An implanted hamster greene melanoma expressing multiple host-tissue differentiation.

After Greene melanoma tissue had been implanted subcutaneously in a Syrian Golden Hamster and allowed to grow there for 10 days, its tissue components were examined by transmission electron microscopy. Amelanotic and melanotic tumor cells, blood vessels and nerve fibers were present. In the amelanotic and melanotic tumor cells the rough endoplasmic reticulum was colonized by spherical endogenous retroviruses, whereas no virus particles could be detected in endothelial or other cells. The cell-type-specific viruses thus mark separate cell lineages and indicate that blood vessels in Greene melanoma are not formed by tumor cells, so that the 'vasculogenic mimicry' currently under controversial discussion for other melanomas is not involved here. The tumor also comprised smooth muscle cells, skeletal muscle fibers and collagen fibers. This remarkable finding implies that tumor growth involves not only the stimulation of angiogenesis and neurogenesis by various growth factors, but also the activation of other cells/tissues in the adjacent host tissues. This influence of the surrounding host tissue produces the observed epiphenomena and can also cause the genotypic character of a tumor to be phenotypically masked (mimicry).