A macrodont is by definition a tooth that is larger than normal [12]. This statement is also found in specialized literature, both veterinary and dental [7, 8, 10, 12,13,14]. In the current study, the presence of characteristic structural changes was found within the macrodontic cheek teeth of the guinea pig. Similarly, one author observed structural deformities and splitting of the teeth with enlarged outlines [7]. The same author used the terms “giant tooth” and “crippled teeth” to describe this pathology. It is difficult to compare the observed structural alterations to possible changes found in human macrodontic teeth, as there are few papers in the available literature describing the histopathology of these teeth. However, according to Komatsu et al., the development of macrodontic teeth in humans occurs in the bell stage. The constantly growing cheek tooth of the guinea pig remains in the bell stage throughout the animal’s life. In the work of Komatsu et al., histological examination of the human macrodontic tooth showed enamel hypoplasia, dentin abnormalities, hypertrophy of the cementum and immature enamel. Similar changes were observed in the examined cheek teeth of the guinea pig in areas where there was thinning or complete loss of the enamel on the dentin surface. In such situations, the cementum filled the areas where dentin had been driven into the pulp cavity. Köstlinger et al. [8, 9] found that 90% of macrodontic cheek teeth are M2 and M3. We obtained similar results during histological evaluation, as 87% of lesions in our study were located within M2 and M3 teeth.
Due to the presence of structural alterations in the examined teeth, the possibility of odontogenic tumours was suspected. Structural changes in macrodontic teeth of guinea pigs are composed of physiologically occurring hard and soft tissue of the tooth with disordered arrangement, whereas the cells do not show any signs of atypia. The possibility of odontogenic cysts was rejected first. Changes of epithelial origin not containing ectomesenchymal components were also eliminated (ameloblastoma). Alterations containing both epithelial and ectomesenchymal components in the structure (ameloblastic fibroma, ameloblastic odontoma, dentinoma, odontoma) were carefully analysed [15]. Among many odontogenic tumours, the posted description is most compatible with an odontoma. Odontomas are benign tumourlike malformations of the hard tissue of the tooth and arise from disorders of the division of the dental lamina. They are built of epithelial and ectomesenchymal components; hence, in the structure, we can see tissues such as enamel, dentin, cementum, and dental pulp, and their arrangement can be normal or disrupted, but the cells do not show changes. Due to their structure, odontomas are more likely to be classified as hamartomas (nonneoplastic tumours). There are 2 types of odontomas: compound odontoma (odontoma compositum), in which altered tissues form structures that anatomically resemble teeth (odontoids), and small remnants of epithelial tissue are found within the changes. The second type is called complex odontoma (odontoma completum), which is a chaotic mass of hard and soft tissue of the tooth that shows no morphological resemblance to a properly formed tooth [13, 16, 17]. Variations of this type have thus far been described for many animal species, including dogs, cats, voles, [18] squirrels [19], prairie dogs [20,21,22], mice [23, 24], rats [25], degus [26], and guinea pigs [27]. However, due to the different structures of brachydontic and elodontic rodent teeth, a different name has been proposed in the literature for the change histologically resembling the odontoma of anelodontic teeth. Boy and Steenkamp proposed the term “elodontoma” for a change diagnosed in a squirrel. In contrast, Pelizzone et al. suggested the term “pseudo-odontoma” to describe a change within the incisor of a prairie dog.
From the work of Capello et al., we only know that the changes diagnosed in 2 guinea pigs showed features of elodontoma and were haphazardly composed of hard and soft tissues of the tooth. We found similar structural changes in histologically examined guinea pig teeth; however, in our case, the alterations were localized within the pulp cavity or at the periphery of the tooth crown, and in the 2 cases described by Capello et al., the changes were localized outside the apex and showed local expansion.
Boy and Steenkamp also described complex odontoma type changes found in squirrels. In such a case, similar to guinea pig [27], the presence of changes composed of hard and soft tissues of the tooth with disordered arrangement located outside the apex was described. Changes in the borders of the tooth crown without the formation of a periapical odontogenic mass were also described. In such cases, deformation of the apex and corrugated enamel were found. Similar changes were noted during histologic evaluation of guinea pig macrodontic teeth in the work conducted (Fig. 4). Moreover, Boy and Steenkamp observed that most of the changes similar to the teeth we examined spread from the apex to the occlusal plane.
Histological description of complex odontoma found in prairie dogs largely resembles changes found in guinea pig macrodontic teeth. In both cases, islands of epithelial and mesenchymal tissue forming tooth-like structures were found. Such changes in guinea pig teeth are shown in Fig. 9. The alterations were built of predentin, dentin and cementum, and the arrangement of these tissues was disordered, similar to the macrodontia of guinea pigs [20].
Dayan et al. described a case of complex odontoma in mice. Histological examination revealed the alteration built, as in previous cases, of enamel, dentin, cementum and connective tissue. Hard tissues formed island-like structures and were mixed with surrounding connective tissue. This image resembles structural changes in guinea pig macrodontic teeth (Fig. 7a). However, for the second time [23, 26], the presence of a fibrous capsule surrounding the change from the outside and beyond the apex was observed. This type of change was not found in the evaluation of guinea pig macrodontic teeth.
An interesting description of the complex odontoma was made by the authors of works conducted on op/op mice [24], op/op rats [25], and ia rats [28] in which the development of this pathology was described as a result of osteopetrosis. Osteopetrosis is an inherited metabolic bone disease in which the process of bone resorption is disrupted. Accumulating bone trabeculae invade the enamel organ through which separate fragments of daughter tooth germs capable of further production of the tooth tissue arise. During the evaluation of macrodontic teeth of guinea pigs, no expansion of bone tissue towards the apex was observed, but detached fragments of the apical bud or cracks in the dentin layer and inflow of pulp cells into the periodontal space or from the periodontal space towards the tooth pulp were observed. The structural alterations described in mice and rats also show considerable similarity to those observed in our study. In the studies by Ida-Yonemochi et al., Philippart et al. and Schour et al. and in our study, disorganized cells of epithelial and mesenchymal origin formed various structures and rings. Trapped pulp cells were found between wavy bands and in the centre of the rings. Moreover, osteodentin was found in the formation of structural changes in rats, as well as in macrodontic teeth of guinea pigs. Osteodentin is mineralized tissue physiologically found in the incisors of rats [29,30,31] and the incisors and cheek teeth of guinea pigs [32]. It is a tissue of mesenchymal origin resembling bone with lacunes in which single cells are trapped. The osteodentin is surrounded by tubular dentin all around. It was observed in the top of the pulp cavity. It is suspected that its function is to seal the pulp cavity on the occlusal surface to protect the pulp cavity from the influx of pathogens from the oral cavity [30, 32]. The presence of this tissue in healthy rat teeth explains its occurrence in the structure of complex odontomas since it is a change built of physiologically existing tissues with a disordered arrangement. This observation does not yet conclusively establish that the structural changes in macrodontic teeth of the guinea pig qualify as odontomas but suggests that osteodentin should be present in the structure of the odontoma in the guinea pig. The papers cited in this paragraph found no connective tissue capsule surrounding the odontoma from the outside.
The presence of a ring-shaped deformity with centrally trapped pulp cells and peripherally located odontoblasts also resembles the description of complex odontoma in voles [18]. In this rodent, the occurrence of osteodentin in the odontoma structure was also found, and the presence of a fibrous capsule was not described. However, the changes in the maxillary incisors showed expansion towards the surrounding bone. In the same work, a description of complex odontoma in a dog and cat was also compiled. Similar to structural alterations in macrodontic cheek teeth of a guinea pig, the presence of tubular and atubular dentin of various shapes, enamel dysplasia and trapped pulp cells were shown.
