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

In an attempt to diagnose a suspected amelanotic melanoma tumour, we examined a variety of tissue and cell samples from one patient at the ultrastructural level, which consisted of single cell suspensions of tumour cells with and without DOPA treatment, tumour cells after culture in agar with and without DOPA treatment, and single tumour cells hetero‐transplanted into a nude mouse. Premelanosomes were not observed in sections of the amelanotic tumour with routine electron microscopy. Osmophilic‐dense bodies, suggestive of melanosomes, were noted in the single cells in suspension treated with DOPA and cells grown in agar without DOPA treatment. Definitive premelanosomes, with an identifiable striated matrix, were only observed in cells grown into colonies in agar and treated with DOPA. Positive L‐DOPA reaction products were noted in the golgi complex, endoplasmic reticulum closely related to the golgi (GERL), and in vacuoles from cells grown in agar. As controls, Cloudman S91 53.1 melanoma cells were evaluated as single cells in suspension or as colonies after culture in agar, both with and without DOPA treatment. Premelanosomes were always observed in this established melanoma cell line while DOPA‐treated cells contained positive L‐DOPA reaction products. The overall findings identified the tumour as amelanotic melanoma and indicated that both DOPA treatment and culture in agar were needed for the demonstration of premelanosomes.