Neuroglia represents the environment surrounding neurocytes and performing supporting, delimiting, trophic and protective functions in the nervous tissue. The selectivity of metabolism between nervous tissue and blood is ensured, in addition to the morphological characteristics of the capillaries themselves (solid endothelial lining, dense basement membrane), also by the fact that the processes of gliocytes, primarily astrocytes, form a layer on the surface of the capillaries that delimits neurons from direct contact with the vascular wall . Thus, the blood-brain barrier is formed.
Neuroglia consists of cells that are divided into two genetically distinct types:
1) Gliocytes (macroglia);
2) Glial macrophages (microglia).
Gliocytes
Gliocytes, in turn, are divided into:
1) ependymocytes; 2) astrocytes; 3) oligodendrocytes.
Ependymocytes form a dense epithelial-like layer of cells lining the spinal canal and all ventricles of the brain.
Ependymocytes are the first to differentiate from the glioblasts of the neural tube, performing demarcation and support functions at this stage of development. On the inner surface of the neural tube, elongated bodies form a layer of epithelial-like cells. On cells facing the cavity of the neural tube, cilia are formed, the number of which on one cell can reach up to 40. Cilia obviously contribute to the movement of cerebrospinal fluid. Long processes extend from the basal part of the ependymocyte, which branch out to cross the entire neural tube and form the apparatus that supports it. These processes on the outer surface take part in the formation superficial glial limiting membrane, which separates the substance of the tube from other tissues.
After birth, ependymocytes gradually lose their cilia; they are retained only in some parts of the central nervous system (midbrain aqueduct).
In the area of the posterior commissure of the brain, ependymocytes perform a secretory function and form a “subcommissural organ” that secretes a secretion, which is believed to take part in the regulation of water metabolism.
The ependymocytes that cover the choroid plexuses of the ventricles of the brain are cubic in shape; in newborns, cilia are located on their surface, which are later reduced. The cytoplasm of the basal pole forms numerous deep folds and contains large mitochondria, inclusions of fat and pigments.
Astrocytes - these are small star-shaped cells, with numerous processes diverging in all directions.
There are two types of astrocytes:
1) protoplasmic;
2) fibrous (fibrous).
Protoplasmic astrocytes
¨Localization - gray matter of the brain.
¨Dimensions - 15-25 microns, have short and thick, highly branched processes.
¨The core is large, oval, light.
¨Cytoplasm - contains a small amount of endoplasmic reticulum cisterns, free ribosomes and microtubules, and is rich in mitochondria.
¨Function - delimitation and trophic.
Fibrous astrocytes.
¨Localization - white matter of the brain.
¨Dimensions - up to 20 microns, have 20-40 smoothly contoured, long, weakly branching processes that form glial fibers that form a dense network - the supporting apparatus of the brain. The processes of astrocytes on blood vessels and on the surface of the brain, with their terminal extensions, form perivascular glial limiting membranes.
¨The cytoplasm is light in electron microscopic examination, contains few ribosomes and elements of the granular endoplasmic reticulum, is filled with numerous fibrils with a diameter of 8-9 nm, which extend into processes in the form of bundles.
¨The nucleus is large, light-colored, the nuclear envelope sometimes forms deep folds, and the karyoplasm is characterized by uniform electron density.
¨Function is support and isolation of neurons from external influences.
Oligodendrocytes - the most numerous and polymorphic group of gliocytes responsible for the production of myelin in the central nervous system.
¨Localization - they surround the bodies of neurons in the central and peripheral nervous system, are part of the sheaths of nerve fibers and nerve endings.
¨The cell sizes are very small.
¨Shape - different parts of the nervous system are characterized by different shapes of oligodendrocytes (oval, angular). Several short and weakly branched processes extend from the cell body.
¨Cytoplasm - its density is close to that of nerve cells, does not contain neurofilaments.
¨Function - perform a trophic function, participating in the metabolism of nerve cells. They play a significant role in the formation of membranes around cell processes; they are called neurolemmocytes (Schwann cells), and participate in water-salt metabolism, the processes of degeneration and regeneration.
Neuroglia- an extensive heterogeneous group of elements of nervous tissue that ensures the activity of neurons and performs nonspecific functions: supporting, trophic, delimiting, barrier, secretory and protective functions. It is an auxiliary component of nervous tissue.
In the human brain, the content of glial cells (gliocytes) is 5-10 times higher than the number of neurons, and they occupy about half of its volume. Unlike neurons, adult gliocytes are capable of division. In damaged areas of the brain, they multiply, filling defects and forming glial scars (gliosis); Tumors from glial cells (gliomas) account for 50% of intracranial neoplasms.
CLASSIFICATION AND FUNCTIONAL MORPHOLOGY OF NEUROGLIA
Neuroglia include macroglia and microglia. Macroglia are divided into: astrocytic glia (astroglia), oligodendroglia and ependymal glia (Fig. 8.7.).
Astroroglia(from the Greek astra - star and glia - glue) is represented by astrocytes - the largest of the glial cells that are found in all parts of the nervous system.
| A | B |
Rice. 8.7. A – Diagram of an astrocyte. The terminal formations of the processes extending from the body radially entwine the blood vessels, participating in the formation of the blood-brain barrier. B – Star-shaped astrocytes are located in the gray matter of the brain, limiting the receptor fields of neurons (x400 impregnation with silver salts).
Astrocytes are characterized by a light oval nucleus, cytoplasm with moderately developed essential organelles, numerous glycogen granules and intermediate filaments. At the ends of the processes there are lamellar extensions (“legs”), which, connecting to each other, surround vessels or neurons in the form of membranes (Fig. 8.7.A)
Astrocytes are divided into two groups:
- Protoplasmic (plasmatic) astrocytes found predominantly in the gray matter of the central nervous system; They are characterized by the presence of numerous branched short relatively thick processes.
- Fibrous (fibrous) astrocytes are located mainly in the white matter of the central nervous system. Long, thin, slightly branched processes extend from their bodies.
Functions of astrocytes:
1. Support- formation of the supporting frame of the central nervous system, within which other cells and fibers are located; During embryonic development, they serve as supporting and guiding elements along which the migration of developing neurons occurs. The guiding function is also associated with the secretion of growth factors and the production of certain components of the intercellular substance, recognized by embryonic neurons and their processes.
2. Demarcation, transport and barrier(aimed at ensuring an optimal microenvironment of neurons): the formation of perivascular limiting membranes by the flattened end sections of the processes, which cover the capillaries from the outside, forming the basis of the blood-brain barrier (BBB). The BBB separates the neurons of the central nervous system from the blood and tissues of the internal environment.
3. Metabolic and regulatory– is considered one of the most important functions of astrocytes, which is aimed at maintaining certain concentrations of K + ions and transmitters in the microenvironment of neurons. Astrocytes, together with oligodendroglial cells, take part in the metabolism of mediators (catecholamines, GABA, peptides, amino acids), actively capturing them from the synaptic cleft after synaptic transmission and then transmitting them to the neuron;
4. Protective (phagocytic, immune and reparative)- participation in various protective reactions when nerve tissue is damaged. Astrocytes, like microglial cells (see below), are characterized by pronounced phagocytic activity. At the final stages of inflammatory reactions in the central nervous system, astrocytes, growing, form a glial scar in place of the damaged tissue.
Ependymal glia, or ependyma(from the Greek ependyma - outer clothing, i.e. lining) is formed by cubic or cylindrical cells (ependymocytes), single-layer layers of which line the cavities of the ventricles of the brain and the central canal of the spinal cord (see Fig. 8.8.). A number of authors also include flat cells that form the lining of the meninges (meningothelium) as ependymal glia.
Rice. 8.8. The electron micrograph shows: Cuboid-shaped ependymal cells form a layer lining the walls of the brain ventricle and the spinal canal (x400). On the free surface of cells are cilia.
The nucleus of ependymocytes contains dense chromatin, the organelles are moderately developed. The apical surface of the ependymocytes bears cilia, which move the CSF with their movements, and a long process extends from the basal pole of some cells, extending to the surface of the brain and being part of the superficial limiting glial membrane (marginal glia).
Functions of ependymal glia:
1. supporting (due to the basal processes);
2. formation of barriers:
Neuro-cerebrospinal fluid (with high permeability),
Hemato-cerebrospinal fluid
3. ultrafiltration of CSF components
Oligodendroglia(from the Greek oligo - little, dendron - wood and glia - glue, i.e. glia with a small number of processes) - a large group of various small cells (oligodendrocytes) with short, few processes that surround the bodies of neurons, includes the composition of nerve fibers and . nerve endings (Fig. 8.9.). Found in the central nervous system (gray and white matter) and PNS; characterized by a dark core; dense cytoplasm with a well-developed synthetic apparatus, high content of mitochondria, lysosomes and glycogen granules.
| A | B |
Rice. 8.9. A – Scheme of an oligodendrocyte. B – oligodendrocyte (O). The cytoplasm contains EPS, ribosomes, microtubules, the Golgi apparatus is well developed (G), the neuron body is nearby (N), the dendrite (D) and myelinated axon (M) are clearly visible (x 13000).
Microglia- a collection of small elongated stellate cells (microgliocytes) with dense cytoplasm and relatively short branching processes, located mainly along the capillaries in the central nervous system (see Fig. 8.10.). Unlike macroglial cells, they are of mesenchymal origin, developing directly from monocytes (or perivascular macrophages of the brain) and belong to the macrophage-monocyte system. They are characterized by nuclei with a predominance of heterochromatin and a high content of lysosomes in the cytoplasm.
Rice. 8.10. Diagram of a microglial cell.
Microglial function– protective (including immune). Microglial cells are traditionally considered as specialized macrophages of the central nervous system - they have significant mobility, becoming activated and increasing in number during inflammatory and degenerative diseases of the nervous system, dead cells (detritus).
The nervous system consists not only of neurons, but also of processes. It contains glial cells necessary for human life. With their help, the nervous system is limited from other environments of the body, which ensures important human functions. Cells have division features, and this is how they differ from neurons.
The collection of cells is called neuroglia or glia. They are considered special cellular structures that are present in the nervous system. They support the brain and spinal cord, as well as the supply of necessary components.
With a blood-brain barrier, immune function is thought to be absent. But when foreign substances penetrate the brain or spinal cord, the cell phagocytizes an analogue of the macrophage. The part of the brain from the peripheral tissues works thanks to neuroglia.
Properties
These structures have many properties that are different from other structures. This is due to the unique conditions that neurons create. Glycotes can divide, but they do not have the function of reproduction and transmission of nerve impulses.
The potential of glia is greater compared to neurons. This is due to the concentration of potassium cations in the cytoplasm. When exposed to stimuli, cells can respond with slow-wave changes.
Immune activity of the brain
Various biochemical reactions occur in the brain, so it must be protected from humoral immunity. It must be taken into account that neuronal tissue is sensitive to diseases, which is why neuronal recovery occurs partially.
It turns out that the formation of areas in the nervous system where a local reaction occurs causes the destruction of many cells. In the periphery of the body, painful places are filled with new cells. In the brain, a lost neuron is not restored. Thanks to neuroglia, the brain is not affected by the immune system.
Classification
Glial cells are divided into 2 types based on morphology and origin. There are microglial and macroglial cells. The first type has many processes with the help of which solid components are phagosed.
Macroglia are derived from ectoderm. Glial cells are divided according to morphology, and therefore they are ependymal and astrocytic, oligodendrocytes. Each type has its own characteristics.
Cell functions
Such structures perform important functions in the body. Astroglia includes many cells, the processes of which are located on the surface of blood vessels. It includes many structures that ensure the normal functioning of the nervous system. Astroglia is used as a support for neurons, normalizes reparative activity, separates nerve fiber, and performs a metabolic function.
Oligodendroglia are presented in the form of cells with processes. It is located under the cerebral cortex. With its help, myelination of axons and metabolism of neurons are performed. Microglia are small cells. They appear from the membranes of the brain, pass into the white and then gray matter. All their functions are important for human development.
Peculiarities
Glial cells can change in size, which is their feature. Moreover, this happens rhythmically with the help of a phase of contraction and relaxation. When the processes swell, their shortening is not observed.
Cell activity occurs thanks to the active components: serotonin and norepinephrine. A physiological feature is the effect on the intercellular space. Cells do not have impulse activity like nerve cells, but they do have a charge to generate membrane activity. Its changes occur slowly, which is determined by the activity of the nervous system.
Glial cells can spread, and this happens in 30-60 ms. The development of activity between them occurs with the help of gap junctions. These contacts exhibit low resistance and also create a sphere for current to flow from one area to another. Since glia are located with neurons, the functioning of the nervous system influences the electrical activity in the glial components.
Pathological processes
Due to exposure to pathologies, neuroglial cells are exposed to various negative consequences.
The following changes may occur:
- swelling and swelling;
- hypertrophy and atrophy;
- hyperplasia;
- amoeboid degeneration;
- homogenizing metamorphosis.
This disease, due to which the cellular structure changes, also occurs in histological examination when it is necessary to identify other human diseases. For a long period of time to examine the nervous system, neuroglial substances were considered secondary. Now they are considered the main components of nervous tissue. Pathologies can cause complex diseases.
Impact of neurons and glial cells
They have common properties and structure, for example, a nucleus that contains genetic information. The exchange between them occurs due to signaling molecules that enter through the membrane using a variety of mechanisms. They have the ability to process signals.
To perform their functions, they have processes that work together. Neurons can transmit an electrochemical signal to the axon, which results in an action. They are connected to each other by synapses.
Some time ago, it was discovered that glia, which were previously used to normalize nervous tissue, are used in signal transmission. They are included in most of the brain, and therefore all their functions are necessary for normal human development.
It was previously believed that glia performed minor roles, but it was later determined that they perform major functions. The signals are transmitted by calcium waves that occur slowly. Neuroglia contact neurons with the help of neurotransmitters. In addition, they are considered the area of the brain where GABA and glutamate are formed.
That is why neuroglia are considered an important element necessary for the full development of a person. Their normal functioning ensures thinking and many other brain processes. If any areas are damaged, effective treatment prescribed by a doctor is required.
Neuroglia (from the Greek neuron - vein, nerve and Greek glia - glue) is the totality of all cellular elements of the nervous tissue, except neurons. Glial cells play an important role in ensuring metabolic processes in neurons. These are cells in the brain, with their bodies and processes filling the spaces between nerve cells - neurons - and brain capillaries.
Each neuron is surrounded by several neuroglial cells, which are evenly distributed throughout the brain and make up about 40% of its volume. Neuroglial cells - their number in the central nervous system (CNS) of mammals is about 140 billion - are 3-4 times smaller than neurons and differ from them in morphological and biochemical characteristics. With age, the number of neurons in the central nervous system decreases, and the number of neuroglial cells increases, because the latter, unlike neurons, retain the ability to divide.
Functions of neuroglia
Main functions: creation of a blood-brain barrier between the blood and neurons, necessary both to protect neurons and mainly to regulate the flow of substances into the central nervous system and their excretion into the blood; ensuring the reactive properties of nervous tissue (formation of scars after injury, participation in inflammatory reactions, in the formation of tumors, etc.). There are astroglia, oligoglia, or oligodendroglia, and ependyma, which together make up macroglia, as well as microglia, which occupy a special position among neuroglial cells.
Astroglia (about 60% of the total number of neuroglial cells) are star-shaped cells with numerous thin processes entwining neurons and capillary walls; main element of the blood-brain barrier; regulates water-salt metabolism of nervous tissue.
Oligoglia (about 25-30%) are smaller, round cells with short processes. Surrounding the bodies of neurons are nerve conductors - axons. They are distinguished by a high level of protein and nucleic acid metabolism; responsible for the transport of substances into neurons. Participate in the formation of myelin sheaths of axons. Ependyma consists of cylindrical cells lining the ventricles of the brain and the central canal of the spinal cord. Acts as a barrier between blood and cerebrospinal fluid; Apparently, it also performs the secretory function of neuroglia (mainly oligoglia) and is involved in the origin of slow spontaneous bioelectrical activity, which includes a-waves of the electroencephalogram. The "neuron - neuroglia" system is a single functional-metabolic complex, characterized by cyclical operation, adaptability of reactions, and the ability to switch certain metabolic processes predominantly into neurons or neuroglia, depending on the nature and intensity of physiological and pathological effects on the central nervous system.
Glial cells are not excitable, that is, PD does not occur in them. However, just like in typical excitable cells, they have a concentration gradient of ions. And when neurons adjacent to them exhibit high activity, the membrane potential of glial cells changes. This occurs as a result of the following morphophysiological features:
A) between glial and nerve cells there is a very small intercellular gap (about 15 nm);
b) there are tight junctions between individual glial cells;
V) the glial membrane is easily permeable to K.
Therefore, when APs occur in neurons, the K concentration in the intercellular fluid increases (the outgoing potassium current ensures membrane repolarization). As a result, K diffuses into the glial cells and their membrane is depolarized. Therefore, an electric current occurs between the depolarized and neighboring glial cells. This current, in turn, further increases K entry into depolarized cells.
As a result, glial cells significantly reduce the extracellular concentration of potassium ions near active neurons. This ensures high “efficiency” of the latter, since active neurons do not have time to pump potassium into the cell (Na, K-Hacoc pumps three sodium ions out of the cell in one “move”, but pumps only two potassium ions) and therefore increasing its concentration by the outside of the membrane can lead to a decrease in the functional activity of neurons. K absorbed by neuroglia in the same way as mediators, then, during “rest”, is transferred from them to the neuron. Astrocytes, performing the above functions, make it easier for neurons to perform their functions, that is, they indirectly participate in the regulation of body functions. Moreover, the role of astrocytes in neuronal function is not limited to this; it is probably more complex. The fact is that receptors for most neurotransmitters are found on the membrane of astrocytes. Although at present the significance of these receptors is not yet entirely clear. It is also very significant that astrocytes synthesize a number of factors classified as growth regulators. Astrocyte growth factors are involved in the regulation of neuronal growth and development.
This function is especially clearly manifested in the process of formation of the central nervous system. in the prenatal and early postnatal periods of development. Astrocytes participate in the immune mechanisms of the brain, protecting it from invading microorganisms. Oligodendrocytes (about 25-30% of all glial cells) form the myelin sheath of neurons. In the periphery, this function is performed by Schwann cells. In addition, they can absorb microorganisms, that is, along with astrocytes, they participate in the immune mechanisms of the brain. Ependymal cells Ependymal cells line the ventricles of the brain, participating in the secretion of cerebrospinal fluid (CSF) and in the creation of the blood-brain barrier (BBB). Microglia make up about 10% of all glial cells. Microglia, being part of the reticuloendothelial system of the body, participates in phagocytosis.
But they make up 10% of the brain volume. Depending on the size and number of processes, they are distinguished astrocytes , oligodendrocytes , microgliocytes .
Neurons and glial cells are separated by a narrow (20 nM) intercellular gap. These slits are interconnected and form the extracellular space of the brain, filled with interstitial fluid. Due to this space, neurons and glia are provided with oxygen and nutrients.
Glial cells rhythmically increase and decrease at a frequency of several oscillations per hour. This promotes the flow of axoplasm along the axons and the movement of intercellular fluid. Thus, glions serve as a supporting apparatus of the central nervous system, ensure metabolic processes in neurons, and absorb excess neurotransmitters and their decay products.
It is assumed that glia are involved in the formation of conditioned reflexes and memory.
In addition to neurons, nervous tissue also includes cells that are companions of neurons - neuroglia (Fig. 1.20). Neuroglial cells (astrocytes, oligodendrocytes, microglia) fill the entire space between neurons, protecting them from mechanical damage (support function). There are about 10 times more of them than neurons, and, unlike them, glial cells retain the ability to divide throughout life. In addition, they form myelin sheaths around nerve fibers. During this process, the oligodendrocyte (in the central nervous system) or its variant, the Schwann cell (in the peripheral nervous system), wraps around a section of the nerve fiber. Then it forms an outgrowth in the form of a tongue, which twists around the fiber, forming layers of myelin (the cytoplasm is squeezed out). Thus, the layers of myelin are, in essence, a tightly compressed cytoplasmic membrane.
Neuroglia also perform a protective function. It lies, firstly, in the fact that glial cells (mainly astrocytes) together with the epithelial cells of the capillaries form a barrier between the blood and neurons, preventing unwanted (harmful) substances from passing through to the latter. This barrier is called the blood-brain barrier. Secondly, microglial cells perform the function of phagocytes in the nervous system. Carrying out a trophic function, neuroglia supplies neurons with nutrients, controls water-salt metabolism, etc.
Rudolf Virchow. 1856. Nerve glue.
Types of neuroglia:
A - protoplasmic astrocytes(in gray matter),
B - fibrous astrocytes(in white matter)
G - oligodendrocytes.
Neuroglia. Astrocytes. Astrocytes:largest&most numerous
A silvered preparation of astrocytes, showing their many fine cytoplasmic processes. Notice their close association with the capillaries (the heavy black structures). Since astrocytes touch both capillaries and neurons they are thought to play an intermediary role in the nutrition and metabolism of neurons.
Functions of astrocytes:
Support of nerve cells
Restoration of nerve fibers when damaged,
Isolation and union of nerve fibers,
Participation in metabolic processes between capillaries and neurons,
Participation in the processes of neuron migration in ebryogenesis.
