A syncytium (plural syncytia; from Greek: σύν (syn) = "together" + κύτος (kytos) = "box, i.e. cell") is a multinucleate cell which 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] A classic example of a syncytium is the formation of skeletal muscle. The term may also refer to cells that are connected by specialized membrane proteins (e.g. gap junctions), like the heart muscle cells.
Another (incorrect but too well-established-to-be-changed) use of the word syncytium is found in animal embryology to refer to the coenocytic blastoderm of invertebrates.[2]
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A syncytium is the normal cell structure for many fungi. Most complex fungi exist as a dikaryon in which threadlike cells are partially partitioned into segments each containing two differing nuclei.
Some examples of plant syncytia, which result during plant development, include:[3]
Large skeletal muscle fibers form by the fusion of thousands of individual muscle cells. The multinucleated (symplastic) arrangement of skeletal muscle 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.
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.
Certain animal immune-derived cells may form aggregate cells also, for example the osteoclast cells responsible for bone resorption.
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. 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.
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 neighbouring 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.