Melanotic paraganglioma of the uterus.

Two cases of melanotic paraganglioma of the uterus are reported. Interpreted as a pigmented paraganglioma, both lesions were an incidental finding in uteri removed for unrelated benign conditions. Microscopically, both tumors were circumscribed, and composed of nests of large round or angulated polygonal cells with abundant clear or granular pale, eosinophilic cytoplasm. Large amounts of coarse intracytoplasmic melanin pigment were present in many cells, whereas other cells were completely devoid of pigment. Both lesions contained psammoma bodies. Electron microscopy showed abundant intracellular melanosomes, including many compound melanosomes and few premelanosomes. A lamellar whorled structure was the dominant infrastructure on which melanin pigment was deposited in many cells. The absence of microvilli or dendritic cell processes and basal lamina, as well as the lack of S-100 protein, pointed against a Schwannian or melanocytic differentiation. The presence of abundant pigment and only rare neuroendocrine-like secretory granules in the cytoplasm of a few cells indicated divergence from classic paraganglioma. Architecturally, this lesion is similar to a paraganglioma; but, functionally, instead of neuroendocrine granules, the tumor cells produce melanin pigment.