Neuroglia

Classification and origin

The systematization of glial elements is based on their localization (CNS or PNS) and embryonic developmental source. The bulk of the cells belong to macroglia, which is of neural origin and develops from embryonic glioblasts.

The exception is microglia. It forms from promonocytes, meaning it has a monocytic (mesenchymal) origin, which largely determines its macrophagic function.

CNS Macroglia: Astrocytes

Astrocytes (astrocytes) are star-shaped cells originating from embryonic radial glia (which initially served as a "guide" for neuroblast migration).

Two morphological types of astrocytes are distinguished:

• Protoplasmic: characterized by thick, short processes. They are mainly localized in the gray matter of the brain.
• Fibrous: have thin, long, and sparsely branching processes. They predominate in the white matter.

The functions of astroglia are realized through their processes. They create a three-dimensional supporting network, participate in neurotransmitter exchange, and secrete neurocyte growth factors. The ends of the processes feature lamellar expansions that form glial limiting membranes—a key element of the blood-brain barrier, which regulates the transport of substances from capillaries to neurons.

Ependymal glia

Ependymocytes line the cerebral ventricles and the spinal canal, forming a tissue called ependyma. Most often, this is a single-layer sheet of cylindrical cells, although in certain areas (cerebral aqueduct, III and IV ventricles) it can be multi-layered.

Ependyma differs from true epithelium by the absence of a basement membrane along most of its length and the absence of keratin filaments in the cytoplasm.

Structural features and functions:

• On the apical surface of the cells (facing the lumen), there are microvrices and motile cilia (kinocilia). Their beating prevents CSF stagnation.
• The ependyma of the choroid plexuses is joined by tight junctions, forming the blood-cerebrospinal fluid barrier. These cells carry out the unidirectional secretion of CSF from the blood.
• A distinct cell type—tanycytes—has long basal processes. They perform a fixing role and ensure the transport of substances between the hypothalamus and the hypophysis (numerous in the floor of the III ventricle).

Oligodendroglia and peripheral glia

Oligodendrocytes are small glial cells with a small number of short and sparsely branching processes (oligo — few). They perform barrier, trophic, and electrical insulation functions.

According to their localization and tasks, they are divided into two types:

1. Satellite cells (mantle gliocytes): surround neuron cell bodies in the gray matter of the CNS and nerve ganglia of the PNS. In ganglia, they are arranged in a single layer (a "mantle") around the perikaryon, ensuring contact with the neuron, and are covered on the outside by connective tissue. In the CNS, they adhere to the neuron in a mosaic pattern, alternating with astrocyte processes.
2. Oligodendrocytes of nerve fibers: are located in the white matter of the CNS and peripheral nerves. They surround axons and are responsible for myelination. These include, for example, neurolemmocytes (Schwann cells) in the PNS.

Microglia (Glial macrophages)

Microgliocytes are small cells with an elongated nucleus that perform the function of phagocytes in nervous tissue. Depending on their functional state, they are divided into three forms:

• Ameboid: motile cells of the developing brain. They actively phagocytose degenerating elements, after which they transform into the resting form.
• Resting (ramified): cells of the formed brain with branching processes and low phagocytic activity.
• Reactive: forms from the resting form after brain injury, regaining a high capacity for phagocytosis.

Clinical significance: due to high mobility in AIDS, microgliocytes can become carriers of the virus into the CNS.

Briefly: answers to topical questions

Class and exam questions

What else to study on this topic

• Blood-brain and blood-cerebrospinal fluid barriers: mechanisms of formation
• The process of axon myelination by oligodendrocytes and Schwann cells
• Histogenesis of nervous tissue: the role of radial glia and glioblasts
• Morphology of mantle gliocytes in the spinal ganglion
• Secretion and outflow pathways of cerebrospinal fluid (CSF)

Other topics in the section "Nervous tissue: neurocytes, gliocytes, nerve fibers"

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