4.2 Odontogenic Cysts, Tumours & Developmental Anomalies

Odontogenic Cysts, Tumours & Developmental Anomalies

Odontogenic cysts and tumours arise from remnants of the tooth-forming apparatus, including the dental lamina (rests of Serres), Hertwig's epithelial root sheath (rests of Malassez), and the reduced enamel epithelium. A precise grasp of their histological characteristics, molecular alterations, and clinical behavior is essential for diagnostic accuracy in the LDS Part 1 exam.

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Inflammatory Odontogenic Cysts

Radicular Cyst (Periapical Cyst)

• Etiology & Origin: The most common odontogenic cyst (~65%). Arises from proliferation of rests of Malassez within the periodontal ligament, stimulated by inflammatory cytokines following pulpal necrosis.
• Histopathology: Lined by non-keratinised stratified squamous epithelium of variable thickness displaying prominent irregular rete pegs. The fibrous cyst wall exhibits intense chronic inflammatory infiltrates (plasma cells, lymphocytes).
• Diagnostic Markers: Features Rushton bodies (hyaline, curved eosinophilic structures within epithelium) and abundant cholesterol clefts surrounded by multinucleated giant cells in the capsule.
• Residual Cyst: A radicular cyst retained in the jaw after extraction of the offending necrotic tooth.

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Developmental Odontogenic Cysts

    OKC Epithelium Architecture:
    Parakeratinised Corrugated Layer (Superficial)
    ├── 5 to 8 Cell Layers (Uniform Thickness)
    ├── Hyperchromatic Palisaded Basal Layer ("Tombstone" alignment)
    └── Flat Basement Membrane (No Rete Pegs) ──► High Recurrence / PTCH1 Mutation

Gorlin-Goltz Syndrome (Nevoid Basal Cell Carcinoma Syndrome)

Autosomal dominant condition caused by germline mutations in the PTCH1 tumor suppressor gene (part of the Sonic Hedgehog signaling pathway). Clinical tetrad:

1. Multiple early-onset Odontogenic Keratocysts of the jaws.
2. Multiple Nevoid Basal Cell Carcinomas of the skin.
3. Bifid ribs and skeletal anomalies.
4. Calcification of the falx cerebri and frontal bossing.

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Benign Epithelial Odontogenic Tumours

Ameloblastoma

Ameloblastoma is the most clinically significant benign odontogenic neoplasm due to its locally invasive, destructive growth and high recurrence rate if inadequately excised.

• Molecular Pathology: Driven by somatic mutations in BRAF V600E (present in ~80–90% of intramedullary/solid types) or SMO mutations.
• Clinical & Radiographic: Most common in the posterior mandible/ramus (80%). Appears radiographically as a soap-bubble or honeycombed multilocular radiolucency with root resorption.

       Follicular Ameloblastoma Island:
       Outer Layer: Columnar Cells with Reverse Polarity (Nuclei away from basement membrane)
       Inner Core: Stellate Reticulum-like Meshwork (Loosely arranged angular cells)

Histological Subtypes

1. Follicular Pattern: Epithelial islands resembling enamel organs within a fibrous stroma. Peripheral cells are tall columnar with hyperchromatic nuclei displaying reverse polarity (nuclei aligned away from the basement membrane, subnuclear vacuolation). Central cells resemble stellate reticulum.
2. Plexiform Pattern: Cords and anastomosing strands of odontogenic epithelium bounded by columnar/cuboidal cells surrounding a delicate vascular stroma.
3. Unicystic Ameloblastoma: Occurs in younger patients (2nd decade). Subclassified into luminal (lined by ameloblastic epithelium), intraluminal (nodular proliferation into lumen), and mural (invasion of cyst wall, requiring aggressive resective management).

Calcifying Epithelial Odontogenic Tumour (CEOT / Pindborg Tumour)

• Histopathology: Sheets and strands of polyhedral epithelial cells with prominent intercellular bridges and nuclear pleomorphism (without mitoses).
• Diagnostic Features: Abundant extracellular amyloid-like deposits that stain positive with Congo Red, exhibiting characteristic apple-green birefringence under polarized light. Concentric calcifications called Liesegang rings develop within the amyloid deposits.

Adenomatoid Odontogenic Tumour (AOT)

• Demographics: Females in 2nd decade; anterior maxilla associated with unerupted canine ("two-thirds rule": 2/3 female, 2/3 teens, 2/3 maxilla, 2/3 canine).
• Histopathology: Duct-like (tubular) structures lined by columnar cells, ductal structures filled with eosinophilic material, surrounded by whorled rosettes of epithelial cells. Extremely low recurrence; cured by simple enucleation.

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Mixed & Mesenchymal Odontogenic Tumours

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Developmental Dental Anomalies

Structural and morphological dental anomalies arise during morphodifferentiation, histodifferentiation, or apposition stages of odontogenesis.

Structural Anomalies (Enamel & Dentine)

Amelogenesis Imperfecta (AI)

Ectodermal hereditary defect affecting enamel formation across primary and permanent dentitions. Classified into four main types:

• Type I (Hypoplastic): Defective matrix formation; enamel is thin, pitted, or smooth but normally mineralised and hard.
• Type II (Hypocalcified): Defective matrix calcification; enamel is of normal thickness at eruption but soft, cheesy, and rapidly lost.
• Type III (Hypomaturation): Defective crystal maturation; enamel is opaque, mottled, snow-capped, and softer than normal.
• Type IV (Hypomaturation-Hypoplastic with Taurodontism).

Dentinogenesis Imperfecta (DI)

Autosomal dominant mesodermal defect affecting dentine matrix production.

• DI Type I: Associated with Osteogenesis Imperfecta (collagen Type I mutation, COL1A1/COL1A2); primary teeth more severely affected, blue sclerae, fragile bones.
• DI Type II (Hereditary Opalescent Dentine): Isolated dentine defect caused by mutations in the DSPP gene (Dentin Sialophosphoprotein). Affects both dentitions equally.
• Radiographic & Histological Hallmarks: Opalescent amber/translucent crowns, marked cervical constriction ("bulbous crowns"), thin short roots, and early total obliteration of pulp chambers and root canals due to rapid disorganized dentine deposition. Dentine tubules are short, irregular, and reduced in number.

       Dentinogenesis Imperfecta (Type II) Characteristics:
       Clinical: Amber / Opalescent Coloration + Severe Attrition
       Radiographic: Bulbous Crowns + Cervical Constriction + Pulpal Obliteration
       Genetics: Autosomal Dominant DSPP Mutation

Morphological Anomalies of Form & Count