DAO 39:49-57 (1999)  -  doi:10.3354/dao039049

ABSTRACT: In the present work we describe the histology of skin tumours of the black plaice Pleuronectes obscurus from Amursky Bay, the Sea of Japan. The epidermis forms numerous papillary folds protruding above the skin surface and supported by delicate branches of connective tissue. This type of neoplasm is classified as epidermal papilloma. The occurrence of severe epidermal hyperplasia and disturbance of the histoarchitecture in some areas, invasion of the adjacent connective tissues by epithelial cells, dystrophic changes of the epithelial cells, and the occurrence of a large number of mitoses point to an increasing malignancy of the papilloma. Moreover, areas with skeletal-muscle differentiation were found within skin tumours. Among the myogenic cells, features of normal somatic myogenesis were observed along with signs of abnormality of this process, suggesting a disturbance of myogenic differentiation. Cellular polymorphism among myogenic cells and invasion of the skin by neoplastic cells are evidence of the malignant character of this type of tumour and allow us to classify it as rhabdomyosarcoma. Due to the position of tumours in the skin, they are ectopic rhabdomyosarcomas. In the skin tumours, atypical small and large rounded cells were identified, the latter having previously been described in flatfish as X-cells. The origin of these cells is discussed and the assumption is put forward that small and large rounded cells can be regarded as cellular elements of rhabdomyosarcomas.

KEY WORDS: Skin tumours · Epidermal papilloma · Rhabdomyosarcoma · X-cell · Flatfish