Animal type melanoma is a rare histopathologic variant of melanoma characterized by sheets and nodules of heavily pigmented epithelioid melanocytes that involve the entire thickness of the dermis. This human neoplasm mimics melanocytic neoplasms seen in gray horses and laboratory animals; thus, is termed animal type melanoma. It is quite rare and, with only a few reported cases, its biological behavior is not well understood. We report an example of animal type melanoma on the back of a 27-year-old man. The lesion showed areas of melanoma in situ, which ruled out the possibility of metastatic melanoma. Features of regression were also seen at dermo-epidermal junction and papillary dermis. In some areas, neoplastic melanocytes exhibited a balloon-cell appearance; in others the neoplasm was composed of sheets and fascicles of heavily pigmented epithelioid melanocytes that permeated the entire dermis and extended into the dermal-subcutaneous interface, mimicking a cellular blue nevus. Epithelioid melanocytes in deeper areas showed abundant, heavily pigmented cytoplasm and pleomorphic nuclei with prominent eosinophilic nucleoli and some mitotic figures. The neoplastic cells did not show evidence of maturation in deeper areas of the lesion. In some sections, a nodule of heavily pigmented epithelioid melanocytes was seen far from the main bulk of the lesion, at the dermal-subcutaneous interface, raising the possibility of a satellite lesion. A lymphoscintigraphy showed a sentinel lymph node in the right axilla and a subsequent axillary lymphadenectomy demonstrated that the architecture of the sentinel lymph node was effaced by metastatic melanoma. The patient received adjuvant chemotherapy with inteferon alfa-2b and four months after this treatment the patient is alive and well, without evidence of recurrences or additional metastases.

The analysis of the complex, multi-step process of metastasis has been facilitated by the use of variant cell lines with differing metastatic or colonization potentials. In order to assess the full extent of the heterogeneity that can be expressed within a single cell line, the results of experiments describing individual ultrastructural, biochemical and behavioral aspects of the metastatic phenotype of B16 mouse melanoma cells have been collected. Of the 84 observations from 60 reports presented, there are 53 positive and 16 negative correlations with colonization after intravenous injection and/or metastatic potential, and 15 cases of no correlation. The tabulated results indicate that colony formation and metastasis can be associated with one or more of many different cellular traits so that genes responsible for each of these many traits could contribute to the metastatic phenotype.