Odontogenic cysts of the anterior mandible of sheep.

Cysts of this site are quite common in mature ewes in New Zealand and the British Isles, although their true prevalence is not yet known. In most examples, an unerupted incisor is embedded in their wall but some do not contain a tooth. The cysts are lined by parakeratinized stratified squamous epithelium and generally exhibit a flat interface between the epithelium and the connective tissue. They may become secondarily infected, in which case they exhibit chronic inflammation. In some cases, foci of cementum are attached to the epithelial lining. When a tooth is involved, the cysts arise from various locations along its crown, which in sheep is normally covered by a thin layer of cementum. Some of these unerupted teeth exhibit irregular hypercementosis and/or external resorption which is repaired by cementum. In some examples there is a fistula between the cyst and the oral cavity, either through a deep periodontal pocket or from the incisal edge of the unerupted tooth piercing the overlying mucosa.