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The structure and growth of epidermanl lesions of Parophrys vetulus : a light and electron microscopic study Majack, Richard Allen

Abstract

Natural history, light microscopic, and electron microscopic studies were made of epidermal lesions afflicting juvenile lemon sole (Parophrys vetulus) in the Strait of Georgia, near Vancouver, British Columbia, Canada. Emphasis was placed on the presentation of natural history and histological evidence for the progressive growth of these lesions from an initial morphological form (the angioepithelial nodule) to a mature and morphologically different form (the epidermal papilloma-like lesion). The study of the histological and cytological aspects of the growth of the lesions necessarily incorporated clarification of the nature of unidentified epidermal and stromal cell types which compose much of the mass of the tumour. It was noted that the histological appearance of the flatfish lesions differs significantly from that of any known fish epidermal disease, including those characterized by cellular hypertrophy and cellular hyperplasia or neoplasia. Natural history studies showed that angioepithelial nodules occurred predominantly on younger fish while epidermal papilloma-like lesions were usually found on larger fish, indicating a progression from the former to the latter. This assumption was supported by the presence of morphological and histological intermediates. The growth of the lesions was characterized by the gradual transformation of normal epidermal cells into "X-cells": ovoid, hypertrophied cells with enlarged nuclei, prominent nucleoli, and necrotic cytoplasm. These "X-cell" types completely dominate the mature lesion and are enclosed, or supported, by abnormal epidermal cells termed "enveloping cells". It was found that the pattern of subcellular necrosis in flatfish tumour cells differs from necrosis caused by non-specific lethal injuries to cells. This observation suggests that the subcellular changes observed may be a result of cellular transformation rather than a non-specific lethal injury to the cells. Other natural history studies indicated a decline in the number of tumours per tumourous fish and a decline in the prevalence of lesions as fish size increases. These results indicate that either the diseased fish are selectively removed from the population or that the tumours necrose and are lost by the host. Both possibilities were discussed. No evidence was found to support the hypothesis that "X-cells" are parasitic protozoans. The presence of virus-like particles in the cytoplasm of the enveloping cells of mature lesions was commonly observed. The relationship between these virus-like particles and the disease process, if any, is unknown.

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