関: plasmodesma、plasmodesmal、plasmodesmata、plasmodesmatal、protoplasmic connection、symplasmic

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出典(authority):フリー百科事典『ウィキペディア（Wikipedia）』「2015/10/20 23:29:35」(JST)

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A syncytium or symplasm (/sɪnˈsaɪtiəm/; 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 animal embryology to refer to the coenocytic blastoderm embryos of invertebrates, such as Drosophila melanogaster.^[2]

Physiological examples

Protists

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

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]

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

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 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 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

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 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.

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 HSV-1, 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 T helper cells are nearby, 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 17806555.
^ 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.
^ 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.
^ 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.
^ 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. doi:10.1100/tsw.2009.90. PMID 19705036.

English Journal

The influence of flavonoid amendment on the absorption of cadmium in Avicennia marina roots.

Li J1, Lu H1, Liu J1, Hong H1, Yan C2.
Ecotoxicology and environmental safety.Ecotoxicol Environ Saf.2015 May 21;120:1-6. doi: 10.1016/j.ecoenv.2015.05.004. [Epub ahead of print]
Flavonoid is a key factor for the tolerance to cadmium in plants. Concentration-dependent kinetics experiment was conducted to investigate the influence of flavonoid amendment on the Cd2+ uptake in Avicennia marina (Forsk) Vierh. roots. We found that compared with the control, saturation concentrati
PMID 26004538

Spread the news: systemic dissemination and local impact of Ca²⁺ signals along the phloem pathway.

van Bel AJ1, Furch AC, Will T, Buxa SV, Musetti R, Hafke JB.
Journal of experimental botany.J Exp Bot.2014 Apr;65(7):1761-87. doi: 10.1093/jxb/ert425. Epub 2014 Jan 30.
We explored the idea of whether electropotential waves (EPWs) primarily act as vehicles for systemic spread of Ca(2+) signals. EPW-associated Ca(2+) influx may trigger generation and amplification of countless long-distance signals along the phloem pathway given the fact that gating of Ca(2+)-permea
PMID 24482370

Auxin-callose-mediated plasmodesmal gating is essential for tropic auxin gradient formation and signaling.

Han X1, Hyun TK1, Zhang M1, Kumar R1, Koh EJ2, Kang BH3, Lucas WJ4, Kim JY5.
Developmental cell.Dev Cell.2014 Jan 27;28(2):132-46. doi: 10.1016/j.devcel.2013.12.008.
In plants, auxin functions as a master controller of development, pattern formation, morphogenesis, and tropic responses. A sophisticated transport system has evolved to allow the establishment of precise spatiotemporal auxin gradients that regulate specific developmental programs. A critical unreso
PMID 24480642

Japanese Journal

Selection and Partial Characterization of a Boron-Tolerant Tobacco Cell Line

Soil science and plant nutrition 47(2), 405-410, 2001-06
NAID 110001718637

Altered Zn compartmentation in the root symplasm and stimulated Zn absorption into the leaf as mechanisms involved in Zn hyper-accumulation in Thlaspi caerulescens