Neuroendocrine (Merkel cell) carcinoma of the vulva.

The clinical, histopathologic, and ultrastructural features of a primary cutaneous vulvar neuroendocrine neoplasm (Merkel cell carcinoma) are presented. This recently described tumor arises in the dermis and is often aggressive with metastasis to regional lymph nodes. Ultrastructural study of this case reveals morphologic similarities to normal cutaneous Merkel cells, including peripherally located dense-core neurosecondary-like granules, and immunohistochemical studies revealed ACTH within neoplastic cells. By light microscopy this tumor is readily confused with other primary or metastatic cutaneous neoplasms; therefore, the importance of electron microscopic examination is emphasized for definitive diagnosis of this unusual tumor.