In addition to typical organelles, lysosomes and vimentin filaments are found here as components of the cytoskeleton. The cell shape is characterized by fine, irregular proliferation.
- Active microglia respond to injuries to the CNS with hypertrophy and proliferation . They differ from the inactive forms by having more developed processes.
Reactive microglia show characteristic behavior. After their activation, the cells accumulate at the location of the lesion , which is made possible by the amoeboid locomotion. Then through phagocytosis or exocytosis of cytotoxic substances such as hydrogen peroxide or nitric oxide, dead cell substances and foreign bodies are removed. After the breakdown of defective endogenous and foreign components, they release specific cytokines (interleukin-1, tumor necrosis factor α, interferon-γ) into the extracellular space , which initiates astrocyte proliferation and the formation of glian scars, which inhibits further immune reactions.
A disturbance in the drainage function of astrocytes or damage to the blood-brain barrier can be a cerebral edema caused. More common tumors are gliomas such as astrocytoma , oligodendroglioma, and glioblastoma .
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