Pathogenesis of diffuse melanosis secondary to malignant melanoma*

The heavily pigmented but otherwise normal appearing skin of a patient with diffuse melanosis secondary to metastatic malignant melanoma was examined by electron microscopy and electron microscopic cytochemistry. The findings indicate that the slate‐blue colour of this melanosis is due, for the most part, to melanosomes derived from melanoma cells that have spread to and throughout the skin of the patient. The dissemination of individual, discrete tumour cells within the patient's dermis represents a unique form of single cell metastasization.