| Summary: | Introduction: Despite numerous investigations the nature and origin of x -cells remain unknown. These ovoid, pale-staining cells with a large, round nucleus and prominent nucleolus are known to occur in the form of lesions, termed x -cell tumours. X -cells may produce large aggregations that resemble true neoplasms. They can be found in quite different organs as skin, pseudobranch, gills, ultimobranchial body, kidney, gonad and spleen (Peters et al. 1978; Dawe, 1981; Diamant & McVicar, 1987). X-celllesions are mainly described in marine species from cold or temperate zones, but a recent publication reported on x -cell tumours in a freshwater species living in warmwater (Diamant et al. 1994).
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