- Email: [email protected]
Hypsodontia in Castor canadensis: an anatomical and histological study
Morphologie, 2006, 90, 33-38 © Masson, Paris, 2006
ORIGINAL ARTICLE
HYPSODONTIA IN CASTOR CANADENSIS: AN ANATOMICAL AND HISTOLOGICAL STUDY castor canadensis
P. ROUAS (1, 2), T. SUAUDEAU (3), Y. DELBOS (3), A. ROUAS (3), J. NANCY (3) (1) LAPP, PACEA, UMR 5199 CNRS, Université de Bordeaux 1, avenue des facultés, 33405 Talence Cedex. (2) Laboratoire de recherche de l’UFR d’Odontologie, Université de Bordeaux 2, 16-20 cours de la Marne 33082 Bordeaux Cedex. (3) Laboratoire de recherche de l’UFR d’Odontologie, Université de Bordeaux 2, 16-20 cours de la Marne 33082 Bordeaux Cedex.
SUMMARY
RÉSUMÉ
Objectives: The aim of this original study is to describe the dental morphology and anatomy of Castor canadensis and to compare results with existing data in the literature relating to this mammal. This will give us a better knowledge of its very distinctive dental system, representative of rodent typology. Material and methods: The authors used an osteological sample that is rarely found in Europe. A series of horizontal histological sections of the labial and jugal dental organs were prepared at different coronal and radicular levels for study under the optic microscope. Coloration was applied for tissue differentiation. Results: Observation of the histological sections reveals an enlargement of the space reserved for the pulp tissue which increases from the crown towards the widely open apex of the incisors. These characteristics are not found in the jugal teeth which, although they also have an open apex, present a different configuration. Conclusion: The dental organs of Rodents in general and Castorides in particular are subject to a constant eruptive force throughout their lifetime. This permanent dental growth occurs as the dental system undergoes intense abrasion, especially the incisors.
L’hypsodontie chez Castor canadensis : étude anatomique et histologique
Key words: tooth. Castor canadensis. continued growth. hypsodont.
Mots-clés : dent. Castor Canadensis. croissance continue. hypsodonte.
INTRODUCTION Castor canadensis is a Vertebrate belonging to the superclass Gnathosomes, class Mammals, superorder Glires, order Rodents, the largest order of Mammals [2], family Castorides, genus Castor, species canadensis. The Glires superorder includes Lagomorphs and Rodents. Lagomorphs or duplicidentes have two pairs of incisors in the upper or lower jaw, whereas the true or simplicidente Rodents, to which Castor canadensis, the subject of our study belongs, have only one pair Correspondence: P. ROUAS, address (2). E-mail: [email protected]
Objectifs : le but de cette étude est de décrire la morphologie et l’anatomie dentaires de Castor canadensis et de mettre en relation les résultats observés avec les données de la littérature relatives à ce Mammifère. Ceci permet une meilleure connaissance de son système dentaire particulier, qui correspond bien à la typologie des rongeurs. Matériel et méthodes : les auteurs utilisent une pièce ostéologique rare en Europe. Différentes coupes histologiques horizontales des organes dentaires labiaux et jugaux sont réalisées à différents niveaux coronaires et radiculaires. Elles sont étudiées en microscopie optique. Des colorations sont utilisées à des fins de différenciations tissulaires. Résultats : l’observation des coupes histologiques met en évidence un élargissement croissant de l’espace dévolu au tissu pulpaire de la couronne vers l’apex, largement ouvert, des incisives. Ces caractéristiques ne sont pas retrouvées sur les dents jugales qui présentent, même si elles ont également un apex ouvert, une configuration différente. Conclusion : les organes dentaires des Rongeurs en général et des Castoridés en particulier sont soumis à une force éruptive permanente tout au long de leur vie. Cette croissance dentaire permanente se fait au fur et à mesure de l’intense abrasion du système dentaire, en particulier au niveau incisif.
per jaw. In the family of Castorides, only the genus Castor has survived, which includes two species with a different geographical distribution: Castor fiber, which can be found in Europe and Asia, and Castor canadensis, found in North America. These two species present very similar phenotypes [8]. Teeth with continuous or prolonged growth can be found in species where horizontal jaw movements tend to predominate [1]. The crowns rise up more or less permanently, which accounts for the use of the terms hypsodontia or hypselodontia, from the Greek upselos, meaning high. Very few studies have been carried out on this subject. Our research will therefore provide a better knowledge of this physiological process in Castor canadensis.
34
P. ROUAS et al.
MATERIAL AND METHODS We used a cephalic skeleton of Castor canadensis from the region of the lower Saint Lawrence 1 (Canada) and which still contained all its teeth. The dental organs were removed in order to produce histological sections for examination under the optic microscope (enlargement ×40). Given the animal’s general morphology when it was caught, its age was estimated at between 8 to 10 years. It was ten months before this item could be imported, as procedures for the import of this type of material are strictly regulated by international conventions. Images of the osseous head and teeth were recorded using an argentic 24×36 camera with a Nikkor 120 mm medical lens. Images of the histological dental sections were taken using an Olympus BH2 camera fixed to a photonic microscope. EPP100 slide film was used in each case. For the medical imaging, we took frontal and profile teleradiographs at a distance of 1m50, giving an enlargement of about 11%, using a Gendex camera (ddp=66 kV). The different histological sections were taken in accordance with a standard protocol which is described below.
Embedding The extracted teeth were immersed in an alcohol bath, then placed at the bottom of circular moulds into which liquid resin was poured. Complete polymerisation of the resin was obtained after 24 hours. Obtaining the sections The resin blocks containing the dental organs were sliced into 1.17 mm sections using a disc microtome. Colouration We applied colourants to some of these sections for a better differentiation of the various tissues observed. Once they were rehydrated, the sections were immersed into a series of different baths in a specific order and for specific durations: fushine (5 minutes), phosphomolybdic acid (10 minutes), aniline blue (1 minute), interspersed with acetified water baths (15 seconds) to fix and rinse.
type and implantation is thecodont, with the teeth set in alveoli in the bone [1, 2, 5]. We suggest that the premolars of Castor canadensis correspond to the fourth premolar P4 of the basic Mammalian dental formula. We believe this can be explained by the isomorphism of this tooth with the molars, and by its position next to the first molar. Another point to notice is the absence of canines, leaving a space between the incisors and the jugal teeth. In this space, the “bar” or diastema, the animal is able to hold branches firmly as if in the jaws of a vice.
Dental morphology Like the majority of upper Vertebrates, Castor canadensis has a dental system known as plexodont which includes multituberculate teeth. This plexodontia is integrated into a heterodont dental system characteristic of the vast majority of Mammals. The morphology of the crowns varies from one end of the jaw to the other. The permanent denture of this species consists of labial teeth and jugal teeth. Tooth differentiation increases from front to back, with the molars thus having the most complex morphology. The labial teeth correspond to the incisors, and they are curved, very long and prominent, protruding outside the oral cavity, with a characteristic orangecoloured pigmentation on the vestibular face [1, 2, 10] (figure 1). Their length and the fact that they are deeply implanted into their supporting bone tissue are an indication of the enormous strength of these incisors (figure 2). The enamel tissue exists only on the vestibular face, which accounts for a lesser amount of wear on the anterior side and the fact that the incisors are bevelled [2, 5]. The jugal teeth consist of premolars and molars. They are separated from the labial teeth by the diastema. They decrease in size from front to back. They are
RESULTS AND DISCUSSION Dental formula Castor canadensis is diphyodont. This means it develops two successive sets of teeth: one which is deciduous, the other which is permanent. Like all rodents, it is a simplicidente with a single pair of incisors in each dental arch. The deciduous dentition consists of only one incisor and one molar per hemi-arch. The permanent dental formula is as follows: I 1/1 P 1/1 M 3/3, which corresponds to data from Anthony [1] and Jordan et al. [2]. Dental replacement is of the vertical
1
Other possibility of script: Saint-Laurent.
FIG. 1. — Anterior view of a Castor canadensis mandible show-
ing the orange coloring of the vestibular face of the labial teeth. FIG. 1. — Vue antérieure d’une mandibule de Castor canadensis objectivant la coloration orangée des faces vestibulaires des dents labiales.
Hypsodontia in castor canadensis: an anatomical and histological study
35
ble. The jugal teeth are very tightly packed one against the other, forming a continuous series where the wear surfaces present these characteristic enamel ridges [6, 9].
Dental histological study
FIG. 2. — Profile teleradiograph showing the length and the deep alveolar embedding of the incisors. The labial teeth are bevelled and come together in characteristic mode during occlusion. FIG. 2. — Téléradiographie de profil montrant la longueur et le
profond enchâssement alvéolaire des incisives. Les dents labiales sont taillées en biseau et présentent un affrontement caractéristique lors de l’occlusion.
shorter and prismatic in shape. They are of the lophodont type [1, 2, 5, 10] which means that the four cusps of the occlusal face combine in pairs to form transverse ridges (figure 3). There are three or four of these ridges, which slope slightly from front to back, from the inside towards the outside on the maxillary, and from the outside towards the inside on the mandi-
FIG. 3. — Left maxillary jugal teeth showing a series of transverse ridges characteristic of the lophodont. D: distal; V: vestibular. FIG. 3. — Dents jugales maxillaires gauches présentant des séries
de crêtes transversales caractérisant le type lophodonte. D : distal ; V : vestibulaire.
After embedding the teeth, histological dental sections were carried out, then studied under an optic microscope. The colouring used, according to the previously defined protocol, helps to determine structures and tissues. For the incisors, we carried out several horizontal sections perpendicular to the corono-apical axis at different levels: at the level of the occlusal bevel, in the median part and at the apex. At occlusal level (figure 4), we found a vestibular enamel, blue in colour, surmounted by a brown layer corresponding to the deposit of ferric oxide on the enamel. At median level (figure 5), the blue coloured enamel was localised on the vestibular face. The dark pink coloured dental surface corresponds to the dentine surrounding the pulp tissue, which appears black. As the enamel is the tissue which is the most mineralised and the hardest, the enamel arc is still the portion of dental tissue that wears least, giving the tooth a bevelled occlusal surface. At the apex (figure 6), blue-coloured enamel tissue can still be seen around the vestibular periphery. Dentine occupies a much smaller volume than in the earlier sections, leaving a much larger central pulp cavity. The increasing volume of the pulp from the occlusal edge to the apex tends to be proof of the hypsodont nature of this labial tooth. The apex there-
FIG. 4. — Transverse section of the free edge of the left maxillary incisor. E: enamel; D: dentin. FIG. 4. — Coupe transversale au niveau du bord libre de l’incisive maxillaire gauche. E : émail ; D : dentine.
36
P. ROUAS et al.
FIG. 5. — Coronal transverse section of the left maxillary incisor. Enamel tissue is only on the vestibular face. E: enamel; D: dentin; P: pulp.
FIG. 6. — Radicular transverse section of the left maxillary incisor. The thickness of the dentinal layer decreases as the volume of the internal pulp increases. E: enamel; D: dentin; P: pulp.
FIG. 5. — Coupe transversale au niveau coronaire de l’incisive maxillaire gauche. La localisation du tissu énamélaire se limite à la face vestibulaire. E : émail ; D : dentine ; P : pulpe.
FIG. 6. — Coupe transversale au niveau radiculaire de l’incisive maxillaire gauche. L’épaisseur de la couche dentinaire se réduit au profit du volume pulpaire interne. E : émail. D : dentine. P : pulpe.
fore provides access to a large number of blood vessels which will generate the formation of secondary dentine. The incisors of Castor canadensis are thus defined as ever-growing hypsodont teeth. All the stages in odontogenesis and eruption are thus combined in a single tooth, which makes this a particularly interesting experimental model. We selected a section from the jugal teeth, from the wear surface (figure 7). On the series of sections we observed the presence of transverse ridges of bluecoloured enamel, separated by dentine which enclosed small areas of pulp tissue (dark brown colour). This is a tubulate arrangement where the enamel invaginates like the fingers of a glove. The reddish brown coloured cement filtered in between the folds of the enamel. At root level, dentine fills almost the entire tooth. A fine peripheral reddish brown edging indicates the presence of cement which is therefore clearly present not only at root level but also at the crown [2]. Small red and black surfaces in the centre represent the areas given over to pulp tissue. Unlike the labial teeth, in the jugal teeth there appears to be less space for the nourishing pulp tissue in the radicular portion. A recent study on rats [3] confirms these results, describing a significant reduction in the volume of pulp in the apical third of the permanent molars of this rodent. The authors also suggest a link with age. We are therefore led to believe that the growth of the jugal teeth is not as intense as that of the labial teeth and that it would be more appropriate to talk of teeth with prolonged growth. However, many factors must be taken into consideration which could affect our argumentation, such as the age of the individual or its activity.
Masticatory physiology In Castor canadensis, the teeth come together in opposition in a specific way [1] (figure 8). In this type of bite, the upper tooth articulates end to end with the
FIG. 7. — Transverse section of the occlusal part of the first right molar of the mandible. E: enamel; D: dentin; C: cement; M: mesial; L: lingual. FIG. 7. — Coupe transversale au niveau de la partie occlusale de la première molaire mandibulaire droite. E : émail ; D : dentin ; C : cément ; M : mésial ; L : lingual.
Hypsodontia in castor canadensis: an anatomical and histological study
FIG. 8. — Right antero-lateral view showing the occlusion of the
jugal teeth. AR: back. BS: bottom. FIG. 8. — Vue antéro-latérale droite présentant le mode d’opposition des dents jugales. AR : arrière. BS : bas.
corresponding lower tooth. As in most rodents, there is a particular modality at incisor level: we find a bevelled articulation in accordance with the very oblique occlusal surfaces. According to Jordan et al. [2], determining the specific features of dental wear tells us a great deal about both the animal’s food regime and way of life. Grinding of the mandibular incisors is behaviour that is specific to rodents. The lingual face of the mandibular incisor of Castor canadensis is brought against the free edge of the maxillary incisor, resulting in wear on this lingual face (figure 2). The grinding cycles of this animal are among the most rapid, with a frequency of about 11 grinding cycles per second. This attrition forms and maintains the specific coronal morphology of these incisors [11]. The term “wear”, describing a loss of dental substance from the incisors, thus seems to be too simplistic to apply to Castorides because in fact this non-pathological phenomenon fulfils a true physiological function which is more than a simple compensatory phenomenon. The nature and extent of mandibular movements are closely dependent on the masticatory type of the species, which in turn depends on the food regime. In non-human Mammals, four types of masticatory movements predominate: vertical (in carnivores), lateral (in primates), transversal (in ungulates), anteroposterior (in rodents). The Canadian beaver presents mandibular movements of predominently the anteroposterior or propalinal type. To these are added vertical movements which help the bevelled articulated labial teeth to bite, and short transversal movements in a brief rotation around the longitudinal axis (helicoidal movements) to assist incision and trituration. These different movements are adapted to the food regime of Castor canadensis which is strictly vegetarian, consisting predominantly of tubers and bark. Three sorts of action are applied: incision, trituration, and the passage from molar occlusion to incisive occlusion. The occlusion of the jugal and labial teeth is never synchronous and, for a molar occlusion for example, there is a corresponding incisive inocclusion. Before incision takes place there is a preparatory
37
phase of “empty” movements, specific to all rodents, and this is in three stages. First, disengagement, which consists of displacing the mandible following a relatively wide rotation. Next, translation towards the front brings the free edge of the mandibular incisor to the level of the palatine groove. After this, the opening determined by a combination of rotation and translation movements brings the labial teeth into occlusion [4, 6]. The crowns of the teeth are worn down considerably and this abrasion is compensated for by the prolongation of dental growth [1]. Because of this, the consequences of the accidental loss of an incisor can threaten an animal’s very survival [11]. When the continuous growth of the opposite incisor is no longer compensated by occlusal wear, this incisor will continue its progression without encountering any obstacle until it perforates the tissues in the mouth, which can bring about the animal’s death [12]. In veterinary practice, the rehabilitation of this incisive occlusion can be life-saving [13]. In the same way, the consequences of a malocclusion of the incisor will be more serious because of the physiological phenomenon of continuous dental growth [7].
CONCLUSION Castor canadensis thus has a specific denture consisting of labial and jugal teeth separated by a diastema. Incisors, premolars and molars undergo an intense non-pathological abrasion, compensated for throughout their lifetime by the force of the erupting teeth. However, these pressures are produced at different rythms, with the incisors seeming to be the teeth that are used the most. The dental organs of Castor canadensis are clearly much more than a simple example of adaptation to a food regime, they are extremely efficient tools without which beavers would certainly be unable to have the same lifestyle.
Acknowledgments We would like to thank Hilary Koziol for her help in the translation of this article.
REFERENCES [1] Anthony J. Cours d’Anatomie Comparée. Paris : Julien Prélat, 1973. [2] Jordan RE, Abrams L, Kraus BS. Kraus’ Dental Anatomy and Occlusion. 2nd ed. Mosby Year Book, 1992. [3] Gomez PA, Cabrini RL. Anatomic variations of the root canal of the rat according to age. Acta Odontol Latinoam 2004; 17: 39-42. [4] Grassé PP. Traité de zoologie. Anatomie, Systématique, Biologie. Mammifères. Paris: Masson et Cie, tome XVII, fascicule 2, 1955. [5] Kardong VK. Vertebrates. Comparative Anatomy, Function, Evolution. 2nd ed. WCB McGraw-Hill, 1998. [6] Kershaw DR. Animal diversity. 5 th ed. London: Chapman & Hall, 1994. [7] Kim JH, Lee JY, Choi SH. Odontoplasty for the treatment of malocclusion of the incisor teeth in a beaver (Castor canadensis). Vet Rec 2005; 156: 114-115.
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[8] Kuehn R, Schwab G, Schroeder W, Rottmann O. Differenciation of Castor fiber and Castor canadensis by noninvasive molecular methods. Zoo Biol 2000; 19: 511-515. [9] Pough FH, McFarland WN. Vertebrate life. 4 th Ed. Upper Saddle River, New Jersey: Prentice Hall International Inc, 1996. [10] Rouas P, Suaudeau T, Delbos Y, Rouas A. Approche morphologique et histologique du système dentaire de Castor canadensis (abstract). Morphologie 2001 ; 85 : 58.
[11] Sicher H. The biology of attrition. Oral Surg 1953; 6: 406412. [12] Zuri I, Terkel J. Reversed palatal perforation by upper incisors in ageing blind mole-rats (Spalax ehrenbergi). J Anat 2001; 199: 591-598. [13] Walmsley AD, Lumley PJ, Laird WRE, Kesterton C, Daft J. Restoration of the anterior dentition of Canadian Beaver (Castor canadensis). J Small Anim Pract 1989; 30: 583-586.
ORIGINAL ARTICLE
HYPSODONTIA IN CASTOR CANADENSIS: AN ANATOMICAL AND HISTOLOGICAL STUDY castor canadensis
P. ROUAS (1, 2), T. SUAUDEAU (3), Y. DELBOS (3), A. ROUAS (3), J. NANCY (3) (1) LAPP, PACEA, UMR 5199 CNRS, Université de Bordeaux 1, avenue des facultés, 33405 Talence Cedex. (2) Laboratoire de recherche de l’UFR d’Odontologie, Université de Bordeaux 2, 16-20 cours de la Marne 33082 Bordeaux Cedex. (3) Laboratoire de recherche de l’UFR d’Odontologie, Université de Bordeaux 2, 16-20 cours de la Marne 33082 Bordeaux Cedex.
SUMMARY
RÉSUMÉ
Objectives: The aim of this original study is to describe the dental morphology and anatomy of Castor canadensis and to compare results with existing data in the literature relating to this mammal. This will give us a better knowledge of its very distinctive dental system, representative of rodent typology. Material and methods: The authors used an osteological sample that is rarely found in Europe. A series of horizontal histological sections of the labial and jugal dental organs were prepared at different coronal and radicular levels for study under the optic microscope. Coloration was applied for tissue differentiation. Results: Observation of the histological sections reveals an enlargement of the space reserved for the pulp tissue which increases from the crown towards the widely open apex of the incisors. These characteristics are not found in the jugal teeth which, although they also have an open apex, present a different configuration. Conclusion: The dental organs of Rodents in general and Castorides in particular are subject to a constant eruptive force throughout their lifetime. This permanent dental growth occurs as the dental system undergoes intense abrasion, especially the incisors.
L’hypsodontie chez Castor canadensis : étude anatomique et histologique
Key words: tooth. Castor canadensis. continued growth. hypsodont.
Mots-clés : dent. Castor Canadensis. croissance continue. hypsodonte.
INTRODUCTION Castor canadensis is a Vertebrate belonging to the superclass Gnathosomes, class Mammals, superorder Glires, order Rodents, the largest order of Mammals [2], family Castorides, genus Castor, species canadensis. The Glires superorder includes Lagomorphs and Rodents. Lagomorphs or duplicidentes have two pairs of incisors in the upper or lower jaw, whereas the true or simplicidente Rodents, to which Castor canadensis, the subject of our study belongs, have only one pair Correspondence: P. ROUAS, address (2). E-mail: [email protected]
Objectifs : le but de cette étude est de décrire la morphologie et l’anatomie dentaires de Castor canadensis et de mettre en relation les résultats observés avec les données de la littérature relatives à ce Mammifère. Ceci permet une meilleure connaissance de son système dentaire particulier, qui correspond bien à la typologie des rongeurs. Matériel et méthodes : les auteurs utilisent une pièce ostéologique rare en Europe. Différentes coupes histologiques horizontales des organes dentaires labiaux et jugaux sont réalisées à différents niveaux coronaires et radiculaires. Elles sont étudiées en microscopie optique. Des colorations sont utilisées à des fins de différenciations tissulaires. Résultats : l’observation des coupes histologiques met en évidence un élargissement croissant de l’espace dévolu au tissu pulpaire de la couronne vers l’apex, largement ouvert, des incisives. Ces caractéristiques ne sont pas retrouvées sur les dents jugales qui présentent, même si elles ont également un apex ouvert, une configuration différente. Conclusion : les organes dentaires des Rongeurs en général et des Castoridés en particulier sont soumis à une force éruptive permanente tout au long de leur vie. Cette croissance dentaire permanente se fait au fur et à mesure de l’intense abrasion du système dentaire, en particulier au niveau incisif.
per jaw. In the family of Castorides, only the genus Castor has survived, which includes two species with a different geographical distribution: Castor fiber, which can be found in Europe and Asia, and Castor canadensis, found in North America. These two species present very similar phenotypes [8]. Teeth with continuous or prolonged growth can be found in species where horizontal jaw movements tend to predominate [1]. The crowns rise up more or less permanently, which accounts for the use of the terms hypsodontia or hypselodontia, from the Greek upselos, meaning high. Very few studies have been carried out on this subject. Our research will therefore provide a better knowledge of this physiological process in Castor canadensis.
34
P. ROUAS et al.
MATERIAL AND METHODS We used a cephalic skeleton of Castor canadensis from the region of the lower Saint Lawrence 1 (Canada) and which still contained all its teeth. The dental organs were removed in order to produce histological sections for examination under the optic microscope (enlargement ×40). Given the animal’s general morphology when it was caught, its age was estimated at between 8 to 10 years. It was ten months before this item could be imported, as procedures for the import of this type of material are strictly regulated by international conventions. Images of the osseous head and teeth were recorded using an argentic 24×36 camera with a Nikkor 120 mm medical lens. Images of the histological dental sections were taken using an Olympus BH2 camera fixed to a photonic microscope. EPP100 slide film was used in each case. For the medical imaging, we took frontal and profile teleradiographs at a distance of 1m50, giving an enlargement of about 11%, using a Gendex camera (ddp=66 kV). The different histological sections were taken in accordance with a standard protocol which is described below.
Embedding The extracted teeth were immersed in an alcohol bath, then placed at the bottom of circular moulds into which liquid resin was poured. Complete polymerisation of the resin was obtained after 24 hours. Obtaining the sections The resin blocks containing the dental organs were sliced into 1.17 mm sections using a disc microtome. Colouration We applied colourants to some of these sections for a better differentiation of the various tissues observed. Once they were rehydrated, the sections were immersed into a series of different baths in a specific order and for specific durations: fushine (5 minutes), phosphomolybdic acid (10 minutes), aniline blue (1 minute), interspersed with acetified water baths (15 seconds) to fix and rinse.
type and implantation is thecodont, with the teeth set in alveoli in the bone [1, 2, 5]. We suggest that the premolars of Castor canadensis correspond to the fourth premolar P4 of the basic Mammalian dental formula. We believe this can be explained by the isomorphism of this tooth with the molars, and by its position next to the first molar. Another point to notice is the absence of canines, leaving a space between the incisors and the jugal teeth. In this space, the “bar” or diastema, the animal is able to hold branches firmly as if in the jaws of a vice.
Dental morphology Like the majority of upper Vertebrates, Castor canadensis has a dental system known as plexodont which includes multituberculate teeth. This plexodontia is integrated into a heterodont dental system characteristic of the vast majority of Mammals. The morphology of the crowns varies from one end of the jaw to the other. The permanent denture of this species consists of labial teeth and jugal teeth. Tooth differentiation increases from front to back, with the molars thus having the most complex morphology. The labial teeth correspond to the incisors, and they are curved, very long and prominent, protruding outside the oral cavity, with a characteristic orangecoloured pigmentation on the vestibular face [1, 2, 10] (figure 1). Their length and the fact that they are deeply implanted into their supporting bone tissue are an indication of the enormous strength of these incisors (figure 2). The enamel tissue exists only on the vestibular face, which accounts for a lesser amount of wear on the anterior side and the fact that the incisors are bevelled [2, 5]. The jugal teeth consist of premolars and molars. They are separated from the labial teeth by the diastema. They decrease in size from front to back. They are
RESULTS AND DISCUSSION Dental formula Castor canadensis is diphyodont. This means it develops two successive sets of teeth: one which is deciduous, the other which is permanent. Like all rodents, it is a simplicidente with a single pair of incisors in each dental arch. The deciduous dentition consists of only one incisor and one molar per hemi-arch. The permanent dental formula is as follows: I 1/1 P 1/1 M 3/3, which corresponds to data from Anthony [1] and Jordan et al. [2]. Dental replacement is of the vertical
1
Other possibility of script: Saint-Laurent.
FIG. 1. — Anterior view of a Castor canadensis mandible show-
ing the orange coloring of the vestibular face of the labial teeth. FIG. 1. — Vue antérieure d’une mandibule de Castor canadensis objectivant la coloration orangée des faces vestibulaires des dents labiales.
Hypsodontia in castor canadensis: an anatomical and histological study
35
ble. The jugal teeth are very tightly packed one against the other, forming a continuous series where the wear surfaces present these characteristic enamel ridges [6, 9].
Dental histological study
FIG. 2. — Profile teleradiograph showing the length and the deep alveolar embedding of the incisors. The labial teeth are bevelled and come together in characteristic mode during occlusion. FIG. 2. — Téléradiographie de profil montrant la longueur et le
profond enchâssement alvéolaire des incisives. Les dents labiales sont taillées en biseau et présentent un affrontement caractéristique lors de l’occlusion.
shorter and prismatic in shape. They are of the lophodont type [1, 2, 5, 10] which means that the four cusps of the occlusal face combine in pairs to form transverse ridges (figure 3). There are three or four of these ridges, which slope slightly from front to back, from the inside towards the outside on the maxillary, and from the outside towards the inside on the mandi-
FIG. 3. — Left maxillary jugal teeth showing a series of transverse ridges characteristic of the lophodont. D: distal; V: vestibular. FIG. 3. — Dents jugales maxillaires gauches présentant des séries
de crêtes transversales caractérisant le type lophodonte. D : distal ; V : vestibulaire.
After embedding the teeth, histological dental sections were carried out, then studied under an optic microscope. The colouring used, according to the previously defined protocol, helps to determine structures and tissues. For the incisors, we carried out several horizontal sections perpendicular to the corono-apical axis at different levels: at the level of the occlusal bevel, in the median part and at the apex. At occlusal level (figure 4), we found a vestibular enamel, blue in colour, surmounted by a brown layer corresponding to the deposit of ferric oxide on the enamel. At median level (figure 5), the blue coloured enamel was localised on the vestibular face. The dark pink coloured dental surface corresponds to the dentine surrounding the pulp tissue, which appears black. As the enamel is the tissue which is the most mineralised and the hardest, the enamel arc is still the portion of dental tissue that wears least, giving the tooth a bevelled occlusal surface. At the apex (figure 6), blue-coloured enamel tissue can still be seen around the vestibular periphery. Dentine occupies a much smaller volume than in the earlier sections, leaving a much larger central pulp cavity. The increasing volume of the pulp from the occlusal edge to the apex tends to be proof of the hypsodont nature of this labial tooth. The apex there-
FIG. 4. — Transverse section of the free edge of the left maxillary incisor. E: enamel; D: dentin. FIG. 4. — Coupe transversale au niveau du bord libre de l’incisive maxillaire gauche. E : émail ; D : dentine.
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FIG. 5. — Coronal transverse section of the left maxillary incisor. Enamel tissue is only on the vestibular face. E: enamel; D: dentin; P: pulp.
FIG. 6. — Radicular transverse section of the left maxillary incisor. The thickness of the dentinal layer decreases as the volume of the internal pulp increases. E: enamel; D: dentin; P: pulp.
FIG. 5. — Coupe transversale au niveau coronaire de l’incisive maxillaire gauche. La localisation du tissu énamélaire se limite à la face vestibulaire. E : émail ; D : dentine ; P : pulpe.
FIG. 6. — Coupe transversale au niveau radiculaire de l’incisive maxillaire gauche. L’épaisseur de la couche dentinaire se réduit au profit du volume pulpaire interne. E : émail. D : dentine. P : pulpe.
fore provides access to a large number of blood vessels which will generate the formation of secondary dentine. The incisors of Castor canadensis are thus defined as ever-growing hypsodont teeth. All the stages in odontogenesis and eruption are thus combined in a single tooth, which makes this a particularly interesting experimental model. We selected a section from the jugal teeth, from the wear surface (figure 7). On the series of sections we observed the presence of transverse ridges of bluecoloured enamel, separated by dentine which enclosed small areas of pulp tissue (dark brown colour). This is a tubulate arrangement where the enamel invaginates like the fingers of a glove. The reddish brown coloured cement filtered in between the folds of the enamel. At root level, dentine fills almost the entire tooth. A fine peripheral reddish brown edging indicates the presence of cement which is therefore clearly present not only at root level but also at the crown [2]. Small red and black surfaces in the centre represent the areas given over to pulp tissue. Unlike the labial teeth, in the jugal teeth there appears to be less space for the nourishing pulp tissue in the radicular portion. A recent study on rats [3] confirms these results, describing a significant reduction in the volume of pulp in the apical third of the permanent molars of this rodent. The authors also suggest a link with age. We are therefore led to believe that the growth of the jugal teeth is not as intense as that of the labial teeth and that it would be more appropriate to talk of teeth with prolonged growth. However, many factors must be taken into consideration which could affect our argumentation, such as the age of the individual or its activity.
Masticatory physiology In Castor canadensis, the teeth come together in opposition in a specific way [1] (figure 8). In this type of bite, the upper tooth articulates end to end with the
FIG. 7. — Transverse section of the occlusal part of the first right molar of the mandible. E: enamel; D: dentin; C: cement; M: mesial; L: lingual. FIG. 7. — Coupe transversale au niveau de la partie occlusale de la première molaire mandibulaire droite. E : émail ; D : dentin ; C : cément ; M : mésial ; L : lingual.
Hypsodontia in castor canadensis: an anatomical and histological study
FIG. 8. — Right antero-lateral view showing the occlusion of the
jugal teeth. AR: back. BS: bottom. FIG. 8. — Vue antéro-latérale droite présentant le mode d’opposition des dents jugales. AR : arrière. BS : bas.
corresponding lower tooth. As in most rodents, there is a particular modality at incisor level: we find a bevelled articulation in accordance with the very oblique occlusal surfaces. According to Jordan et al. [2], determining the specific features of dental wear tells us a great deal about both the animal’s food regime and way of life. Grinding of the mandibular incisors is behaviour that is specific to rodents. The lingual face of the mandibular incisor of Castor canadensis is brought against the free edge of the maxillary incisor, resulting in wear on this lingual face (figure 2). The grinding cycles of this animal are among the most rapid, with a frequency of about 11 grinding cycles per second. This attrition forms and maintains the specific coronal morphology of these incisors [11]. The term “wear”, describing a loss of dental substance from the incisors, thus seems to be too simplistic to apply to Castorides because in fact this non-pathological phenomenon fulfils a true physiological function which is more than a simple compensatory phenomenon. The nature and extent of mandibular movements are closely dependent on the masticatory type of the species, which in turn depends on the food regime. In non-human Mammals, four types of masticatory movements predominate: vertical (in carnivores), lateral (in primates), transversal (in ungulates), anteroposterior (in rodents). The Canadian beaver presents mandibular movements of predominently the anteroposterior or propalinal type. To these are added vertical movements which help the bevelled articulated labial teeth to bite, and short transversal movements in a brief rotation around the longitudinal axis (helicoidal movements) to assist incision and trituration. These different movements are adapted to the food regime of Castor canadensis which is strictly vegetarian, consisting predominantly of tubers and bark. Three sorts of action are applied: incision, trituration, and the passage from molar occlusion to incisive occlusion. The occlusion of the jugal and labial teeth is never synchronous and, for a molar occlusion for example, there is a corresponding incisive inocclusion. Before incision takes place there is a preparatory
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phase of “empty” movements, specific to all rodents, and this is in three stages. First, disengagement, which consists of displacing the mandible following a relatively wide rotation. Next, translation towards the front brings the free edge of the mandibular incisor to the level of the palatine groove. After this, the opening determined by a combination of rotation and translation movements brings the labial teeth into occlusion [4, 6]. The crowns of the teeth are worn down considerably and this abrasion is compensated for by the prolongation of dental growth [1]. Because of this, the consequences of the accidental loss of an incisor can threaten an animal’s very survival [11]. When the continuous growth of the opposite incisor is no longer compensated by occlusal wear, this incisor will continue its progression without encountering any obstacle until it perforates the tissues in the mouth, which can bring about the animal’s death [12]. In veterinary practice, the rehabilitation of this incisive occlusion can be life-saving [13]. In the same way, the consequences of a malocclusion of the incisor will be more serious because of the physiological phenomenon of continuous dental growth [7].
CONCLUSION Castor canadensis thus has a specific denture consisting of labial and jugal teeth separated by a diastema. Incisors, premolars and molars undergo an intense non-pathological abrasion, compensated for throughout their lifetime by the force of the erupting teeth. However, these pressures are produced at different rythms, with the incisors seeming to be the teeth that are used the most. The dental organs of Castor canadensis are clearly much more than a simple example of adaptation to a food regime, they are extremely efficient tools without which beavers would certainly be unable to have the same lifestyle.
Acknowledgments We would like to thank Hilary Koziol for her help in the translation of this article.
REFERENCES [1] Anthony J. Cours d’Anatomie Comparée. Paris : Julien Prélat, 1973. [2] Jordan RE, Abrams L, Kraus BS. Kraus’ Dental Anatomy and Occlusion. 2nd ed. Mosby Year Book, 1992. [3] Gomez PA, Cabrini RL. Anatomic variations of the root canal of the rat according to age. Acta Odontol Latinoam 2004; 17: 39-42. [4] Grassé PP. Traité de zoologie. Anatomie, Systématique, Biologie. Mammifères. Paris: Masson et Cie, tome XVII, fascicule 2, 1955. [5] Kardong VK. Vertebrates. Comparative Anatomy, Function, Evolution. 2nd ed. WCB McGraw-Hill, 1998. [6] Kershaw DR. Animal diversity. 5 th ed. London: Chapman & Hall, 1994. [7] Kim JH, Lee JY, Choi SH. Odontoplasty for the treatment of malocclusion of the incisor teeth in a beaver (Castor canadensis). Vet Rec 2005; 156: 114-115.
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[8] Kuehn R, Schwab G, Schroeder W, Rottmann O. Differenciation of Castor fiber and Castor canadensis by noninvasive molecular methods. Zoo Biol 2000; 19: 511-515. [9] Pough FH, McFarland WN. Vertebrate life. 4 th Ed. Upper Saddle River, New Jersey: Prentice Hall International Inc, 1996. [10] Rouas P, Suaudeau T, Delbos Y, Rouas A. Approche morphologique et histologique du système dentaire de Castor canadensis (abstract). Morphologie 2001 ; 85 : 58.
[11] Sicher H. The biology of attrition. Oral Surg 1953; 6: 406412. [12] Zuri I, Terkel J. Reversed palatal perforation by upper incisors in ageing blind mole-rats (Spalax ehrenbergi). J Anat 2001; 199: 591-598. [13] Walmsley AD, Lumley PJ, Laird WRE, Kesterton C, Daft J. Restoration of the anterior dentition of Canadian Beaver (Castor canadensis). J Small Anim Pract 1989; 30: 583-586.