Ultrastructural characterization of the spontaneous papilloma of Japanese newts.

The spontaneous epithelioma of the Japanese newt, Cynops pyrrhogaster, is a useful animal model for study of development and regression of papillomata of suspected viral origin. Therefore, ultrastructural studies of this cutaneous tumor were undertaken in order to characterize this model further. The ultrastructure of normal Cynops pyrrhogaster epidermis resembled teleost more than anuran epidermis. Surface cells of the bilayered stratum corneum were covered by micro-ridges and contained prominent nuclei, unlike the stratum corneum of mammals. Capillary endothelial cells contained unique small dense granules. Principal changes in tumors consisted of hyperplasia of stratum granulosum cells, increased melanin bodies in corneal cells, distorted configurations and aberrant location of outer corneal cells, which also demonstrated irregularity of micro-ridges and increased mucin and vacuole content. Intercellular spaces were enlarged between all cell types. Virus-like particles were observed in approximately 40 percent of tumors, particularly in the middle of the tumor.