Syncytium

A syncytium or symplasm (/sɪnˈsɪʃiəm/; plural syncytia; from Greek: σύν syn 'together' and 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. 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. A syncytium or symplasm (/sɪnˈsɪʃiəm/; plural syncytia; from Greek: σύν syn 'together' and 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. 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. The field of embryogenesis uses the word syncytium to refer to the coenocytic blastoderm embryos of invertebrates, such as Drosophila melanogaster. In protists, syncytia can be found in some rhizarians (e.g., chlorarachniophytes, plasmodiophorids, haplosporidians) and acellular slime moulds, dictyostelids (amoebozoans) and acrasids (Excavata). Some examples of plant syncytia, which result during plant development, include: 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. 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 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. 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. Certain animal immune-derived cells may form aggregate cells, such as 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 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.