Nodular melanoma demonstrates a confluent proliferation of atypical single and nested melanocytes in the dermis. The lesion is poorly circumscribed and asymmetric, and the dermal cells often show mitoses and nuclear pleomorphism. Although melanomas may comprise spindle cells, the cells typically do not have the wavy nuclei characteristic of a neurofibroma.
Piloleiomyoma is a benign smooth muscle tumor derived from the arrector pili muscle. Histologic appearance demonstrates poorly circumscribed fascicles of spindle cells in the dermis. As opposed to the wavy nuclei of neural cells, smooth muscle cells can be identified by their blunt-ended, cigar-shaped nuclei with a perinuclear vacuole.
Neurofibroma demonstrates a well-circumscribed, nonencapsulated proliferation of spindle cells with wavy nuclei and scant cytoplasm. The lesion is embedded in a pale, loose collagen stroma. Mast cells are often present.
Dermal nevi can appear clinically similar to neurofibromas. However, on histologic examination, a dermal nevus demonstrates uniform nests of banal, cuboidal melanocytes within the dermis. Spindle cell proliferation is not seen.
Dermatofibromas manifest as hyperpigmented, firm nodules. On histologic examination, there is proliferation of spindle cells in the dermis that often wrap around collagen bundles at the periphery of the lesion, forming collagen balls. Epidermal hyperplasia and basal layer pigmentation are usually present.
