Neuroglia – common features, classification and function

This article has been translated from WikiSkripta; ready for the editor's review.

Neuroglia (glia, glial tissue) is a supporting tissue that together with neuronss make up the nervous system. Glial cells represent about 90% of all cells in the nervous system and have a wide range of functions. They form the support of the neuronal network, ensure the nutrition of the neurons, have the ability of phagocytosis and by forming myelin, they aid insulation.

The cells have argyrophilic properties, so they can be very well impregnated with silver. This technique is suitable for imaging the cytoplasm of both neuroglia and neurons. When stained with hematoxylin-eosin (HE), only the nuclei of the glial cells are visible, which are small compared to the nuclei of the surrounding neurons.

Neuroglia are divided into:

1. Central:
   • Macroglia – astrocytes, oligodendroglia, ependyma, tanycytes, Müller cells, pituicytes;
   • Microglia;
2. Peripheral:
   • Schwann cells
   • Satellite cells.

Central glia

Astrocytes

Protrusion of astrocyte on capillary endothelium

Astrocytes belong to macroglia and are the largest glial cells. Astrocyte processes are attached to blood vessels and to the pia mater by means of extended ``glial feet. Around the vessels in the CNS, projections of astrocytes form the ``membrana limitans gliae perivascularis, which is part of the hematoencephalic barrier (barrier between blood and CNS tissue). On the surface of the CNS, projections of astrocytes form the ``membrana limitans gliae superficialis'. Another function of astrocytes is the nutrition of neurons. Astrocytes therefore separate nervous tissue from the surroundings and mediate the exchange of substances with blood or cerebrospinal fluid. At the same time, astrocytes have an insulating function in relation to synapses, as they prevent the spread of excitement outside the synapse.

If some neurons die, the dead focus is replaced by astrocytes, which create a so-called glial scar.

We distinguish two types of astrocytes, plasma and fibrillar.

Plasma astrocytes

• They have voluminous cytoplasm and numerous broad projections.
• Cytoplasm contains numerous bundles of gliofibrils.
• They occur mainly in the gray matter CNS, where they nourish neurons and at the same time provide them with mechanical support.

Fibrillar astrocytes

• They have long, thin and little branched projections.
• Cytoplasm contains numerous bundles of gliofibrils.
• They occur mainly in the white matter of the CNS (many axonss here → they nourish axons).

Oligodendroglia (oligodendrocytes)

Processes of an oligodendrocyte form a myelin sheath around axons in the CNS

They belong to macroglia, but are functionally divided. They have a small body and nucleus and a small number of projections (hence the name ``oligo-). Their main function is the formation of myelin around axons in the CNS. During development, the oligodendrocyte sends out processes that wrap around the axons. One oligodendrocyte myelinates a large number of axons (or 10–50 internodal segments) with its projections. It is divided into interfascicular and perineural. Interfascicular oligodendrocytes form rows between myelinated fibers in the white matter, while perineural ones are located in close proximity to the neuronal bodies in the gray matter. The Obersteiner-Redlich zone is a line that separates the section of the nerve myelinated by oligodendroglia and Schwann cells, thus forming an imaginary interface between the CNS and the PNS.

• They have thin and innumerable, sparsely branched projections.
• The cytoplasmic body is small, the cell contains abundant mitochondria, GK and GER.
• They are found in the white and gray matter of the CNS.

Ependyma

Ependymal cells form the lining of the CNS cavities. In shape and arrangement, they resemble a single-layered epithelium ("pseudoepithelium"), which has a cubic to cylindrical character, but there is no basement membrane under the ependyma. The apical surface of the cells is provided with microcilia and microvilli. The movement of microcilia and microvilli contributes to the circulation of the cerebrospinal fluid. A special type of ependymal cells are 'tanycytes. A long process emerges from the base of the tanycyte and sinks into the nervous tissue. Tanycytes are found mainly on base III. cerebral ventricles. Ependymal cells are also part of the choroid plexus, which produces cerebrospinal fluid.

• They have kinocilia and microvilli on the free surface; a process emerges from the tanycytes and sinks into the nerve tissue.
• The core is round to oval with a prominent nucleolus.
• Lines the cavities of the CNS.

Other Macroglia

Other macroglia include Müller cells' (in the retina), Bergmann cells (in the cerebellum cortex), pituicytes (in neurohypophysis) and pinealocytes' (in epiphysis).

Microglia (Horteg's glia)

Microglia are small cells with an oval body and a large number of projections. They have phagocytic ability, they are part of the monocyto-macrophage system'. When the CNS is damaged, they enlarge, migrate to the site of damage, phagocytize and transform into so-called granule cells'. They are the only cells of the nervous system that do not differentiate from ectoderm but from mesoderm.

• They have a small elongated body with richly branched short spiny projections.
• The nucleus is oval and rich in heterochromatin, the cytoplasm is rich in lysosomes, phagosomes, residual bodies and free ribosomes.
• They occur in the CNS mainly along blood vessels.

Peripheral glia

Schwann cells

Schwann cells

They resemble oligodendrocytes. Forms the myelin sheaths of axons in the peripheral nervous system. However, unlike an oligodendrocyte, one Schwann cell myelinates only one axon, and not only the processes, but the entire cell participates in the formation of myelin. They provide mechanical and metabolic support to axons, ensuring their isolation from the endoneurium.

• They have an elongated shape and rest on the basement membrane.
• The nucleus contains abundant heterochromatin and an indistinct nucleolus, micropinocytotic vesicles are abundant in the cytoplasm.
• They wrap the axons of PNS neurons.

Satellite cells (amphycytes)

Satellite cells are small cells with short processes. They surround the bodies of neurons in sensitive and vegetative ganglia, where they have an important metabolic role.

• They have a flattened shape and rest on the basement membrane.
• The nucleus is round and rich in heterochromatin, the cytoplasm contains abundant free ribosomes, a distinct Golgi apparatus and small lysosomes.
• Surround ganglion cells in the cerebrospinal and autonomic ganglia.

Links

Related Articles

• General structure of nervous tissue
• Neuron
• Nervous tissue (physiology)

External links

• RADIM, Jančálek – PETR, Dubový. Základy neurověd v zubním lékařství [online]. MEFANET, ©2011. The last revision 27.10.2011, [cit. 26.11.2011]. <http://portal.med.muni.cz/clanek-560-zaklady-neuroved-v-zubnim-lekarstvi.html>.

• Neuroglie (česká wikipedie)
• Glial cell (anglická wikipedie)

References

• no. . Základy histologie. 7. edition. Jinočany : H & H, 1997. pp. 502. ISBN 80-85787-37-7.

• yes. . Histologie : celost. učebnice pro lék. fakulty. 1. edition. Avicenum, 1986. 

• yes. . Lékařská fyziologie. 4. edition. Praha : Grada, 2003. pp. 771. ISBN 80-247-0512-5.

• VÁCLAVA, Konrádová – JIŘÍ, Uhlík – LUDĚK, Vajner. Funkční histologie. 1. edition. Jinočany : H & H, 2000. ISBN 80-86022-80-3.