New phenotypical and ultrastructural findings in spindle cell (desmoplastic/neurotropic) melanoma

Spindle cell melanoma is a rare and distinctive variant of malignant melanoma that is composed of spindled neoplastic cells and includes desmoplastic and neurotropic melanoma. The lack of expression of several melanoma markers may result in a delayed or wrong diagnosis. In this study, we have analyzed in detail the phenotype of the tumor cells in 9 spindle cell melanomas on both paraffin-embedded and frozen material, using melanocytic, neural, and mesenchymal markers. The neoplastic cells expressed the melanocytic markers S-100, Mel-CAM, and NKIC3, but lacked gp100 and Melan-A; tyrosinase and c-Kit were expressed in 2 of 7 cases. Most cases expressed the neural markers p75-nerve growth factor receptor, neural cell adhesion molecule, and NSE. All cases expressed vimentin but lacked the mesenchymal markers CD34 and alpha-smooth muscle actin. Remarkably, all spindle cell melanomas strongly and diffusely expressed the fibroblastic markers Thy1 (CD90) and aminopeptidase N (CD13) and variably expressed the enzyme prolyl-4-hydroxylase, involved in procollagen formation. The coexpression of melanocytic, neural, and fibroblastic markers suggests bidirectional differentiation of neoplastic melanocytes toward (myo)fibroblasts and Schwann cells, a feature that was confirmed by electron microscopy. Furthermore, the lack of CD90 and CD13 staining in a wide range of melanocytic lesions suggests specificity of these markers for spindle cell melanoma.