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a mass of cytoplasm containing several nuclei and enclosed in a membrane but no internal cell boundaries (as in muscle fibers)

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2014/07/03 10:16:39」(JST)

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A syncytium (/sɪnˈsaɪtiəm/; plural syncytia; from Greek: σύν (syn) = "together" + κύτος (kytos) = "box, i.e. cell") is a multinucleate 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 that are 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 animal embryology to refer to the coenocytic blastoderm embryos of invertebrates, such as Drosophila melanogaster.^[2]

Physiological examples

Fungi

A syncytium is the normal cell structure for many fungi. Most fungi of Basidiomycota exist as a dikaryon in which threadlike cells are partially partitioned into segments each containing two differing nuclei.^{[citation needed]}

Plants

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

Developing endosperm^[3]
The non-articulated laticifers
The amoeboid tapetum,^[4] and
The "nucellar plasmodium" of the family Podostemaceae^[5]

Animals

Skeletal muscle

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 death 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

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 in the fact that the cells are not long and multi-nucleated. Cardiac tissue is therefore described as a functional syncytium, as opposed to the true syncytium of skeletal muscle.

Osteoclasts

Certain animal immune-derived cells may form aggregate cells also, for example 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 multi-nucleated barrier - the syncytiotrophoblast. This is probably important in order 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.

Pathological examples

Viral infection

Syncytium caused by HSV-1 infection in Vero cells

Syncytia can also form when cells are infected with certain types of viruses, notably HIV and paramyxoviruses, e.g. respiratory syncytial virus (RSV). 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.

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 there are nearby T helper cells, the gp41 HIV receptors displayed on the surface of the T helper cell will bind to other similar lymphocytes.^[6] 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.

References

^ Daubenmire, R. F. (1936). "The Use of the Terms Coenocyte and Syncytium in Biology". Science 84 (2189): 533–534. doi:10.1126/science.84.2189.533. PMID 17806554. edit
^ Willmer, P. G. (1990). Invertebrate Relationships : Patterns in Animal Evolution. Cambridge University Press, Cambridge.
^ Płachno, B. J.; Swiątek, P. (2010). "Syncytia in plants: Cell fusion in endosperm—placental syncytium formation in Utricularia (Lentibulariaceae)". Protoplasma 248 (2): 425–435. doi:10.1007/s00709-010-0173-1. PMID 20567861. edit
^ 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. edit
^ 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. edit
^ 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". TheScientificWorldJOURNAL 9: 746–763. doi:10.1100/tsw.2009.90. PMID 19705036. edit

UpToDate Contents

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リンク元	「syncytium」「合胞体」
拡張検索	「respiratory syncytial virus infection」「respiratory syncytial virus vaccine」「giant syncytial trophoblast」
関連記事	「syncytia」