Answer: Radicular cyst

Histology: This cyst is lined by a non-keratinizing stratified squamous epithelium and is supported by a cyst wall composed of mature collagenous stroma. Within the wall is a focally intense inflammatory infiltrate composed of a polymorphous cell population including neutrophils, lymphocytes, macrophages and plasma cells. The epithelial lining varies in thickness from 3 to 4 cell layers in relatively uninflamed areas to several layers in thickness with prominent rete ridge formation in regions adjacent to inflammation. Foreign body type giant cells and foamy macrophages may be noted adjacent to areas where cholesterol crystal clefts are noted. Occasionally mucous cells may be noted along the inner lining of the cyst as well as dystrophic mineralization of the individual epithelial cells. The connective tissue lining will be variably infiltrated with acute and chronic inflammatory cells with a liberal distribution of macrophages containing hemosiderin and lipid material.

Discussion: Radicular cyst development is preceded by a periapical granuloma, which in essence represents an intense inflammatory focus at the root surface with an early absence of proliferating odontogenic epithelium. Subsequent to development of a well defined inflammatory focus within the granuloma there is a proliferation of resident epithelial odontogenic rests (Rests of Malassez). These rests reside within the periodontal ligament or membrane, which invests the root of the tooth including the apex region. Subsequently there is epithelial residue proliferation adjacent to the inflammatory milieu with a subsequent coalescence of proliferating epithelium. This lining along with the connective tissue of the cyst wall forms a semi- permeable membrane, which will enable the establishment of an oncotic gradient across this membrane, permitting fluid ingress into the cyst lumen. Lateral expansion of the cyst follows by this mechanism.

Unusual structures which can be noted either within the cyst wall or epithelial lining include the presence of hyaline bodies which tend to be linear to curved as well as ring-shaped and are generally brightly eosinophilic, refractile and hyaline in nature. Focal collections of cholesterol crystal clefts likewise will be noted within the cyst lumen and cyst wall. The lining of the cyst is often discontinuous where the epithelium will abruptly terminate, forming a cyst lining ulceration.

The histological differentiation of this cyst from other jaw cysts must rely upon integration of the clinical and radiographic findings as well as an understanding of the epithelial lining characteristics. The radicular cyst lining is nonkeratinizing and varies in thickness from 2 to 3 cell layers to many cell layers where chronic inflammation is present within the cyst wall.

The odontogenic keratocyst lining is thin and flattened with a prominent low columnar to cuboidal basal cell layer, which is highly regimented (palisaded). The thickness of the lining epithelial layer is generally 6 to 10 cells. Basal layer contour parallels the surface with the epithelial-stromal interface being flat rather than forming ridges. A critical feature is the presence of parakeratin at the luminal surface, a feature not found with the radicular cyst. On occasion the cyst lumen will be filled with thick, pasty keratin material. The contour of the epithelial surface is generally corrugated in contrast to the smoothly contoured surface of the radicular cyst.

Separation of the nasopalatine duct cyst relies upon radiographic and histologic features. Radiographically this cyst is located in the paramedian region of the anterior maxilla with a well-defined hyperostotic margin and is associated with vital incisor teeth, unlike the radicular cyst. Histologically, a simple cuboidal cell lining only a few layers thick, a pseudostratified columnar ciliated epithelium, columnar stratified epithelium, stratified squamous epithelium, or most commonly, combinations of these types characterize this cyst. An important distinguishing feature is contained within the wall of the cyst where there are prominent veins, arteries, and nerves given the relationship to the incisive canal.