Mast cells with bilobed or multilobed nuclei in a nodular lesion of a patient with urticaria pigmentosa.

Mast cells with bilobed or multilobed nuclei have only rarely been observed in the bone marrow of patients with systemic mastocytosis and in a case of subdural mast cell sarcoma. To our knowledge, they have not been reported in cutaneous mast cell disease. We report a rare occurrence of mast cells with bilobed or multilobed nuclei (atypical mast cell type II) in a nodular lesion of a 24-year-old woman with urticaria pigmentosa. The typical and atypical mast cells were confirmed by Giemsa and Leder's naphthol-AS-D-chloroacetate esterase stains and by immunohistochemical staining for tryptase and KIT protein (CD117). Although the nodular lesion with atypical mast cells did not appear to be cytologically malignant, the occurrence of atypical mast cells in a nodular lesion but not in a papular lesion might denote progression of the disease as suggested by the emergence of cells positive for p53.