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Odontogenic Myxoma of the Mandible in a Filly

Odontogenic tumors are uncommon in domestic animals and have been broadly classified as epithelial or mesodermal (stromal) odontogenic tumors.4 Other than the canine fibromatous epulis, stromal neoplasms are very rare in animals. The 2 main types of mesodermal tumors are odontogenic myxoma (fibromyxoma) and cementoma.4 In the only report of an odontogenic myxoma in a domesticated animal, no details were given.3 This present report includes a light microscopic, ultrastructural, and immunohistochemical description of an odontogenic myxoma in the mandible of a horse. A 1.5-year-old mixed-breed filly was referred to the Veterinary Medicine Teaching Hospital, University of Florida, Gainesville, because of marked left mandibular swelling, profuse salivation, and inability to eat or drink. On physical examination, a painful, malodorous mass was present on the left mandible. Radiologic examination revealed malalignment of the horizontal ramus of the left mandible and expansion of the ramus by a mottled area of uneven radiopacity. Because of a guarded prognosis and financial constraints, the filly was euthanized. At necropsy, the only abnormality was a glistening, bosselated, tan and off-white mass (12 3 11 cm) in the horizontal ramus of the left mandible, protruding into the oral cavity (Fig. 1). The surface of the mass was partially covered by strands of green fibrous ingesta, with multifocal areas of necrosis and hemorrhage. The mass completely replaced 2 premolar teeth. On radiologic examination of the dissected mandible, the mass had multiloculated radiolucency and was expanding and destroying the mandibular bone (Fig. 2). On cut surface, the mass was variably firm, glistening, and offwhite to light tan. Sagittal section of the mandibular ramus revealed extensive invasion and replacement of the mandibular bone by the mass. A thin band of remnant bone was present on the ventral aspect of the ramus. Tissue samples obtained at necropsy were fixed in 10% neutral buffered formalin, embedded in paraffin, and stained with hematoxylin and eosin. Paraffin-embedded sections were histochemically stained with alcian blue at pH 2.6 for sulfated and nonsulfated glycosaminoglycans (GAG) and at pH 1.0 for sulfated GAG. Formalin-fixed, paraffin-embedded sections were immunohistochemically stained for localization of cytokeratin (rabbit polyclonal antibody against cytokeratins 5, 6, 8, 13, and 16), vimentin, S-100 protein, lysozyme, and actina using an avidin–biotin–peroxidase complex (ABC) method.b For ultrastructural examination, sam-