Gastric leiomyomas in the Japanese newt, Cynops pyrrhogaster; ultrastructural observations.

Nineteen of 700 outwardly healthy captive adult Japanese newts, Cynops pyrrhogaster, had nodular neoplasms protruding on the gastric serosal surface. These tumours were characterized as sharply circumscribed leiomyomas of the muscularis externa with occasional submucosal involvement. Ultrastructural analysis of tumour myocytes revealed irregular-shaped, well-differentiated cells with cytoplasmic processes, and reduced quantities of intracellular myofibrils. They were surrounded by enlarged intercellular spaces filled with disorganized and increased amounts of collagen. Vacuolation was seen both in tumour myocytes and in mucous epithelial cells. Virus-like particles were observed in nuclei of some tumour myocytes. Interstitial cells with laminated-type granules were seen amongst tumour cells. These relatively benign leiomyomas shared some morphologic features with human gastric leiomyomas. Their cause and prognosis remain unknown.