Syncytium

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A syncytium or symplasm (/sɪnˈstiəm/; plural syncytia; from Greek: σύν (syn) = "together" + κύτος (kytos) = "box, i.e. cell") is a multinucleated cell that can result from multiple cell fusions of uninuclear cells (i.e., cells with a single nucleus), in contrast to a coenocyte, which can result from multiple nuclear divisions without accompanying cytokinesis.[1] The term may also refer to cells interconnected by specialized membrane with gap junctions, as seen in the heart muscle cells and certain smooth muscle cells, which are synchronized electrically in an action potential.

Another (correct and well-established) use of the word syncytium is found in embryogenesis to refer to the coenocytic blastoderm embryos of invertebrates, such as Drosophila melanogaster.[2]

Physiological examples[edit]

Protists[edit]

In protists, syncytia can be found in some rhizarians (e.g., chlorarachniophytes, plasmodiophorids, haplosporidians) and acellular slime moulds, dictyostelids (amoebozoans) and acrasids (excavates).

Plants[edit]

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

Fungi[edit]

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[edit]

Skeletal muscle[edit]

A classic example of a syncytium is the formation of skeletal muscle. Large skeletal muscle fibers form by the fusion of thousands of individual muscle cells. The multinucleated (symplastic) arrangement is important in pathologic states such as myopathy, where focal necrosis (death) of a portion of a skeletal muscle fibers 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", but with some of the surviving segments being functionally cut off from their nerve supply via loss of continuity with the neuromuscular junction.

Cardiac muscle[edit]

The syncytium of cardiac muscle is important because it allows rapid coordinated contraction of muscles along their entire length. Action potentials propagate along the surface of the muscle fiber from the point of synaptic contact, through 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.

Osteoclasts[edit]

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

Placenta[edit]

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[edit]

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.[6]

Pathological examples[edit]

Viral infection[edit]

Syncytium caused by HSV-1 infection in Vero cells

Syncytia can also form when cells are infected with certain types of viruses, notably HSV-1, HIV, MeV, 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, syncytium are also known as multinucleated giant cells or polykaryocytes.[7] 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[edit]

Typically, the viral families that can cause syncytium are enveloped because viral envelope proteins on the surface of the host cell are needed to fuse with other cells.[8] But, certain members of the Reoviridae family are notable exceptions due to a unique set of proteins known as fusion-associated small transmembrane (FAST) proteins.[9] 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.[10]

HIV[edit]

HIV infects CD4+ T cells and makes the cell produce viral proteins, including fusion proteins. Then, the cell begins to display surface HIV glycoproteins, which are antigenic. Normally, a cytotoxic T cell will immediately come to "inject" lymphotoxins, such as 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.[11] 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.

See also[edit]

References[edit]

  1. ^ Daubenmire, R. F. (1936). "The Use of the Terms Coenocyte and Syncytium in Biology". Science. 84 (2189): 533–534. PMID 17806555. doi:10.1126/science.84.2189.533. 
  2. ^ Willmer, P. G. (1990). Invertebrate Relationships : Patterns in Animal Evolution. Cambridge University Press, Cambridge.
  3. ^ Płachno, B. J.; Swiątek, P. (2010). "Syncytia in plants: Cell fusion in endosperm—placental syncytium formation in Utricularia (Lentibulariaceae)". Protoplasma. 248 (2): 425–435. PMID 20567861. doi:10.1007/s00709-010-0173-1. 
  4. ^ Tiwari, S. C.; Gunning, B. E. S. (1986). "Colchicine inhibits plasmodium formation and disrupts pathways of sporopollenin secretion in the anther tapetum ofTradescantia virginiana L". Protoplasma. 133 (2–3): 115. doi:10.1007/BF01304627. 
  5. ^ Murguía-Sánchez, G. (2002). "Embryo sac development in Vanroyenella plumosa, Podostemaceae". Aquatic Botany. 73 (3): 201–201. doi:10.1016/S0304-3770(02)00025-6. 
  6. ^ http://palaeos.com/metazoa/porifera/hexactinellida.html
  7. ^ Albrecht, Thomas; Fons, Michael; Boldogh, Istvan; Rabson, Alan S. (1996-01-01). Baron, Samuel, ed. Medical Microbiology (4th ed.). Galveston (TX): University of Texas Medical Branch at Galveston. ISBN 0963117211. PMID 21413282. 
  8. ^ "ViralZone: Syncytium formation induced by viral infection". viralzone.expasy.org. Retrieved 2016-12-16. 
  9. ^ Salsman, Jayme; Top, Deniz; Boutilier, Julie; Duncan, Roy (2005-07-01). "Extensive Syncytium Formation Mediated by the Reovirus FAST Proteins Triggers Apoptosis-Induced Membrane Instability". Journal of Virology. 79 (13): 8090–8100. ISSN 0022-538X. PMC 1143762Freely accessible. PMID 15956554. doi:10.1128/JVI.79.13.8090-8100.2005. 
  10. ^ Duncan, Roy; Corcoran, Jennifer; Shou, Jingyun; Stoltz, Don (2004-02-05). "Reptilian reovirus: a new fusogenic orthoreovirus species". Virology. 319 (1): 131–140. doi:10.1016/j.virol.2003.10.025. 
  11. ^ Huerta, L.; López-Balderas, N.; Rivera-Toledo, E.; Sandoval, G.; Gȑmez-Icazbalceta, G.; Villarreal, C.; Lamoyi, E.; Larralde, C. (2009). "HIV-Envelope–Dependent Cell-Cell Fusion: Quantitative Studies". The Scientific World Journal. 9: 746–763. PMID 19705036. doi:10.1100/tsw.2009.90.