Multinucleated cells, often simply called giant cells, are found in a variety of situations in teleost fish.
They are not uncommon in granulomatous inflammatory responses, such as bacterial kidney disease, proliferative kidney disease, or as a response to vaccine, and are a result of fusion of macrophages or epithelioid cells.
They are also found in virus infections, so-called syncytial giant cells, in which a number of non-macrophage cells, such as hepatocytes, fuse together. But giant cells are also found in normal fish as osteoclasts.
Although osteoclasts in fish can be mononuclear, the multinucleated ones are more easily seen histologically.
These cells are responsible for removing or remodelling damaged tissue or for resorbing bone, as seen during periods of high calcium demand, for example at spawning time.
In skin biopsies, it is important to differentiate osteoclasts removing scales damaged, for example in dermatitis, from foreign-body type giant cells, although both can occur at the same time.
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