Spontaneous regressive epitheliomas in the Japanese newt, Cynops pyrrhogaster.

Spontaneous tumors in urodele amphibians have been considered uncommon, and this resistance has sometimes been associated with the natural regenerative capacity of tissues in such species. This report describes spontaneous, nonpigmented, benign epitheliomas which were found in 44 of 1586 (2.8%) adult newts, Cynops pyrrhogaster, captured in central Japan. Both sexes were affected equally, usually with single tumors occurring at any epidermal site. Under laboratory conditions, these large tumors rapidly regressed or disappeared. Lesions were histologically noninvasive, hyperplastic epidermal reactions accompanied by loss of basal, subdermal melanocytes. Ultrastructurally, enlargement of intercellular spaces between tumor cells, increased pigment granules and membrane-bound cytoplasmic aggregates within the spaces, swollen rough endoplasmic reticula, degenerating pigment granules, and altered corneal cells were noted. Virus-like particles were observed in one tumor cell. Prelminary attempts failed to demonstrate transmissibility of the tumor, and no new cases arose under laboratory conditions. The cause of these tumors in the Japanese newt remains unknown, and it is suggested that if a viral agent is involved then other environmental cofactors (diet, temperature, or water constituents) are required for its expression.