On differences in skin tumours of Pacific and Atlantic Flatfish

. Pacific flatfish from North American and Japanese coastal waters regularly manifest skin tumours. Those are generally papillomas, which develop from primary inflammatory nodules (vascularized nodules) on fish of a few months's age. Inside the epidermal part of the tumour slightly dedifferentiated malpighian cells change to rounded enlarged cells showing evidence of considerable degeneration (vacuolization and destruction of the cell organelles). These ‘X‐cells’, which compose the major part of the mature tumour, always remain characteristically separated from the surface, from the basal lamina, and from each other, by non‐enlarged ‘enyelope cells’. Oval, similarly degenerate cells also occur in the tumour stroma, especially in the nodule‐like pre‐papillomatous stages and here they originate apparently from fibroblasts. In the Atlantic, on the other hand, skin papillomas and other neoplastic skin growths occur relatively infrequently. The skin papillomas of Atlantic flatfish are considerably less complex. They belong to the relatively simple type which predominates in fishes in general–hyperplastic epidermal papillae which are supported and natured by branched folds of the dermal tissue (stroma). Considering the close relationship between the Pacific and Atlantic flatfishes, the distinctive structure of the skin tumours of Pacific fish is considered to be due probably to the influence of a specific virus.