Osseous choristoma of the buccal mucosa: A case report with immunohistochemical study of bone morphogenetic protein-2 and -4, and a review of the literature

Osseous choristomas are benign lesions that are characterized by ectopic bone formation in the soft tissue of the head and neck region. However, the pathophysiological mechanism responsible for ossification remains unclear. In this paper, we report a case of an osseous choristoma arising in the buccal mucosa and present a review of the relevant literature. We also carried out an immunohistochemical study of bone morphogenetic protein (BMP)-2 and -4, the potent inducers of ectopic bone formation, to identify the characteristics of bone-forming cells in the lesion. An 86-year-old woman was referred to our department because of an asymptomatic mass in the buccal mucosa. The lesion was 7 mm in size, and was excised under general anesthesia. The postoperative course was uneventful, with no signs of recurrence during the 24 months after surgery. Histologically, the lesion contained a lobular-shaped osseous tissue with a fibrous capsule, and the pathological diagnosis of osseous choristoma was made in consideration of the ectopic bony tissues that were localized away from the maxillo-mandibular bone. Immunohistochemical examination showed the first evidence of expression of BMP-2 and -4 in the osseous choristoma. BMPs were expressed in the osteoblasts surrounding the ectopic bone. Our results indicated that BMP-2 and -4 was associated with the ossification of the osseous choristomas.