The nervous system consists not only of neurons and their processes. For 40% it is represented by glial cells, which play an important role in its life. They literally limit the brain and nervous system from the rest of the body and ensure its autonomous operation, which is really important for humans and other animals that have a central nervous system. Moreover, neuroglial cells are able to divide, which distinguishes them from neurons.
General concept of neuroglia
The collection of glial cells is called neuroglia. These are special cell populations that are located in the central nervous system and on the periphery. They support the shape of the brain and spinal cord, and also supply it with nutrients. It is known that there are no immune reactions in the central nervous system due to the presence of the blood-brain barrier. However, when a foreign antigen enters the brain or spinal cord, as well as in the cerebrospinal fluid, the gli althe cell, a reduced analog of the peripheral tissue macrophage, phagocytizes it. Moreover, it is the separation of the brain from peripheral tissues that provides neuroglia.
Immune brain protection
The brain, where many biochemical reactions take place, which means that a lot of immunogenic substances are formed, must be protected from humoral immunity. It is important to understand that the neuronal tissue of the brain is very sensitive to damage, after which neurons recover only partially. This means that the appearance of a place in the central nervous system where a local immune reaction will take place will also lead to the death of some surrounding cells or demyelination of neuronal processes.
On the periphery of the body, this damage to somatic cells will soon be filled with newly formed ones. And in the brain, it is impossible to restore the function of a lost neuron. And it is neuroglia that limits the brain from contact with the immune system, for which the central nervous system is a huge amount of foreign antigens.
Classification of glial cells
Glial cells are divided into two types depending on morphology and origin. Separate microglial and macroglial cells. The first type of cells originates from the mesodermal sheet. These are small cells with numerous processes capable of phagocytizing solids. Macroglia is a derivative of the ectoderm. The macroglial glial cell is divided into several types depending on the morphology. Allocate ependymal and astrocytic cells, as well as oligodendrocytes. These types of cell populations are also divided into sever altypes.
Ependymal glial cell
Ependymal glial cells are found in specific areas of the central nervous system. They form the endothelial lining of the cerebral ventricles and the central spinal canal. They take their origin in embryogenesis from the ectoderm, and therefore represent a special type of neuroepithelium. It is multi-layered and performs a number of functions:
- support: makes up the mechanical frame of the ventricles, which is also supported by the hydrostatic pressure of the CSF;
- secretory: secretes some chemicals into the cerebrospinal fluid;
- delimiter: separates the medulla from the cerebrospinal fluid.
Types of ependymocytes
Among ependymocytes, there are some types. These are ependymocytes of the 1st and 2nd order, as well as tanycytes. The former form the initial (basal) layer of the ependymal membrane, and the ependymocytes lie in the second layer above them. It is important that the ependymal glial cell of the 1st order is involved in the formation of the hematoglyphic barrier (between the blood and the internal environment of the ventricles). Ependymocytes of the 2nd order have villi oriented towards the CSF flow. There are also tanycytes, which are receptor cells.
They are located in the lateral sections of the bottom of the 3rd cerebral ventricle. Having microvilli on the apical side and one process on the basal side, they can transmit information to neurons about the composition of the CSF fluid. At the same time, the cerebrospinal fluid itself through small numerous slit-like holes between the ependymocytes of the 1st and2nd order can go directly to neurons. This allows us to say that ependyma is a special type of epithelium. Its functional, but not morphological counterpart at the periphery of the body is the endothelium of blood vessels.
Oligodendrocytes
Oligodendrocytes are types of glial cells that surround a neuron and its processes. They are found both in the central nervous system and near the peripheral mixed and autonomic nerves. The oligodendrocytes themselves are polygonal cells equipped with 1-5 processes. They interlock with each other, isolating the neuron from the internal environment of the body and providing conditions for nerve conduction and generation of impulses. There are three types of oligodendrocytes that differ in morphology:
- central cell located near the body of a brain neuron;
- satellite cell surrounding the body of a neuron in a peripheral ganglion;
- Schwann cell, covering the neuronal process and forming its myelin sheath.
Oligodendrocyte glial cells are found both in the brain and spinal cord and in peripheral nerves. Moreover, it is not yet known how the satellite cell differs from the central one. Considering that the genetic material of all cells of the body, except for the sex cells, is the same, it is likely that these oligodendrocytes can mutually replace each other. The functions of oligodendrocytes are as follows:
- reference;
- insulating;
- separating;
- trophic.
Astrocytes
Astrocytes are the glial cells of the brain that make up the medulla. They are star-shaped and small in size, although they are larger than microglial cells. There are only two types of astrocytes: fibrous and protoplasmic. The first type of cells is located in the white and gray matter of the brain, although there are significantly more of them in the white.
This means that they are most common in areas where there is a significant number of neuronal myelinated processes. Protoplasmic astrocytes are also glial cells: they are found in the white and gray matter of the brain, but they are much more numerous in the gray matter. This means that their function is to create a support for the bodies of neurons and the structural organization of the blood-brain barrier.
Microglia
Microglial cells are the last type of neuroglia. However, unlike all other cells of the central nervous system, they are of mesodermal origin and are special types of monocytes. Their precursors are stem blood cells. Due to the structural features of neurons and their processes, it is glial cells that are responsible for immune reactions in the central nervous system. And their functions are almost similar to those of tissue macrophages. They are responsible for phagocytosis and antigen recognition and presentation.
Microglia contain special types of glialcells that have receptors of differentiation clusters, which confirms their bone marrow origin and the implementation of immune functions in the central nervous system. They are also responsible for the development of demyelinating diseases, Alzheimer's disease and Parkinson's syndrome. However, the cell itself is only a way of implementing the pathological process. Therefore, probably, when it is possible to find the mechanism of microglia activation, the development of these diseases will be stopped.
