Spontaneous amelanotic melanomas of the uveal tract in F344 rats

Five intraocular amelanotic melanomas were identified in the National Toxicology Program's database consisting of records from more than 60,000 female and 60,000 male F344 rats, which were used as control and treated animals in 2-year carcinogenicity studies. The five spontaneous melanomas were grossly observed as white or yellow, unilateral nodules, which originated in the region of the iris and ciliary body, often also involving the choroid. These amelanotic melanomas were composed predominantly of spindle cells arranged in a whorled pattern often with perivascular orientation. Mitotic figures were common in five tumors. The spindle cells had a positive immunoreactivity for S-100 protein but were negative for desmin. Electron microscopic studies provided clear evidence that these tumors originated from the uveal melanocytes. Ultrastructurally, the spindle cells contained numerous cytoplasmic premelanosomes (stage II melanosomes) that were not associated with melanin. Special histochemical studies showed that the spindle cells had a negative reaction for melanin. Although electron microscopic features are critical in the diagnosis of amelanotic melanomas of the uveal tract, the whorled pattern of spindle cells is a useful histologic criterion in differential diagnosis of this tumor in F344 rats.