Ameloblastoma classification
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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]Associate Editor(s)-in-Chief: Simrat Sarai, M.D. [2]
Overview
Ameloblastoma may be classified based on histopathology into six subtypes: follicular, plexiform, acanthomatous, basal cell, unicystic, and desmoplastic.
Classification
The following six histhologic subtypes have been identified for ameloblastoma:
- Follicular
- Plexiform
- Acanthomatous
- Basal cell
- Unicystic
- Desmoplastic
Mixtures of different patterns are commonly observed and the lesions are usually classified based on the predominant pattern present.
- Follicular ameloblastoma: These are most readily recognizable and common type of ameloblastoma histologically. The follicular pattern consist of islands of epithelial cells with a central mass of polyhedral cells or loosely arranged angular cells resembling stellate reticulum, surrounded by well organized single layer of cuboidal or tall columnar cells with nuclei placed at the opposite pole of basement membrane resembling pre-ameloblasts. This peripheral cell layer tends to show cytoplasmic vacuolization. Cystic formation is often seen in the center of the epithelial islands due to degeneration of stellate reticulum like cells.
- Plexiform ameloblastoma: The tumor epithelium is arranged in form of network/tabeculae which is bound by a layer of cuboidal or columnar cells and stellate reticulum like areas are usually minimal. Cyst formation occurs but is usually due to stromal degeneration rather than cystic change in the epithelium. The stroma consists of loose, vascular sparsely cellular connective tissue.
- Acanthomatous ameloblastoma: It resembles a typical follicular ameloblastoma except it shows extensive squamous metaplasia sometimes with keratin formation within the epithelial islands.
Granular cell ameloblastoma: When the central stellate reticulum cells show extensive granular cell transformation i.e. in the form of sheets of large eosinophilic granular cells, the tumors are referred to as granular cell ameloblastoma. Sometimes this change, may be so extensive that the peripheral columnar cells may also be replaced making the diagnosis difficult especially in a small biopsy. Ultrastructurally, it is seen that the granules consist of pleomorphic, osmiophilic, lysosome like organelles. Basal cell ameloblastoma: This variant shows predominant basaloid pattern consisting of darkly stained cells with minimal cytoplasm and little evidence of palisading at the periphery resembling those seen in basal cell carcinoma. Desmoplastic ameloblastoma: This variant was first described in detail by Eversole et al. in 1984 and is characterized by extensive stromal collagenization or desmoplasia surrounding compressed small/irregular islands of odontogenic epithelium making it a distinct entity. Cyst formation is common and ameloblast like areas are present only in small foci. Calcification in the fibrous stroma and occasional bone formation can also be seen. Histochemical evaluation of the collagen suggests that the dense stroma is not scar tissue but represents active denovo synthesis of extracellular matrix proteins. It typically presents radiographically as a mixed radiolucency and radiopacity mimicking a fibrosseous lesion. In contrast to typical ameloblastoma, this variant frequents the maxilla and the anterior region of the jaws. Unicystic ameloblastoma: This is considered as an in-situ or superficially invasive form of ameloblastoma and consists of a single cyst lined by ameloblastic epithelium. It presents clinically similar to a dentigerous cyst and is usually associated with an impacted tooth (usually 3rd molars). They are usually seen in younger patients with an average age of diagnosis being 22 years and generally involve the mandible. This variant histologically presents as a single cyst lined by ameloblastomatous epithelium and is divided into several subgroups based on pattern and extent of ameloblastomatous proliferation in relation to cyst wall. Reichart et al. classification modified from Ackermann et al.: -1. Luminal UA: Cystic lesion lined by epithelium which exhibits columnar differentiation and reverse polarization of the basal cell layer. The connective tissue adjacent to the epithelium often exhibits a uniform, thin band like area of hyalinization. -1.2. Luminal and intraluminal UA: Is the simple type but exhibits one or more nodules projecting into the lumen. No extension is seen into the surrounding connective tissue wall. Occasionally, this form of UA can produce an intraluminal plexiform pattern of odontogenic epithelium that lacks typical ameloblastomatous differentiation and is called as unicystic plexiform ameloblastoma. -1.2.3. Luminal, intraluminal and intramural UA: Here there is occurrence of intramural proliferation of ameloblastoma along with subgroup 1.2 features. -1.3. Luminal and intramural UA: exhibits a cyst with a luminal lining in combination with intramural nodules of SMA tissues. The intramural ameloblastoma tissue may be seen as an infiltration from the cyst lining or as free islands of follicular ameloblastoma. Peripheral ameloblastoma: They present histologically with follicular or plexiform pattern as well as acanthomatous pattern. In most cases, the tumor is well separated from the overlying epithelium but many are confluent with the overlying mucosa. Other rare variants: Clear cell variant which may contain clear PAS positive cells are localized in the stellate reticulum like areas. Papilliferous keratoameloblastoma show occurrence of areas of ameloblastoma with keratinisation, tumor islands with papilliferrous appearance along with cystic areas resembling odontogenic keratocysts. Mucous cells can also be demonstrated rarely in ameloblastoma.