The ultrastructure of congenital nevocytic nevi. I. Nevus cells in walls of blood vessels and lymphatics.

As congenital nevocytic nevi have an increased risk of malignant degeneration, nevus-cell involvement in blood vessels and lymphatics is of particular interest. The present histological and ultrastructural studies revealed nevus-cell nests in the walls of venules in 1 out of 11 patients with medium-sized nevi, and in subcutaneous veins in 3 out of 8 patients with garment nevi. In all cases, the nests histologically consisted of benign-appearing subendothelial B-type cells. Ultrastructurally, the features of these nevocytes essentially corresponded to those of nevocytes in the surrounding area. In 1 patient with a garment nevus, in whom affected vessels of the lumbar area as well as of the neck were examined, the nevus-cell nests were exclusively situated between the endothelium and its basal lamina. These nevocytes appeared to be more electron dense, but had no unequivocally atypical features. In the region of these nests, the endothelium was often discontinuous; thus, the nevus-cells were in direct contact with the lumen. Some of these cells exhibited slight degenerative changes. The lymphatics were affected in 6 of the 18 cases of garment nevi and in 2 of the 11 cases of medium-sized nevi. The morphological findings were comparable to those for blood vessels. It is concluded that, in garment nevi, morphologically benign nevocytes may be carried off hematogenously as well as lymphogenously.