syncytia

Syncytia (singular syncytium) are large multinucleated cells formed when two or more individual cells fuse to create a single cytoplasmic mass that contains multiple nuclei. This fusion results in a shared plasma membrane enclosing several nuclei, rather than discrete, separate cells.

Physiological examples of syncytia occur during normal development. In skeletal muscle, myoblasts fuse to form long multinucleated muscle fibers. In the placenta, cytotrophoblasts fuse to form the syncytiotrophoblast layer, which covers placental villi and participates in nutrient exchange and barrier function between mother and fetus.

In addition to these developmental instances, many viruses induce syncytium formation. Enveloped viruses often express fusion proteins on infected cell surfaces that trigger fusion with neighboring cells, creating multinucleated giant cells known as syncytia. This cytopathic effect is characteristic of several paramyxoviruses, such as measles and respiratory syncytial virus, and can also be observed with other viruses, including HIV, Nipah, and Hendra viruses. Syncytia formation can facilitate rapid cell-to-cell viral spread and may contribute to tissue damage and immune evasion.

Detection and significance vary by context. In tissue culture, the appearance of syncytia is a readily recognized cytopathic effect used in virology assays. In clinical settings, the presence of syncytial giant cells can aid in diagnosing certain viral infections or indicate specific placental or muscular tissue changes.