Cutaneous mesenchymal hamartoma.

A case of a cutaneous mesenchymal hamartoma occurring on the thigh of a newborn infant is reported. Microscopically, the solitary tumorous lesion consisted of an intimate mixture of fibrocollagenous bundles, islands of mature fat cells, distorted blood vessels, and eccrine sweat glands. Many mast cells were scattered throughout the fibrocollagenous bundles. Ultrastructurally, most tumor cells were suggestive of undifferentiated mesenchymal cells.