Signalment:  

8-month-old female spayed domestic shorthair cat, (Felis cats).An oral mass was noted when the cat yawned.

Gross Description:  

As per the clinician: the mass is red, inflamed and appears to extend along the gumline.

Histopathologic Description:

Gingiva: The tissue is composed of a mass that is characterized by fronds and islands of odontogenic epithelium that are separated by and often cradling loose or compact mesenchymal tissue. In some areas the mesenchymal tissue is forming compact round aggregates and these are the areas in which the epithelium appears to form a rim or cradle around the mesenchymal tissue. The epithelial fronds are lined by columnar cells with basal nuclei and tend to have prominent intercellular bridges. Mitotic cells are rare and primarily seen in the more compact areas of both the mesenchymal and the epithelial components. The overlying oral epithelium is segmentally ulcerated. Large numbers of neutrophils were present in the subjacent necrotic stroma and infiltrated the mesenchymal tissue of the neoplastic mass.

Morphologic Diagnosis:  

Gingiva: Feline inductive odontogenic tumor (Feline inductive fibroameloblastoma).

Condition:  

Feline inductive odontogenic tumor

Contributor Comment:  

Feline inductive odontogenic tumor (also known as inductive fibroameloblastoma) is grouped as part of feline ameloblastic fibroma, which is characterized by epithelial and mesenchymal proliferation as a manifestation of the inductive properties of the odontogenic epithelium. Ameloblastic fibroma consists of poorly organized and more or less diffuse mesenchymal induction around the epithelial islands. A subtype of ameloblastic fibroma is the inductive fibroameloblastoma, characterized by well-formed cup-shaped epithelial structures that partially encircle stroma that resembles dental pulp.(10) The terminology inductive is used based on the resemblance of the epithelial islands to the cup stage of odontogenesis in which the dental lamina has an inductive effect on the stroma producing dental pulp.(6)

Feline inductive odontogenic tumors (FIOT) are rare dental tumors specific to cats.(6,7) The neoplasm is the most common dental tumor of young kittens of either sex.(10) Feline inductive odontogenic tumor is described exclusively in cats under 3 years of age1 and is most often found on the rostral maxilla,(7,10) occasionally causing tooth loss or partial distortion. It has been confused with ameloblastoma in cats.(5) Unlike FIOT, which occurs in young cats, up to three years of age with most being younger than 18 months,(6) ameloblastomas occur in adult cats, chiefly over the age of six years and affects the maxilla and the mandible.(6)

Feline inductive odontogenic tumor is a rare and interesting odontogenic neoplasm in which the odontogenic epithelium has inductive potential to form aggregated foci of dental pulp-like mesenchymal cells.(9) Its biological behavior is not well elucidated. The neoplasm is thought to have a benign behavior. Some studies indicate that Type IV collagen and laminin were constantly positive around the foci of epithelial cells, and Ki-67 positive indices were extremely low. These findings are consistent with the benign clinical presentation.(9) Although, it is a benign neoplastic mass histopathologically, feline inductive odontogenic tumor grows by expansion and can infiltrate the underlying bone and cause considerable local destruction.(1) Local recurrence has been reported in incompletely excised masses; however, metastasis does not appear to occur. This tumor differs microscopically from human ameloblastic fibromas in that it is not well-circumscribed but rather originates multifocally within the supporting connective tissue as characteristic, spherical condensations of fibroblastic connective tissue (ectomesenchyme) associated with islands of odontogenic epithelium.(6)

JPC Diagnosis:  

Gingiva: Feline inductive odontogenic tumor.

Conference Comment:  

Understanding normal tooth development is helpful in understanding the various odontogenic tumors and their classifications. Teeth develop from two embryonic tissues: the buccal cavity squamous epithelium (BCSE) and the embryonic mesenchyme (EM). BCSE invaginates into the EM to become the dental lamina, which develops into the enamel organ. The enamel organ is composed of the outer enamel epithelium, inner enamel epithelium, stellate reticulum and stratum intermedium. Ameloblasts originate from the inner enamel epithelium and form a cap enclosing a nodule of mesenchyme, known as the dental papilla. Ameloblasts induce the dental papilla mesenchyme to condense at the site of the future tooth and differentiate into odontoblasts. Odontoblasts produce dentin, and dentin production re-induces ameloblasts to synthesize enamel. Dentin formation always precedes enamel formation. Odontoblasts, which initially lay down pre-dentin and then move backward toward the pulp cavity as they produce dentin, remain active throughout life, so the process of making dentin continues after eruption and the pulp cavity shrinks as the animal ages. Ameloblasts, induced by odontoblasts, differentiate as a row of palisading columnar cells facing the odontoblasts. They lay down an uncalcified matrix, and then harden it, backing away as the enamel layer is built up. In brachydonts, cementum covers dentin where it is not covered by enamel. Cementum formation over the root occurs when there is degeneration of Hertwigs epithelial root sheath allowing mesenchymal cells to come in contact with dentin. Those epithelial cells differentiate into cemtoblasts which produce cementum. In brachydonts, such as dogs and cats, once the tooth erupts through the gumline, the cells of the stellate reticulum die and the stimulus is lost, hence the ameloblasts die and enamel cant be renewed. On the other hand, in hypsodont teeth, found in ruminants, rodents and horses, ameloblasts survive and enamel is continuously renewed. Overall, odontoblasts, dentin, cementum and pulp derive from EM (mesenchyme), while ameloblasts and enamel derive from epithelium (BCSE). The periodontal ligament, which anchors teeth into alveolar bone, is produced by ligament fibroblasts which stem from dental follicle cells.(2,3,11)

Odontogenic epithelium is characterized by 1) peripheral palisading of columnar epithelial cells, 2) apical hyperchromatic nuclei with basilar cytoplasmic clearing and 3) prominent intercellular bridging between internal epithelial cells (stellate reticulum).(2) Tumors of odontogenic epithelium are broadly divided into two categories: non-inductive tumors without odontogenic mesenchyme, including ameloblastoma, amyloid-producing odontogenic tumor and canine acanthomatous ameloblastoma, and inductive tumors with the presence odontogenic mesenchyme, including feline inductive odontogenic tumor, ameloblastic fibroma, complex odontoma and compound odontoma.(2,8,9,10)

Ameloblastoma, the least differentiated of the non-inductive odontogenic tumors, has been reported in dogs, cats, horses and humans. Ameloblastoma is classified as central (within the bone) or peripheral (within the gingival soft tissue) and is characterized by islands of poorly differentiated odontogenic epithelium, occasionally admixed with foci of metaplastic bone. Ameloblastic epithelial cells often undergo squamous differentiation and tumors are occasionally so heavily keratinized that it becomes difficult to differentiate ameloblastoma from squamous cell carcinoma. Central ameloblastomas are more common in animals.(2,8,10) The neoplastic islands of odontogenic epithelium in amyloid-producing odontogenic tumors (APOT) are separated by abundant waxy eosinophilic material (amyloid) which exhibits strong apple-green birefringence under polarized light when stained with Congo red. Recent studies have suggested the protein in APOT is not actually amyloid, but is derived from an ameoblastin-like peptide, or AAmel. Ameloblastin (formerly known as sheathlin) is an enamel matrix protein that is essential for enamel formation. Ameloblastin and another enamel protein, amelogenin, are both produced by ameloblasts during amelogenesis (see WSC 2012-13, conference 8, case 2).(2,4) Canine acanthomatous ameloblastoma is more locally aggressive than the other non-inductive odontogenic tumors and, like the ameloblastoma, is generally composed of interconnecting cords and sheets of odontogenic epithelium. It can be differentiated from ameloblastoma by an increased amount of stroma with abundant fibrillar collagen, regularly-positioned stellate mesenchymal cells, and regularly dispersed empty blood vessels, reminiscent of periodontal ligament connective tissue (see WSC 2012-13, conference 8, case 1). Cyst formation is common in acanthomatous ameloblastomas, however, in contrast to ameloblastomas, keratinization is rare.(2,8)

A comprehensive review of feline inductive odontogenic tumors is provided above by the contributor. Another inductive odontogenic tumor, ameloblastic fibroma, is composed of islands and cords of odontogenic epithelium on an abundant, collagen-poor fibrous stroma resembling dental pulp (see WSC 2011-12, conference 1, case 4). This is the most common odontogenic tumor in cattle, but has also been reported in young horses and dogs.(2,7) Complex/compound odontomas, the most differentiated odontogenic tumors, are composed of tooth-like structures known as denticles. Denticles contain enamel, dentin, cementum and pulp, arranged in a manner similar to a normal tooth; discrete islands of odontogenic epithelium are not present. These are reported in dogs, horses and cattle and can disrupt surrounding, normal teeth.(2)

References:

1. Beatty JA, Charles JA, Malik R, France MP, Hunt GB. Feline inductive odontogenic tumour in a Burmese cat. Aust Vet J. 200;78(7):452-455.

2. Brown CC, Baker DC, Barker IK. The alimentary system. In: Maxie MG, ed. Jubb, Kennedy and Palmers Pathology of Domestic Animals, 5th ed. Vol 2. Philadelphia, PA: Elsevier Saunders; 2007:5-7, 24-28.

3. Caceci T. Digestive System I: Oral Cavity & Associated Structures. VM8054: Veterinary Histology website. http://www.vetmed.vt.edu/education/Curriculum/VM8054/Labs/labtoc.htm. Accessed October 29, 2013.

4. Delaney MA, Singh K, Murphy CL, Solomon A, Nel S, Boy SC. Immunohistochemical and biochemical evidence of ameloblastic origin of feline amyloid-producing odontogenic tumors in cats. Vet Pathol. 2013;50(2):238-242.

5. Gardner DG. Ameloblastomas in cats: a critical evaluation of the literature and the addition of one example. J Oral Pathol Med. 1998;27(1):39-42.

6. Gardner DG, Dubielzig RR. Feline inductive odontogenic tumor (inductive fibroameloblastoma) a tumor unique to cats. Journal of Oral Pathology & Medicine. 1995;24:185190.

7. Head KW, Else RW, Dubielzig RR. Tumors of the alimentary tract. In Meuten DJ, ed. Tumors in Domestic Animals. 4th ed. Ames, IA: Iowa State Press; 2002:401-481.

8. Head KW, et al. Tumors of odontogenic epithelium without odontogenic mesenchyme. In: Tumors of the Alimentary System of Domestic Animals. Washington DC: AFIP and CL Davis DVM Foundation and WHO Collaborating Center for Worldwide Reference on Comparative Oncology; 2003:49-51. 

9. Hiroki Sakai, Takashi Mori, Tsuneyoshi Iida, Yanai Tokuma, Kouji Maruo and Toshiaki Masegi. Immunohistochemical features of proliferative marker and basement membrane components of two feline inductive odontogenic tumours. Journal of Feline Medicine & Surgery. 2008;10(3):296-299. 

10. Poulet FM, Valentine BA, Summers BA. A Survey of epithelial odontogenic tumors and cysts in dogs and cats. Vet Pathol. 1992;29:369-380.

11. Tutt C. Tooth development (odontogenesis). In: Small Animal Dentistry: A Manual of Techniques. 2006:1-32.

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3-1. Gingiva

3-2. Gingiva

3-3. Gingiva

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