Dysplastic melanocytic nevus. Electron-microscopic observation as a diagnostic tool.

Dysplastic melanocytic nevi (DMN) are distinctive cutaneous nevomelanocytic lesions that possess unique clinical and histopathological features. In our previous study, we showed that the fine structure of melanosomes in epidermal melanocytes of DMN are abnormal and reveal deranged melanogenesis. This study is an extension of our previous study and clarifies the fine structure of melanosomes in both epidermal melanocytes and keratinocytes with observations in an additional 10 cases of DMN with and without a marked mesenchymal response. We found that a poor mesenchymal response does not exclude fine structural abnormality of melanosomes in DMN; that abnormal melanosomes are manifested by a spherical shape with either fine granules or incompletely developed lamellae and/or both; that melanization occurs unevenly on the spherical granular and/or incompletely on the lamellar matrices; and that these abnormal melanosomes are transferred to keratinocytes before being completely melanized, and they reveal marked degradation. We suggest that the fine structural characterization of abnormal melanosomes is a new adjunct for histopathological diagnosis of DMN.