The developmental biology of induced malignant melanoma in guinea pigs and a comparison with other neoplastic systems.

Malignant melanoma has been induced in the Weiser-Maple guinea pig by prolonged application of 7,12-dimethylbenz(a)anthracene. The tumor shows a biphasic growth pattern analogous to the radial and vertical growth phase of human cutaneous malignant melanoma. It evolves through a predictable series of cellular events classified as intraepidermal melanocytic hyperplasia, dermal melanocytosis, dermal melanocytoma, malignant melanoma without intralesional transformation, and, finally, malignant melanoma with intralesional transformation, which is characterized by the appearance of "new kinds of cells" and is associated with widespread metastases and massive lymph node involvement. Clinically, the lesions evolve from diffuse hyper-pigmentation to brown-black macules, to nodules of increasing size, to overt malignant melanoma associated with metastases, wasting, and death. Examples of intralesional transformation analogous to that in guinea pigs are found not only in human malignant melanomas, but in other human neoplastic systems, and such analogous cellular events are discussed in this paper.