Neuroglia – common features, classification and function

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Neuroglia (glia, glial tissue) is a supporting tissue, that other than neurons is an integrative part of the nervous system. Glial cells represent about 90% of all cells in the nervous system, and have a wide range of functions. They are the support of the neuronal network, ensure the nutrition of the neurons, have phagocytic ability, and form myelin, which aids insulation.

These cells have argyrophilic properties, so they can be very well stained with silver impregnation. 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 are classified as macroglia, and are the largest glial cells. Astrocyte processes are attached to blood vessels and to the pia mater by means of extended glial pedicles. 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, or the BBB). 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 separate nervous tissue from its 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 excitation outside the synapse.

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

We distinguish two types of astrocytes, plasma and fibrillar.

Plasma astrocytes

• Voluminous cytoplasm and numerous broad projections.
• Cytoplasm containing numerous bundles of gliofibrils.
• They occur mainly in the gray matter CNS, where they nourish neurons and provide them with mechanical support.

Fibrillar astrocytes

• Long, thin and small branched projections.
• Cytoplasm contains numerous bundles of gliofibrils.
• They occur mainly in the white matter of the CNS (many axons 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 are a specialized type of neuroglia that form the lining (ependyma) of the fluid-filled cavities within the Central Nervous System (CNS)—namely, the cerebral ventricles and the central canal of the spinal cord.

Structure and Location:

• Epithelial-like Arrangement: Ependymal cells are arranged in a single layer, giving them the appearance of a simple cuboidal to columnar epithelium (sometimes referred to as a "pseudoepithelium"). Crucially, however, they are classified as glia and lack a true underlying basement membrane.
• Apical Modifications: The surface facing the cavity (the apical surface) is equipped with both cilia (microcilia) and microvilli.
  • The cilia are motile structures whose rhythmic beating facilitates the circulation and movement of the cerebrospinal fluid (CSF) within the CNS cavities.
  • The microvilli are non-motile projections that increase the surface area for absorption and exchange.
• Specialized Type (Tanycytes): A unique subset of ependymal cells are the tanycytes. These cells are predominantly found lining the floor of the third cerebral ventricle. Tanycytes possess long, basal processes that extend deeply into the underlying nervous tissue, suggesting a role in transporting substances between the CSF and the brain parenchyma, particularly in regions lacking a complete blood-brain barrier.

Function:

The primary functions of ependymal cells relate to the production and circulation of CSF:

1. CSF Circulation: The beating of the apical cilia aids in the directional flow and mixing of CSF.
2. Barrier and Exchange: They form a permeable barrier between the neural tissue and the CSF, regulating the exchange of solutes and fluid.
3. CSF Production: Ependymal cells are integral components of the choroid plexus, a specialized vascular structure found in the ventricles. The cells of the choroid plexus actively filter plasma to produce the majority of the CSF.

• 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, and form the myelin sheaths of axons in the peripheral nervous system. However, unlike oligodendrocytes, 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.