Fig. 4.9. Epithelial lining of a keratocyst. The basal palisading and the corrugated parakeratinised surface are unique to this lesion. Moreover, there is a striking parallel between the basal layer and the surface
Fig. 4.10. In the event of inflammation, the epithelial lining of a keratocyst loses its typical pattern to transform into a lattice of spongiotic squamous epithelium

The odontogenic keratocyst shows a thin connective tissue wall lined by stratified squamous epithelium with a well-defined basal layer of palisading columnar or cuboidal cells and with a surface of a corrugated layer of parakeratin (Fig. 4.9). Mitotic figures can be identified in parabasilar and midspinous areas [12]. Rushton bodies similar to those seen in radicular cysts may also be present. The underlying cyst wall may contain tiny daughter cysts and solid epithelial nests. Also, epithelial proliferations similar to ameloblastoma have been reported. Daughter cysts and intramural epithelial nests are more common in cysts associated with the nevoid basal cell carcinoma syndrome [185]. When inflamed, the odontogenic kera-tocyst loses its typical histologic features, but shows a non-keratinising stratified epithelium exhibiting spongiosis and elongated rete pegs supported by a con nective tissue containing a mixed inflammatory infiltrate (Fig. 4.10). Rarely, odontogenic keratocysts show development of epithelial dysplasia and squamous cell carcinoma [87].

Immunohistochemical studies have not yielded data of diagnostic or prognostic significance [148, 149].

Odontogenic keratocysts may also contain mucous cells, melanin-producing cells, dentinoid and intramural cartilage [12, 74, 86, 105]. Ciliated cells may be seen, but in maxillary cases, they could also be the result of a communication with the maxillary sinus [122]. In addition, the cyst wall may contain intramural odontogenic epithelial remnants. Occasionally, intraosseous cysts are lined by orthokeratinised epithelium, thus having the appearance of an epidermoid cyst. Such cysts are known as orthokeratinised odontogenic cysts. Their differentiation from the odontogenic keratocyst with parake-ratinisation is clinically important as recurrence of or-thokeratinised cysts is rare [188]. Differential diagnosis with unicystic ameloblastoma (see Sect. may be difficult. Odontogenic keratocyst exhibits a compact spinous layer and a corrugated superficial parakeratin layer, and ameloblastoma a spinous layer with intercellular oedema.

Keratocysts do not cause symptoms unless concomitant inflammation causes pain and swelling. Radiographs may reveal extensive uni- or multilocular ra-diolucent lesions that occupy the major part of the jaw without having caused any appreciable cortical expansion. When the odontogenic keratocyst forms part of the nevoid basal cell carcinoma syndrome, patients may show any of the other features of this syndrome [72].

Odontogenic keratocysts tend to recur after enucle-ation [147, 148, 149]. Sometimes, a partial jaw resection is needed to a provide permanent cure [182]. If associated with the nevoid basal cell carcinoma syndrome, the chance of recurrence is even higher [18]. If located in the gingiva, the behaviour of the odontogenic keratocyst is less aggressive [65].

As there is sufficient evidence that this lesion actually represents a cystic neoplasm, the most recent WHO classification proposes the diagnostic designation kera-tocystic odontogenic tumour [181].

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