Mixed epithelial and mesenchymal
Calcifying epithelial odontogenic tumour Adenomatoid odontogenic tumour Squamous odontogenic tumour Odontogenic myxoma Odontogenic fibroma Cementoblastoma Ameloblastic fibroma Ameloblastic fibro-odontoma Odontoma - complex type Odontoma - compound type
Calcifying odontogenic cyst (calcifying cystic odontogenic tumour/
dentinogenic ghost cell tumour )
Primary intraosseous carcinoma
Clear cell odontogenic carcinoma
Malignant epithelial odontogenic ghost cell tumour
Odontogenic sarcoma an overview of the various entities encompassed under this heading.
Epithelial odontogenic tumours are supposed to be derived from odontogenic epithelium: dental lamina, enamel organ and Hertwig's root sheath. As there is no contribution, either proliferative or inductive, from the odontogenic mesenchyme, these lesions do not contain dental hard tissues or myxoid tissue resembling the dental papilla.
Ameloblastomas closely resemble the epithelial part of the tooth germ. They behave aggressively locally, but do not metastasise.
It is the most common odontogenic tumour  and may occur at any age, although cases in the first decade are rare. Maxillary cases are far outnumbered by mandibular ones. Rarely, the sinonasal cavities are involved [130, 139].
The intraosseous lesions are solid, solid with cystic parts, multicystic or unicystic. In the gingiva, the tumours have a white fibrous appearance on the cut sur face, due to the preponderance of fibrous stroma of lesions at this site.
Ameloblastomas consist of either anastomosing epithelial strands and fields or discrete epithelial islands. The former pattern is called the plexiform type, the other the follicular (Figs 4.11, 4.12). Both may occur within one and the same lesion [73, 181]. The peripheral cells at the border with the adjacent fibrous stroma are columnar with nuclei usually in the apical half of the cell body away from the basement membrane. The cells lying more centrally are fusiform to polyhedral and loosely connected to each other through cytoplasmic extensions. Especially in the follicular type, an increase in intercellular oedema may cause cysts that coalesce to form the large cavities responsible for the multicystic gross appearance ameloblastomas may show. In the plexiform type, cyst formation is usually the result of stromal degeneration. Condensation of collagenous fibres may cause a juxta-epithelial eosinophilic hyaline band. At the periphery of the lesion, the tumour infiltrates the adjacent cancellous bone. The lower cortical border of the mandible and the periosteal layer usually expand, but will not be perforated, the periosteum in particular forming a barrier . Spread into soft tissues is highly unusual; when observed, it is probably an ameloblastic carcinoma, a lesion to be discussed later on (see Sect. 18.104.22.168). Mitotic figures may occur within the peripheral columnar as well as in the stellate reticulum-like cells. In the absence of cytonuclear atypia and with a normal configuration, they are without prognostic significance.
4.4.1 Odontogenic Tumours -Epithelial
Acanthomatous and granular cell type ameloblasto-ma are variants of follicular ameloblastoma with squa-mous metaplasia and granular cells respectively. If ke-ratinisation is abundant, leading to large cavities filled with keratin, lesions are called keratoameloblastoma . In these tumours acantholysis may lead to a pseu-dopapillary lining that characterises the variant called papilliferous keratoameloblastoma.
The basal cell (basaloid) ameloblastoma is composed of nests of basaloid cells with a peripheral rim of cuboi-dal cells and does not display a well-developed, loose oe-dematous centre.
Desmoplastic ameloblastoma shows a dense collage-nous stroma, the epithelial component being reduced to narrow, compressed strands of epithelium. When these strands broaden to form larger islands, a peripheral rim of dark staining cuboidal cells and a compact centre in which spindle-shaped epithelial cells assume a whorling
pattern may be discerned (Fig. 4.13). Within the stro-mal component, active bone formation can be observed .
Unicystic ameloblastoma represents a cyst that is lined by ameloblastomatous epithelium (Fig. 4.14) . This epithelium may proliferate to form intraluminal nodules with the architecture of plexiform ameloblastoma. Downward proliferation of this epithelium may lead to infiltration of the fibrous cyst wall by ameloblastoma nests. Sometimes, the cyst lining itself lacks any features indicative of ameloblastoma, these being confined to intramural epithelial nests . Inflammatory alterations may obscure the specific histologic details to such an extent that none are left.
Ameloblastomas may also contain clear cells as well as mucous cells [100, 184].
Epithelial nests resembling ameloblastoma may be found in calcifying odontogenic cysts and ameloblas-
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