Histopathogenesis of malignant skin melanoma induced in genetically susceptible transgenic mice

Animal models of human malignant skin melanoma were created in melanoma-susceptible inbred-strain transgenic mice by grafting skin from donors of high-susceptibility lines to hosts of a low-susceptibility line, thereby overcoming the problem of early death of the more susceptible animals from eye melanomas. As already described [Mintz, B. & Silvers, W. K. (1993) Proc. Natl. Acad. Sci. USA 90, 8817-8821], melanocytes within the grafts selectively proliferated in close proximity to areas of greatest wound healing, presumably in response to mitogenic factors from cells contributing to wound repair. An orderly sequence of externally visible events culminated in malignant melanoma. We examine here the histogenetic concomitants of these changes and find that they define a stepwise sequence strikingly comparable to that leading to human cutaneous melanoma. Moreover, the histological details suggest some of the underlying mechanisms. While the early lesions are first seen in the superficial dermis in the mouse, and in the basal layer of the epidermis in the human, both progress by radial growth followed by vertical growth. Melanocytic hyperplasia resulted in nests of densely melanized fusiform cells which were losing their dendrites. Some discrete lesions in the deep dermis appeared as blue nevi. As radial proliferation advanced, cellular atypia increased and the previously independent melanocytes cohered closely and formed a small solid tumor; the cells were usually then hypomelanotic or amelanotic. Ulceration of tumor through the epidermis occurred early. The tumor mass grew rapidly in the deep dermis and invaded and destroyed subcutaneous tissue and muscle. Primary tumors in the skin were often heterogeneous, with lobules or regions differing in pigmentation or atypia. However, the cells in circulating emboli, or in metastases in lymph nodes and lungs, appeared relatively homogeneous. These genetically uniform transgenic mouse models provide experimental access to the multistage genesis of melanoma.