Palpebral amelanotic melanomas in F344 rats

Spontaneous amelanotic melanomas in the eyelids of F344 rats were found in one of 1/926 (0.11%) male and 5/925 (0.54%) female F344 rats that were used as control and treated animals in five different carcinogenicity studies conducted by the National Toxicology Program (Research Triangle Park, NC). These melanomas were grossly recognized as single, tan or white, well-circumscribed masses of the right or left eyelid. These melanomas primarily occurred in the dermis of the skin of the eyelids and consisted of poorly differentiated spindle cells characteristically arranged in interlacing fascicles. Rarely, epithelioid tumor cells were also observed, and these tumor cells showed a negative histochemical reaction for melanin. The epidermis and dermal-epidermal junction were usually uninvolved. The diagnosis of amelanotic melanoma could only be established by electron microscopic examination. The most striking ultrastructural feature of the tumor cells was a large number of intracytoplasmic premelanosomes (stage II melanosomes without melanin), which nearly filled the cytoplasm of most tumor cells. Giant premelanosomes and melanophagosomes were also seen. The tumor cells did not possess the ultrastructural features characteristics of Schwann cells (thin, long cell processes and pericytoplasmic basal laminae). The histologic and ultrastructural features of these palpebral tumors were similar to those of cutaneous amelanotic melanomas of the pinna in F344 rats.