ASTROCYTES: HISTOLOGY, FUNCTIONS, TYPES - NEUROPSYCHOLOGY - 2024

The astrocytes are one of four types of glial cells that function to the physical and metabolic support of neuronal cells, therefore, are part of the central nervous system of humans and many other vertebrate animals.

Together with oligodendrocytes, microglial cells, and ependymal cells, astrocytes form what is known as "neuroglia." The neuroglial cells are usually found in much greater numbers than neurons, but they do not participate in the reaction and / or propagation of nerve impulses.

Immunofluorescence microscopy of an astrocyte (Source: GerryShaw via Wikimedia Commons)

The terms "neuroglia" and "astrocyte" were proposed in 1895 by Mihaly von Lenhossek to identify the cell group that supports neurons and a special class of these cells, characterized by their stellate shape.

Astrocytes have been shown to increase the number of functional neuronal synapses in neurons of the central nervous system, which means that they are required for the transmission of nerve stimuli.

Diagram of the different types of cells that make up glia in the central nervous system. Ependymal cells, oligodendrocytes, astrocytes and microglial cells are observed (Source: BruceBlaus. When using this image in external sources it can be cited as: Blausen.com staff (2014). «Medical gallery of Blausen Medical 2014 ». WikiJournal of Medicine 1 (2). DOI: 10.15347 / wjm / 2014.010. ISSN 2002-4436. Via Wikimedia Commons)

These cells comprise between 20 and 25% (and sometimes up to 50%) of the volume in many brain areas and are known to have special roles in responding to injury, although it has recently been proposed that they are involved in many diseases of the system. central nervous.

Histology

Astrocytes are "stellar" or star-shaped cells, as they have cytosolic projections of different sizes that make them similar to children's drawings of a space star.

These cells are distributed throughout the brain and throughout the spinal cord and constitute more than 50% of all glial cells.

When viewed under a light microscope after routine staining, astrocytes (depending on the type) have large oval or lobular nuclei with little cytosolic content.

The characteristic cytosolic projections of astrocytes are known as “glial fibrils”, and they are mainly composed of the glial-fibrillar acidic protein (GFAP), specific for the astrocytes of the central nervous system and that is commonly used as a marker protein.

Astrocytes from a cell culture. Color is the product of glial-fibrillar acidic protein (GFAP) staining (Source: The original uploader was GrzegorzWicher at Polish Wikipedia. Via Wikimedia Commons)

The glial fibers of astrocytes are closely related to the cell body and axons of neurons, they surround the sites of nerve synapses and also the well-known nodules of Ranvier, present in axons covered by a myelin sheath.

Although they are not excitable cells, astrocytes express specific sodium and potassium channels that are very important for their functions in maintaining homeostasis in the nervous system.

Membrane specializations

Astrocytes have two types of specializations in their membranes known as gap junctions and orthogonal assemblies.

Gap junctions are made up of transmembrane proteins called connexons, which join with homologous proteins in nearby cells to form hydrophobic channels through which small molecules can exchange between cells.

There are numerous gap junctions between astrocytes and astrocytes and between astrocytes and oligodendrocytes. Among the molecules that are exchanged through these bonds are small ions, oligosaccharides, and certain trophic factors.

Orthogonal assemblies, on the other hand, are "paracrystalline" arrangements that are made up of 7nm particles. They are numerous in the more distal portions of the cytosolic projections, especially in the region facing the blood vessels.

These structures participate in cell adhesion and in the transport of substances between astrocytes and between astrocytes and the cerebrospinal fluid.

Types

There are two well-defined types of astrocytes that differ in their morphology and anatomical location. These are protoplasmic astrocytes and fibrous astrocytes.

However, many researchers consider that they are the same type of cells that acquire different functions depending on the environment where they are.

Other bibliographic documents, however, establish the existence of a third type of astrocytes, characterized by their elongated cell bodies and commonly known as the Bergmann glial cells of the cerebellum and the Müller cells in the retina of the eyes.

Only the astrocytes present in the brain and spinal cord will be described here.

Protoplasmic astrocytes

The existence of such cells was demonstrated by silver staining techniques. These are typical of the gray matter of the brain and are cells with a stellar appearance (similar to a star).

They have an abundant cytosol where a large nucleus is found and they differ from fibrous astrocytes in that they have short processes.

The ends of some of the cytosolic projections are composed of "vascular feet" or pedicels that interact with adjacent blood vessels.

Some protoplasmic astrocytes are close to the cell bodies of some neurons, as if they were "satellite" cells.

Fibrous astrocytes

Fibrous astrocytes are cells with few internal organelles, rich in free ribosomes and storage molecules such as glycogen. They have longer cytosolic projections or projections than protoplasmic astrocytes, which is why they are known as "fibrous" astrocytes.

These cells are associated with the white matter of the brain and their processes also connect with blood vessels, but are separated from these by their own basal lamina.

Features

As neuroglial cells, astrocytes play a vital role in the physical support and metabolic support of neurons in the central nervous system in vertebrate animals.

In addition, these cells are responsible for the elimination of ions and other waste substances from neuronal metabolism that are typical of the neuronal microenvironment, especially the axonal region, such as:

- Potassium ions (K +)

- Remains of glutamate and

- Traces of gamma aminobutyric acid (GABA)

In charge of, among other things, the energy metabolism of the cerebral cortex, as they release glucose from glycogen molecules stored in their cytosol.

This release occurs only when astrocytes are stimulated by neurotransmitters such as norepinephrine and vasoactive intestinal peptide or VIP peptide, which are released by nearby neurons.

Astrocytes also participate in neuronal development and in the transport and release of neurotrophic factors, which is why some authors consider them to be cells that maintain homeostasis in the central nervous system.

These cells can also play important roles in healing damaged areas of the brain. They control brain pH and regulate multiple neural functions by maintaining a relatively constant microenvironment.

Implications for the blood-brain barrier

Some astrocytes participate in the formation and maintenance of the blood-brain barrier, since they have the ability to form a continuous layer on the blood vessels in the periphery of the central nervous system.

The blood-brain barrier is a kind of “structure” that limits the entry of circulating blood elements into the central nervous system.

The relationship of these nerve cells with this function in such that it has been experimentally demonstrated that epithelial cells can induce the differentiation of astrocytic precursors.

Immune functions of astrocytes

Some literature reviews highlight astrocytes as immunocompetent cells of the central nervous system, as they are capable of expressing proteins of the Major Histocompatibility Complex (MHC), which have important functions in antigen presentation.

These cells, then, participate in T-cell activation, not only by the expression of antigen-presenting proteins, but also by their ability to express co-stimulatory molecules that are critical to the process per se.

However, the participation of astrocytes in the immune system is not limited to the presentation of antigens, but it has also been shown that these cells can secrete a wide variety of cytokines and chemokines, which may mean that they are involved in inflammatory processes and immune reactivity in the brain.

Clinical significance

In view of experimental data suggesting that the suppression of astrocytes in the central nervous system results in substantial neuronal degeneration in adults, it is clear that these cells have valuable clinical significance.

Astrocytes, among their multiple functions, have been linked to the long-term recovery of patients with brain injuries. They are also involved in the regeneration of neurons, mainly due to their ability to express and release trophic factors.

In other words, the survival of neurons is highly dependent on their association with astrocytes, such that any massive damage that occurs in these cells will directly affect normal brain functions.

Astrogliosis

Many neurodegenerative diseases are distinguished by proliferation, morphological change, and increased expression of glial-fibrillar acidic protein (GFAP) in astrocytes; condition known as "astrogliosis".

This process, depending on the context where it occurs, can be beneficial or deleterious, since it can mean neuronal survival due to the production of growth factors or the formation of “glial scars”, respectively.

Astrogliosis is not a random or “all or nothing” process. Rather, it is a highly controlled event that depends on multiple cellular signals and the particular context in which the cell in question is found.

References

1. Chen, Y., & Swanson, RA (2003). Astrocytes and brain injury. Journal of Cerebral Blood Flow and Metabolism, 23 (2), 137–149.
2. Dong, Y., & Benveniste, EN (2001). Immune function of astrocytes. Glia, 36 (2), 180–190.
3. Gartner, LP, & Hiatt, JL (2012). Color atlas and text of histology. Lippincott Williams & Wilkins.
4. Kimelberg, HK, & Nedergaard, M. (2010). Functions of Astrocytes and their Potential As Therapeutic Targets. Neurotherapeutics, 7 (4), 338–353.
5. Montgomery, DL (1994). Astrocytes: Form, Functions, and Roles in Disease. Veterinary Pathology, 31 (2), 145–167.
6. Ransom, B., Behar, T., & Nedergaard, M. (2003). New roles for astrocytes (stars at last). Trends in Neurosciences, 26 (10), 520–522.
7. Sofroniew, MV, & Vinters, HV (2010). Astrocytes: Biology and pathology. Acta Neuropathologica, 119 (1), 7–35.