Melanocytic lesions in lymph nodes associated with congenital naevus.

Two cases of melanocytic lesions in lymph node associated with congenital naevus are presented. The first was a 30-year-old man with a nodular melanoma arising in a small congenital naevus. The second was a 2-year-old male infant with a giant congenital naevus. In both cases, naevus cell aggregates were observed in the capsule, trabeculae, perisinusoidal areas and lymphatic vessels surrounding the nodes. In the first case, clusters of large atypical melanocytes were present amongst naevus cell aggregates in the perisinusoidal areas as well as in the lymphoid parenchyma. Between the naevus cells and large atypical melanocytes, transitional forms were observed which supports the idea that the presence of large atypical melanocytes is indicative of benign naevus cells. In the second case, marginal sinuses were packed with clusters of large melanin-rich cells. Immunohistochemically, these cells were S-100 protein negative, but ultrastructural studies proved them to be melanocytes. They were considered indicative of spread of benign naevus cells via lymphatic vessels. Arrested migration of naevus cells during embryogenesis and benign spread of naevus cells are possible explanations for the histogenesis of naevus cell aggregates in lymph nodes associated with congenital naevus.