A spontaneous compound odontoma in an adult Sprague Dawley rat (Rattus norvegicus).

Reports of compound odontomas in rats are very rare. A 14-month-old adult male Sprague Dawley rat was found to have a hard mass associated with the caudal aspect of the left mandible. After 2 weeks of observation, the rat was euthanized due to the mass growing significantly in size and the rat losing >20% of its body weight. Grossly, the mass was well-circumscribed, 3.7 × 3 × 1.2 cm, hard and heterogeneously coloured white, tan and red. The mass was restricted to the mandibular bone and did not involve surrounding subcutaneous tissue. On cut surface, the mass was a similar colour and brittle. Histologically, there were numerous proto-teeth embedded in ossified stroma. Each proto-tooth had a central mesenchyme pulp surrounded by columnar odontoblasts and dentine matrix. The dentine was often bordered by enamel matrix, which was occasionally bounded by ameloblasts. These histological findings were consistent with a compound odontoma. This is the first report of a spontaneous compound odontoma in the caudal mandible of a rat.