Pathology in Practice

Caroline Andrews Comparative Biomedical Scientist Training Program, National Cancer Institute, National Institutes of Health, Bethesda, MD 20892.
Diagnostic Center for Population and Animal Health, Michigan State University, Lansing, MI 48910.

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Barbie J. Gadsden Diagnostic Center for Population and Animal Health, Michigan State University, Lansing, MI 48910.

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Elizabeth A. Carr Department of Large Animal Clinical Sciences, College of Veterinary Medicine, Michigan State University, East Lansing, MI 48824.

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Matti Kiupel Diagnostic Center for Population and Animal Health, Michigan State University, Lansing, MI 48910.

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History

A 2-year-old crossbred Quarter Horse filly was evaluated at the Michigan State University Veterinary Teaching Hospital because of dyspnea, epiphora of the right eye, and unilateral swelling over the right maxillary sinus.

Clinical and Gross Findings

Radiographic views of the head revealed a well-circumscribed mass (approx 10 cm in diameter) that obstructed the right maxillary sinus and displaced the second right maxillary molar tooth (tooth 110 as defined on the basis of the Triadan system of dental nomenclature). Multiple clearly demarcated radiopaque areas consistent with mineralized tissue were evident within the area of the mass. The horse was euthanized and submitted for necropsy. On gross examination, there was a firm swelling rostroventral to the right eye. The head was cut transversely at the level of the maxillary sinus, revealing a well-demarcated, unencapsulated, expansile mass (approx 10 cm in diameter) that expanded the right maxillary sinus and displaced the second maxillary molar tooth. The mass was composed of numerous (> 100) mineralized structures of various sizes and shapes embedded in a clear, gelatinous material (Figure 1).

Figure 1—
Figure 1—

Photograph of a transverse section through a maxillary mass in the head of a 2-year-old crossbred Quarter Horse filly. The well-demarcated, unencapsulated, expansile mass has expanded the right maxillary sinus and is composed of numerous tooth-like structures embedded in a clear, gelatinous material.

Citation: Journal of the American Veterinary Medical Association 244, 4; 10.2460/javma.244.4.417

Formulate differential diagnoses from the history, clinical findings, and Figure 1—then turn the page →

Histopathologic Findings

Histologic examination of sections of the mass revealed multiple, well-differentiated tooth-like structures of various shapes (Figure 2). These structures had features of normally developing teeth (denticles) and were embedded in a fibrovascular stroma of variable density. Denticles had a central core of mesenchymal tissue (dental pulp) bordered by a single layer of pseudostratified columnar cells (odontoblasts). Adjacent to the odontoblastic layer was a thin, pale eosinophilic predentin layer surrounded by homogeneous, bright eosinophilic material containing numerous 7- to 10-μm-wide, evenly distributed, parallel tubules (mature dentin). The outer enamel layer was acellular and composed of a deeply eosinophilic, fine fibrillar material oriented in a wavy pattern. A linear row of palisading tall columnar cells with apical nuclei and variably distinct, basilar foci of clear cytoplasm (ameloblasts) lined the outer enamel surface (Figure 3).

Figure 2—
Figure 2—

Photomicrograph of a section of the maxillary mass in the head of the horse in Figure 1. The mass is composed of well-differentiated tooth-like structures (denticles [brackets]) of various shapes embedded in a fibrovascular stroma of variable density. H&E stain; bar = 1,000 m.

Citation: Journal of the American Veterinary Medical Association 244, 4; 10.2460/javma.244.4.417

Figure 3—
Figure 3—

Photomicrograph of a section of a denticle in the maxillary mass in the head of the horse in Figure 1. In the main image, notice the layer of odontoblasts (arrow) that separates dental pulp (P) from pale eosinophilic predentin, which transitions into brightly eosinophilic mature dentin (D). Enamel (E) with a layer of ameloblasts (arrowhead) is present. H&E stain; bar = 500 m. Inset A—Portion of the main image at higher magnification. The odontoblasts are visible as a single layer of pseudostratified columnar cells between the dental pulp (right of image) and predentin (pale eosinophilic layer) and mature dentin (brightly eosinophilic region; left of image). H&E stain; bar = 50 μm. Inset B—Another portion of the main image at higher magnification. Acellular, deeply eosinophilic enamel oriented in a wavy pattern (top of image) is bordered by a linear row of palisading tall columnar ameloblasts with apical nuclei and variably distinct, basilar foci of clear cytoplasm. Stratum intermedium cells are visible along the apical surface of the ameloblast layer (bottom of image). H&E stain; bar = 50 μm.

Citation: Journal of the American Veterinary Medical Association 244, 4; 10.2460/javma.244.4.417

Morphologic Diagnosis and Case Summary

Morphologic diagnosis and case summary: compound odontoma in a horse.

Comments

For the horse of the present report, differential diagnoses based on the clinical signs included neoplasia, abscess, fracture, or a cyst. However, the denticles observed within the mass at necropsy were pathognomonic for a compound odontoma.

Neoplasms of odontogenic epithelium are subdivided on the basis of the presence or absence of odontogenic mesenchyme. Odontomas, ameloblastic fibromas, ameloblastic fibro-odontomas, and feline inductive odontogenic tumors contain odontogenic mesenchyme, whereas ameloblastomas, canine acanthomatous ameloblastomas, and amyloid-producing odontogenic tumors do not.1–3 Odontomas are composed of completely differentiated dental tissues and are considered by some authors to be more accurately described as hamartomas rather than neoplasms.1 Odontomas are further classified as either complex or compound. Compound odontomas contain denticles, which result from complete odontogenesis but lack normal tooth shape.1–3 Tissues of complex odontomas are less organized and do not form tooth-like structures.1

Odontogenic tumors are rare in domestic animals.2 Odontomas in young animals1,4,5 are most commonly reported and are most likely to develop in horses and cattle.3 Odontomas do not metastasize but are expansile masses that may disrupt adjacent tissues.3 Resection is curative but is complicated by the size and location of the mass as well as the requirement for intensive postoperative care.4

The clinical signs in horses with odontomas are nonspecific and are mostly related to the expansile, space-occupying nature of the mass.4–7 A hard, nonpainful swelling over the maxilla or mandible that does not affect mastication is the most consistent finding,4–7 and may be the only clinical abnormality.4,5 Unilateral nasal discharge,4,7 unilateral ocular discharge,7 a unilateral decrease in nasal airflow,4,7 intermittent respiratory noise,7 and difficulty with the bit during training6 have also been observed in horses with odontomas. The unilateral epiphora described by Rubio-Martinez et al7 and observed in the horse of the present report was likely attributable to obstruction of the nasolacrimal duct by the mass. The clinical signs associated with the odontoma in this horse were similar to those previously reported. Although rare, odontoma should be considered as a differential diagnosis for maxillary or mandibular swellings in horses.

References

  • 1. Head KW, Cullen JM, Dubielzig RR, et al. Histological classification of tumors of odontogenic origin of domestic animals. In: Schulman FY, ed. Histological classification of tumors of the alimentary system of domestic animals. Washington, DC: Armed Forces Institute of Pathology, 2003;4657.

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  • 2. Head KW, Else RW, Dubielzig RR. Tumors of the alimentary tract. In: Meuten DJ, ed. Tumors in domestic animals. 4th ed. Ames, Iowa: Iowa State Press, 2002; 402407.

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  • 3. Brown CC, Baker DC, Baker IK. Alimentary system. In: Maxie MG, ed. Jubb, Kennedy, and Palmer's pathology of domestic animals. 5th ed. Philadelphia: Saunders Elsevier, 2007; 2427.

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  • 4. Brounts SH, Hawkins JF, Lescun TB, et al. Surgical management of compound odontoma in two horses. J Am Vet Med Assoc 2004; 225: 14231427.

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  • 5. Snyder C, Dubielzig RR, Gengler W, et al. Surgical treatment of a rostral mandibular complex odontoma in a 3-year-old horse. Equine Vet Educ 2008; 20: 647651.

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  • 6. Dubielzig RR, Beck KA, Levine S, et al. Complex odontoma in a stallion. Vet Pathol 1986; 23: 633635.

  • 7. Rubio-Martínez LM, Nykamp S, Trout D. What is your diagnosis? J Am Vet Med Assoc 2011; 238: 695696.

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