Diagnostic electron microscopy for amelanotic melanoma: correlation of patient biopsy, soft agar assay, and xenograft

Monoclonal antibody MAT-1 against human tyrosinase can detect melanogenic cells on formalin-fixed paraffin-embedded sections

Immunohistochemical localization of tyrosinase was examined with a monoclonal antibody (MoAb MAT-1) against human tyrosinase on routine formalin-fixed paraffin-embedded sections of 3 normal skin specimens, 15 melanocytic tumors (6 pigmented nevi, 3 juvenile melanomas and 6 malignant melanomas) and 3 non-melanocytic tumors. In the melanotic melanomas, almost all tumor cells were clearly stained with the antibody. In the nevocytic nevi, the nevus cells in lower epidermis and upper dermis were positive for MoAb MAT-1, but negative in middle and lower dermis. All three juvenile melanomas, one amelanotic melanoma, and three non-melanocytic tumors were entirely negative for MoAb MAT-1. Thus, MoAb MAT-1 could recognize the cells with melanogenic activity on routine formalin-fixed paraffin-embedded sections. However, the staining quality was not adequate for normal epidermal melanocytes, indicating that small technical innovations in the immunostaining process such as formalin fixation after PBS washing are required. Nevertheless, MoAb MAT-1 can be expected to be very useful for identifying melanogenic cells on paraffin-embedded sections, because we have to date no other antibody available for it.

Amelanotic melanoma: cases studied by Fontana stain, S-100 immunostain, and ultrastructural examination

We studied 15 cases of amelanotic melanoma by using light microscopy (in conjunction with Fontana staining and S-100 immunostaining) and electron microscopy. In 14 of the 15 cases, metastatic melanomas were present; 11 were associated with a history of pigmented primary melanoma. The Fontana stain was positive in 4 cases and negative in 11. The S-100 stain was positive in 13 cases and negative in 2. Of the 11 cases with negative Fontana stains, 9 were positive with the S-100 immunostain and 2 were negative. Electron microscopy showed melanosomes in 13 cases. Melanosomes were seen in the two cases with negative Fontana and S-100 staining. Although all stages of melanosomes were identified, generally the majority were immature melanosomes. This study demonstrates the difficulties associated with the histologic diagnosis of amelanotic melanoma. The Fontana stain may be helpful, and the S-100 stain is generally positive. Electron microscopy provided the only evidence for melanoma in two cases and is considered the most definitive method of diagnosis of amelanotic melanomas. These tumors generally produce some pigment and thus are called amelanotic by convention, not because pigmentation is totally absent.

The natural history of a family of transplantable melanomas in hamsters

We have characterized a family of transplantable melanomas in Syrian (golden) hamsters, which originated in 1959 as a spontaneous melanoma of hamster skin, and which has been maintained since then by serial passage. Emphasis has been placed on using the same method of transplantation, keeping strict records on all passages, and applying the same investigative techniques, in order to trace tumor behavior over long periods of time. This tumor family consists of five variants linked by common origin, but which differ with respect to differentiation level, malignancy, intermediary metabolism, chromosome number, and cell surface properties. Once established, these melanomas possessed a considerable degree of phenotypic stability over decades of passaging. One tumor line in this family is emphasized. The Ab amelanotic melanoma lost its differentiated functions (the ability to synthetize melanin) a quarter of a century ago, and since then has remained dedifferentiated in serial passage in hamsters. After transfer to primary cell culture, the Ab melanoma cells differentiate readily and lose much of their proliferative potential. This process is reversible by reimplantation of the cells into a hamster. Inasmuch as this hamster melanoma system meets many of the conditions required for an experimental tumor model, five melanoma variants are characterized concisely and compared to other melanomas in humans and animals. Mechanisms by which new melanoma variants arise are discussed and compared to some phenomena in the evolution of the species.