Syncytium

Source: Wikipedia, the free encyclopedia.

A syncytium (

heart muscle cells and certain smooth muscle cells, which are synchronized electrically in an action potential
.

The field of

embryogenesis uses the word syncytium to refer to the coenocytic blastoderm embryos of invertebrates, such as Drosophila melanogaster.[2]

Physiological examples

Protists

In

acrasids (Excavata) and Haplozoon
.

Plants

Some examples of plant syncytia, which result during plant development, include:

Fungi

A syncytium is the normal cell structure for many fungi. Most fungi of Basidiomycota exist as a dikaryon in which thread-like cells of the mycelium are partially partitioned into segments each containing two differing nuclei, called a heterokaryon.

Animals

Nerve net

The neurons which makes up the subepithelial nerve net in comb jellies (Ctenophora) are fused into a neural syncytium, consisting of a continuous plasma membrane instead of being connected through synapses.[6]

Skeletal muscle

A classic example of a syncytium is the formation of

multinucleated arrangement is important in pathologic states such as myopathy, where focal necrosis (death) of a portion of a skeletal muscle fiber does not result in necrosis of the adjacent sections of that same skeletal muscle fiber, because those adjacent sections have their own nuclear material. Thus, myopathy is usually associated with such "segmental necrosis", with some of the surviving segments being functionally cut off from their nerve supply via loss of continuity with the neuromuscular junction
.

Cardiac muscle

The syncytium of

intercalated discs
. Although a syncytium, cardiac muscle differs because the cells are not long and multinucleated. Cardiac tissue is therefore described as a functional syncytium, as opposed to the true syncytium of skeletal muscle.

Smooth muscle

platelet-derived growth factor receptor alpha (PDGFRα) that are electrically coupled and work together as an SIP functional syncytium.[7][8]

Osteoclasts

Certain animal immune-derived cells may form aggregate cells, such as the osteoclast cells responsible for bone resorption.

Placenta

Another important vertebrate syncytium is in the placenta of placental mammals. Embryo-derived cells that form the interface with the maternal blood stream fuse together to form a multinucleated barrier – the syncytiotrophoblast. This is probably important to limit the exchange of migratory cells between the developing embryo and the body of the mother, as some blood cells are specialized to be able to insert themselves between adjacent epithelial cells. The syncytial epithelium of the placenta does not provide such an access path from the maternal circulation into the embryo.

Glass sponges

Much of the body of Hexactinellid sponges is composed of syncitial tissue. This allows them to form their large siliceous spicules exclusively inside their cells.[9]

Tegument

The fine structure of the

vesicles.[10] The innermost layer is bounded by a layer of connective tissue known as the "basal lamina". The basal lamina is followed by a thick layer of muscle.[11]

Pathological examples

Viral infection

Vero cells

Syncytia can also form when cells are infected with certain types of

SARS-CoV-2, and pneumoviruses, e.g. respiratory syncytial virus (RSV). These syncytial formations create distinctive cytopathic effects when seen in permissive cells. Because many cells fuse together, syncytia are also known as multinucleated cells, giant cells, or polykaryocytes.[12] During infection, viral fusion proteins used by the virus to enter the cell are transported to the cell surface, where they can cause the host cell membrane to fuse
with neighboring cells.

Reoviridae

Typically, the viral families that can cause syncytia are enveloped, because viral envelope proteins on the surface of the host cell are needed to fuse with other cells.

Reoviridae family are notable exceptions due to a unique set of proteins known as fusion-associated small transmembrane (FAST) proteins.[14] Reovirus induced syncytium formation is not found in humans, but is found in a number of other species and is caused by fusogenic orthoreoviruses. These fusogenic orthoreoviruses include reptilian orthoreovirus, avian orthoreovirus, Nelson Bay orthoreovirus, and baboon orthoreovirus.[15]

HIV

HIV infects Helper CD4+

perforin or granzyme, that will kill the infected T helper cell. However, if T helper cells are nearby, the gp41 HIV receptors displayed on the surface of the T helper cell will bind to other similar lymphocytes.[16] This makes dozens of T helper cells fuse cell membranes into a giant, nonfunctional syncytium, which allows the HIV virion to kill many T helper cells by infecting only one. It is associated with a faster progression of the disease[17]

Mumps

The mumps virus uses HN protein to stick to a potential host cell, then, the fusion protein allows it to bind with the host cell. The HN and fusion proteins are then left on the host cell walls, causing it to bind with neighbour epithelial cells.[18]

COVID-19

Mutations within SARS-CoV-2 variants contain spike protein variants that can enhance syncytia formation.[19] The protease TMPRSS2 is essential for syncytia formation.[20] Syncytia can allow the virus to spread directly to other cells, shielded from neutralizing antibodies and other immune system components.[19] Syncytia formation in cells can be pathological to tissues.[19]

"Severe cases of

pneumocytes. The viral and cellular mechanisms regulating the formation of these syncytia are not well understood,"[21] but membrane cholesterol seems necessary.[22][23]

The syncytia appear to be long-lasting; the "complete regeneration" of the lungs after severe

flu "does not happen" with COVID-19.[24]

See also

References

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  2. ^ Willmer, P. G. (1990). Invertebrate Relationships: Patterns in Animal Evolution. Cambridge University Press, Cambridge.
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  9. ^ "Palaeos Metazoa: Porifera:Hexactinellida".
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  13. ^ "ViralZone: Syncytium formation is induced by viral infection". viralzone.expasy.org. Retrieved 2016-12-16.
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  15. PMID 14967494
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  17. ^ National Institutes of Health (2019-12-27). "Syncytium | Definition | AIDSinfo". Retrieved 2019-12-27.
  18. ^ "MUMPS, Mumps Virus, Mumps Infection". virology-online.com. Retrieved 2020-03-12.
  19. ^
    PMID 34606831
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  20. PMID 34953136
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  21. PMID 33051876
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  22. PMID 33890572
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  23. ^ "SARS-CoV-2 needs cholesterol to invade cells and form mega cells". phys.org. Retrieved 2021-01-22.
  24. ^ Gallagher, James (23 October 2020). "Covid: Why is coronavirus so deadly?". BBC News.
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