Syncytia

Many enveloped viruses possess a fusion protein in their envelopes. This confers the ability of the virion to fuse with the host cell membrane and thus allow entry of the infectious genomic material into the cell cytoplasm. During replication of the virus, expression of the fusion protein at the cell membrane can result in the fusion of neighbouring cells, and the formation of multi-nucleate cells or syncytia.

Measles virus

Very large syncytia can be formed during replication of measles virus in cell culture.
An additional distinguishing feature of measles is the presence of distinct eosinophilic inclusions in the nuclei of infected cells.

In the syncytium shown on the right, multiple nuclei are clustered around an eosinophilic cytoplasmic mass that probably represents the Golgi compartments of the fused cells.

Intra-nuclear inclusions are clearly visible.

Respiratory syncytial virus

As its name implies, respiratory syncytial virus (RSV) causes large syncytia.
Nuclei do not contain inclusions, but pale eosonophilic inclusions can often be seen in the cytoplasm (see below).

Mumps virus

Mumps virus CPE is indistinguishable from that of RSV (shown above).
However, mumps virus encodes a haemagglutin protein which is incorporated in the virus envelope, and appears at the cell surface from which progeny virions will bud. If erythrocytes (red blood cells) are added to infected cell sheets, they will adhere to the cell surface.
This process is termed haemadsorption
and allows differentiation between mumps and RSV infections.

Haemadsorption of erythrocytes on the surface of cells infected with mumps virus

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