Structure of basement membranes in malignant melanoma and nevocytic nevi

Basement membranes found around tumor cells in nevocytic nevi, Spitz's nevi, and malignant melanomas were analyzed by electron microscopy and antibody staining for several basement membrane proteins. Nevocytic nevi and Spitz's nevi showed a distinct, occasionally discontinuous lamina densa regardless of whether they were located in junctional zones of the epidermis or within the dermis. All basement membranes around nests of aggregated nevus cells, however, lacked anchoring fibrils. This correlated with the absence of type VII collagen. In contrast, type IV collagen, laminin, and nidogen were present at the periphery of the nevus cell clusters in agreement with the presence of an intact lamina densa. Aggregated tumor cells in malignant melanomas were bordered by a lamina densa when located in a junctional position and lacked this structure when they had migrated into the dermis. This process was accompanied by a drastically reduced staining for collagen type IV and nidogen, whereas laminin was still detectable. Anchoring fibrils and their molecular correlate, type VII collagen, were consistently absent. These observations demonstrate major alterations in the composition of basement membranes around malignant melanomas, which can be an important factor for the invasive growth and formation of metastases of these tumors.