Frank J.M. Verstraete, DrMedVet, DAVDC, DECVS
Cysts are pathologic cavities that are lined by epithelium and contain fluid or semi-solid material. A thin fibrous capsule separates the epithelium from the surrounding normal tissues. Odontogenic cysts are derived from the odontogenic epithelium, and can be developmental in origin or the result of an inflammatory process. A number of types have been recognized in humans, the most important being the radicular (periapical) cyst, eruption cyst, dentigerous (follicular) cyst, and odontogenic keratocyst (or primordial cyst). The radicular cyst is inflammatory in origin, and is classified as such, separate from the odontogenic cysts by the World Health Organization. The radicular cyst derives however its epithelial lining from the cells of Malassez in the periodontal ligament, and is therefore also a true odontogenic cyst.
By definition, a dentigerous cyst is attached to the cemento-enamel junction and encloses the crown of an unerupted tooth. The cyst wall is composed of a thin layer of connective tissue and an unkeratinized epithelial lining that is only 2–6 cell layers thick and resembles reduced enamel epithelium.
The odontogenic keratocyst has very distinct histopathologic features, which include a uniform thickness of 8 to 10 cell layers. The basal cells are palisaded. The luminal epithelial cells are typically parakeratinized. The irregularly folded cyst lumen is filled with keratin. Odontogenic keratocysts are not necessarily associated with teeth, and may be uniloculated or multiloculated. They may exhibit an aggressive clinical behavior and have a high recurrence rate.
The radicular or periapical cyst develops from a preexisting periapical granuloma. The periapical granuloma is a well-circumscribed mass of granulation tissue and inflammatory cells that replaces bone at the apex of a nonvital tooth. Stimulated by the necrotic dental pulp, this inflammatory tissue may undergo cystification (periapical or radicular cyst) when rests of Malassez, normally found in the periodontal ligament, are stimulated to proliferate Presumably, this epithelial lining serves as a partially protective barrier between the nonvital tooth apex and the surrounding viable tissue. The cystic epithelium is non-keratinized and the cyst wall contains a mixed inflammatory cell infiltrate.
A retrospective study to identify the types of odontogenic cysts and their clinical-pathologic correlations in the dog was recently completed at University of California, Davis. Information was collected from records of 41 dogs including breed, age, sex and location of lesion in the oral cavity. Histological slides and full-mouth radiographs pertaining to each patient were reviewed. Statistical analysis was applied to the clinical-pathologic features of the most common lesion, the dentigerous cyst. Forty-one dogs with odontogenic cysts were identified between 1995 and 2010. The included cases consisted of 29 dogs with dentigerous cysts, 1 radicular cyst, 1 periodontal cyst, and 1 gingival inclusion cyst. In addition, 9 odontogenic cysts were identified whose clinical behavior and histological features were suggestive of, but not diagnostic for odontogenic keratocyst (keratocystic odontogenic tumor) in humans based on strict pathologic criteria; these lesions were all found on the maxilla surrounding the roots of normally erupted teeth. Six dentigerous cysts were associated with unerupted canine teeth and 30 were associated with unerupted first premolar teeth, predominately of the mandible. One dog had a dentigerous cyst associated with an unerupted canine and first premolar tooth. Four dogs had bilateral first premolar dentigerous cysts and 1 dog had 3 dentigerous cysts. Dentigerous cysts were identified in a variety of breeds but brachycephalic dog breeds were overrepresented. It was concluded that the dentigerous cyst has a distinct clinical pattern in the dog.
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