Light and electron microscopic studies of a transplantable melanoma associated with virus-like particles.

A transplantable melanoma of Golden hamsters has been carried by the chunk method from the 15th through the 52nd generation and examined for growth characteristics and for light and electron microscopic appearance as well as for light microscopy histochemistry. Grossly, tumors were heavily pigmented and inva sive, and grew rapidly to kill the animals in 4 to 6 weeks. The growth rate remained stable within the 3 years of study. Histologically, malignant cells formed Fontanapositive pigment and grew as a solid tumor about dilated vascular channels with a supporting reticulum. Very little inflammatory infiltrate occurred, but necrosis and hemorrhage were common. 3,4-Dihydroxy-L-phenylalanine oxidase and acid phosphatase activity were seen together in most viable cells. Ultrastructurally, in addi tion to premelanosomes and melanosomes, large polyribosomes and rough endoplasmic reticulum abounded in most cells. Large autophagic vacuoles containing melanin granules were seen frequently, suggesting that the cells did not discharge their melanin. Virus-like particles were observed in the rough endoplasmic re ticulum in a majority of cells of every generation of tu mor; they did not occur in inflammatory or endothelial cells, or in surrounding normal structures. Transplanta tion of the tumor in albino hamsters resulted in an identi cal pigmented tumor, except that it grew more slowly. Virus-like particles occurred in large amounts. The rela tionship of virus-like particles to melanoma formation remains unknown. They may transform cells to increase growth rate and account for the very rapid growth of heavily pigmented melanomas.