Proliferative activity of cutaneous melanocytic neoplasms defined by a proliferating cell nuclear antigen labelling index.

To evaluate the proliferative activity of benign, borderline and malignant cutaneous melanocytic neoplasms, 30 cases of malignant melanoma (MM) and 41 cases of naevi were studied by immunostaining using a monoclonal antibody against proliferating cell nuclear antigen (PCNA). PCNA is a nuclear antigen expressed in the late G1 and S phase and serves as a marker of proliferating cells. Invasive MM and MM in situ showed much higher PCNA positivity rates than melanocytic naevi (invasive MM, 18.0%; MM in situ, 11.3%; ordinary melanocytic naevi, 2.6%). The PCNA positivity rate did not increase significantly with the thickness of MM. Among ordinary melanocytic naevi, junctional naevi had a higher PCNA positivity rate than compound or intradermal naevi. Mean PCNA positivity rates for Spitz's naevi and sporadic dysplastic naevi were within the range for ordinary melanocytic naevi, indicating the benign nature of both types of naevus. Contrary to some previous studies, MM in situ showed high proliferative activity, indicating that cells of MM in situ are actively proliferating. This study clearly demonstrates that MM and various types of naevi can be separated according to differences in proliferative activity defined by the PCNA labeling index.