Issue 1 (193)/2023

ALSE and ACS Style
Spataru, C.; Spataru, M.-C. Peculiarities of the masticatory apparatus of guinea pigs (Cavia porcellus). Journal of Applied Life Sciences and Environment 2023, 56 (1), 127-138.
https://doi.org/10.46909/alse-561090

AMA Style
Spataru C, Spataru M-C. Peculiarities of the masticatory apparatus of guinea pigs (Cavia porcellus). Journal of Applied Life Sciences and Environment. 2023; 56 (1): 127-138.
https://doi.org/10.46909/alse-561090

Chicago/Turabian Style
Spataru, Constantin, Mihaela-Claudia Spataru. 2023. “Peculiarities of the masticatory apparatus of guinea pigs (Cavia porcellus)” Journal of Applied Life Sciences and Environment 56, no. 127-138.
https://doi.org/10.46909/alse-561090

Peculiarities of the Masticatory Apparatus of Guinea Pigs (Cavia porcellus)

Constantin SPATARU¹ and Mihaela-Claudia SPATARU^2,*

¹Department of Preclinics, Faculty of Veterinary Medicine, “Ion Ionescu de la Brad” Iasi University of Life Sciences, 8, Mihail Sadoveanu Alley, 700489, Iasi, Romania; email: cspataru@yahoo.com
²Department of Public Health, Faculty of Veterinary Medicine, “Ion Ionescu de la Brad” Iasi University of Life Sciences, 8, Mihail Sadoveanu Alley, 700489, Iasi, Romania

*Correspondence: mspataru@yahoo.com

Received: Jun. 14, 2023. Revised: Jul. 10, 2023. Accepted: Jul. 15, 2023. Published online: Jul. 19, 2023

ABSTRACT. Guinea pigs (Cavia porcellus) are rodents that feed on grassy plants, buds and sprouts, with cheek teeth having specialised abrasive surfaces for plant grinding. In analysing the prehension and trituration ways of guinea pigs, many differences concerning mandible conformation, the positioning of cheek teeth and the morphology of the masticatory muscles compared to other rodents were found. Masticatory muscles of guinea pigs are predominant compared to the mimetic muscles which are reduced. Compared to other rodents, in guinea pigs, inside the tendon thickness of the superficial part of the masseter muscle there are two rounded cartilaginous structures such as sesamoids. The dorsal one is larger, measuring about 3–4 mm in diameter having the role of reducing pressure on the tendon when it passes over the mandible ridge. The other is ventrally placed, about 2–3 mm in size and protects the tendon of the superficial part of the masseter muscle when it passes over the ventral tubercle of the mandible.

Keywords: guinea pig; mandible; masseter; masticator muscles; skull.

 

INTRODUCTION

Guinea pigs or domestic guinea pigs (Cavia porcellus), also known as cavy or domestic cavy, are a species of Hystricomorpha rodent belonging to the family Caviidae (Álvarez and Pérez, 2019; Álvarez et al., 2023; Graur et al., 1991; Spotorno et al., 2005). Guinea pigs are diurnal, being most active during dawn and dusk. The guinea pig’s food by diet consists of grassy plants, buds and sprouts (Noguchi et al., 1994; Spotorno et al., 2005). The cheek teeth have specialised occlusion surfaces for grinding both by propulsion/retropulsion and lateral movements (Noguchi et al., 1994; Spataru and Spataru, 2019).

Guinea pigs are the most publicised experimental animal. Louis Pasteur and Robert Koch used them in investigations of infectious diseases, the same contributing to the work of several Nobel Prize awards research. Since the nineteenth century, guinea pigs have been used in research in the field of physiology, dermatology, pharmacological tests, metabolism (Westropp and Buffington, 2002, West and Fernindez, 2004), asthma (Ricciardolo et al., 2008), study of foetal and placental evolution and aspects of birth (Carter et al., 2007; Mitchell and Taggart, 2009), in the diagnosis of some diseases such as tuberculosis, diphtheria (Dharmadhikari and Nardell, 2008; Obregon-Henao et al., 2011) or in infectious diseases with common, nosocomial causes, such as Staphylococcus aureus (Padilla-Carlin et al., 2008), etc. For example, guinea pigs share many similarities to humans, both hormonally, immunologically and physiologically. Unlike other rodents, but similar to humans, guinea pigs are prone to scurvy if they do not receive vitamin C in their diet (Clarke et al., 1980).

Guinea pigs are monophyodont, the teeth erupting continuously (hypsodontic) (Harkness et al., 2002), the lower incisors being longer than the upper ones, and the molars have no cusps presenting a groove or deep indentation (Cooper and Schiller, 1975; Pereira et al., 2020). In all mammals , the muscles directly involved in mastication are the masseter, temporalis and pterygoideus (Cooper and Schiller, 1975; Myers et al., 2006).  Some differences have been identified in the terms of skull conformation and masticatory apparatus among Myomorpha (mice and rats), Scuiromorpha (squirrels), Hystricomorpha (South American caviomorph rodents and porcupines) and Protogomorpha (mountain beaver), based on differences in the muscles of mastication and other characteristics of the masticatory apparatus (Druzinsky, 2010; Vassalo and Verzi, 2001; Hautier et al., 2012; Pereira et al., 2020; Álvarez et al., 2023).

 

MATERIALS AND METHODS

The study of the skull and head muscles was performed on five corpses of specimens of adult guinea pigs. The muscles were highlighted using layer by layer dissections, following the place of origin and insertion of each muscle and the direction of fibres and analysing the role of each muscle in the mastication process. The skulls were prepared by scraping the adjacent tissues and through light boiling. The identified features were photographed, processed and interpreted, being compared to other information from the specific literature (Woods, 1972; Popesko et al., 1992; Constantinescu, 2018; Álvarez and Pérez, 2019; Álvarez et al., 2023).

 

RESULTS AND DISCUSSION

Viewed from the side, the guinea pig’s skull fits into a rectangle with a small side of about 2 cm and a large one of 6.5 cm. The viscerocranium represents 2/3 of the entire skull, being the dominant portion of the skull (Pereira et al., 2020; Spataru et al., 2013).

In guinea pigs, the premaxillary bones (os incisivum) are very developed, showing a curved appearance laterally. Each of them has a narrow incisor alveolus, the incisors having continuous growth (Cooper and Schiller, 1975; Druzinsky, 2010; Harkness et al., 2002; Isotupa and Rönning, 1977; Spataru, 2016; Stan, 2014). Each alveolar process of the incisive bone is deep, reaching the base of the alveolar processes of the maxillary bone, thus increasing the resistance of the incisors. The incisors have a chisel appearance, being shorter and triangular in cross-section and slightly curved ventrally, reaching the plane of the root of the molars.

From each maxilla, a zygomatic process in the form of a triangular blade is laterally detached, giving  attachment point for the masseter muscle (Figure 1).

The maxillary bone has one premolar and three molar alveolar processes. The dental formula is 2x (I1/1, C 0/0, P 1/1, M 3/3), with a total of 20 teeth showing an open rooted aspect (hypsodont) (Isotupa and Rönning, 1977; Kruska and Steffen, 2013; Spataru and Spataru, 2019; Stan, 2014).

In guinea pigs, the oblique rostro-aboral position of the molars is easily noticeable. The alveolar processes of the premolar and the first molars are close to the median plane of the oral cavity, being separated only by the median intermaxillary suture. The rest of the molars have a divergent position, widening the oral cavity. The molars have no cusps, the abrasion surface being formed by two transverse ridges each; taken as a whole, the abrasion surface being oblique mid-laterally. In guinea pigs, all cheek teeth have short and uniform dental crowns, while the crowns decrease from the premolar to the last lower molar (Boivin et al., 2022; Butler, 1980, 1985; Clarke et al., 1980; Harkness et al., 2002; Isotupa and Rönning, 1977; Stan, 2014).

 

 

On the other hand, the oblique position of the molars and the slight mid-lateral obliquity of their abrasion surface show that, in addition to the propulsion and retropulsion movements of the mandible, lateral movements are also used for chewing food (Figure 2, Figure 3, Figure 8) (Álvarez and Pérez, 2019; Álvarez et al., 2023; Byrd, 1981; Spataru, 2016; Stan, 2014).

In guinea pigs, the mandible is impressive by its length, being as long as the cranium. If in most rodents the caudal angle of the mandible has the appearance of a trident, being formed by the three obvious processes: the coronoid process, the articular condyle and the angular process. In guinea pigs a reduction in height of the coronoid process that appears as a triangular bony blade is notable, being 2–3 mm high or sometimes absent (Anthwal et al., 2012, 2015; Cox et al., 2012; Spataru et al., 2013). The condyle has a lamellar appearance, being thin (Figure 3, Figure 4) (Spataru, 2016).

The mandibular condyle is placed in the plane of the lower molar level, similar to carnivores, being slightly deviated medially, an aspect that increases the mandible`s resistance to mechanical stress (Spataru, 2016). The angular process is lamellar and exceeds the aboral plane of the skull, with a wide pterygoid fossa on its medial face (Figure 3). During resting, the tip of the incisors and the mandibular condyle are placed in a plane that passes dorsally to the mandibular molar  level. This indicates a powerful force of the incisors in sectioning food (Butler, 1980, 1985; Christiansen and Adolfssen, 2005; Boivin et al., 2022). The masticatory muscles in guinea pigs are predominant, being represented by strong muscles. The mimetic muscles are reduced and simpler, being related to precise oronasal movements and regional sensitivity, with only the malaris and buccinator muscles being more prominent, which are involved in chewing the bolus (Álvarez et al., 2023). The masseter, temporal and pterygoid muscles have strong bellies formed by muscle bundles, being distributed in different directions, increasing the muscle strength and the sectioning force in  chewing food between the dental plates (Byrd, 1981; Cox and Jeffery, 2011).

Within the masticatory muscles, the masseter muscle is more developed in guinea pigs, formed by three parts consisting of fibers distributed in different planes and angles (Cox and Jeffery, 2011; Spataru, 2019; Álvarez et al., 2023;). The superficial part has an aponeurotic aspect and forms a strong and wide tendon which originates on the facial tubercle (Figure 4, Figure 5, Figure 6). The musculo-aponeurotic fibres have an accentuated ventro-aboral obliquity, which then merges into a massive muscular belly that is briefly inserted on the angle and the ventral edge of the angular process of the mandible (Figure 4, Figure 5). Thus, Cox and Jeffery (2011) indicated that this part represents about 45% of the masticatory musculature, compared to about 30% in the squirrel and rat.

In adult guinea pigs, there are two cartilaginous thickenings, similar to sesamoids, in the thickness of the tendon. The first is larger, about 3–4 mm in length, protecting the tendon when it passes over the mandibular ridge. The second is placed ventrally to the first, about 2–3 mm in length, protecting the tendon of the superficial part of the masseter muscle when it passes over the ventral tubercle of the mandible.

During mastication, the propulsion of the mandible is mainly produced by the deep part of the masseter muscle. It originates on the zygomatic arch being inserted over a wide area of the mandible, from the rostral limit of the premolar and the first molar, the whole angular process ventrally towards the ridge that delineates both the masseter and the pterygoid fossa.

 

 

 

 

 

 

These aspects regarding the origin on a tubercle and its wide insertion with the oblique orientation of the fibers as well as the presence of the two cartilaginous nuclei, mean that the superficial part of the masseter muscle is involved both in mandible propulsion and its efficient and fast raising, considering that the temporal muscle is reduced (Anthwal and Tucker, 2012; Cox et al., 2012; Cox and Jeffery, 2011; Spataru et al., 2013).

The oblique position of fibers, the long tendon of origin and the presence of cartilaginous structures represent a factor of adaptation for grinding food (Álvarez et al., 2023), the contraction of the muscle permitting trituration for a long time, avoiding muscle fatigue.

The middle (zygomatic) part of the masseter muscle has similar tilting of fibers to other rodents. In guinea pigs, the origin is large, extending over the entire zygomatic arch, both on its lateral and medial faces. Fibres have a ventro-aboral oblique position, being crowded towards the dorsal edge of the angular process and the base of the mandibular condyle (Cooper and Schiller, 1975; Pereira et al., 2022). This plays a main role in the rapid and strong lifting of the mandible, due to the placement of the articular condyle at the level of the mandibular molar plate. The smaller deep (infraorbital) part of the masseter muscle originates on the infraorbital ridge. The muscle passes through the infraorbital foramen (which appears as a deep ditch),  being inserted on an ovoid area of the masseter fossa, which is delimited by the tuberculum mandibularis dorsalis (Figure 6).

As in other species of rodents, the temporal muscle consists of two small parts (Álvarez and Pérez, 2019; Byrd, 1981; Cox and Jeffery, 2011; Kruska and Steffen, 2013). The rostral part is smaller having a triangular appearance (Figure 7) (Spataru et al., 2013). The origin occupies the spaces of the parietal bone, caudally to the orbit, and the insertion reaches the rostral edge of the coronoid process. The deep part (the aboral part) originates on the whole temporal fossa up to the external occipital protuberance. It has a fan-like shape; the fibers being inserted on top of the coronoid process (Figure 7).

Compared with other rodents, guinea pigs present a strongly developed digastricus (occipito-mandibularis) muscle, appearing as a cord of fibres (Figure 8) (Spataru et al., 2011). It originates on the paracondylar process (2–3 mm long in guinea pigs) and on the caudo-ventral edge of the tympanic bulla. The massive muscular belly is ventro-rostrally obliquely oriented to the caudal edge of the mandible body. The bellies of the digastricus muscles fuse into the intermandibular space, inserting together with the mylohyoideus and geniohyoideus (reduced) muscles on the mandible. The dorso-ventral position of fibers allows a large extension of the temporo-mandibular joint, as well as performing a strong retropulsion of the mandible.

The lateral (external) and medial (internal) pterygoid muscles arise from the entire surface of the pterygoid ridges of the basisphenoid and palatine bones, having a wide insertion on the mandible, throughout the pterygoid fossa (medial pterygoid muscle) and mandible pterygoid fovea and articular disc (lateral pterygoid muscle) (Woods, 1972; Constantinescu, 2018).

 

 

 

Besides raising the mandible, the muscles produce the lateral movements which are very efficient in chewing food, an aspect shown by the ventro-medial obliquity of the occlusal surface of the lower molars. Some authors suggest that the lateral pterygoid muscle ensures long time and continuous contraction such as in retention of jaw position (Abe et al., 2008; Cox and Jeffery, 2011; Easton and Carlson, 1990; Spataru et al., 2013).

 

CONCLUSIONS

 

In guinea pigs, the mandible is impressive by its length,. The jointing condyle is placed in the plane of the mandibular molar level, similar to carnivores.

In guinea pigs, the oblique displacement of the molars and the slight mid-lateral obliquity of their abrasion surface show that, in addition to the propulsion and retropulsion movements of the mandible, lateral movements are also used for chewing food. In guineea pigs, the masseter muscle is predominant, being formed by three parts having its fibers distributed in different planes and angles. The superficial part is the most developed, two cartilaginous thickenings protecting the tendon when it passes over the ridges of the mandible. The oblique aspect of the deep part of the masseter muscle, the long tendon of origin and the presence of a cartilaginous structure represent a factor of adaptation for grinding food, the contraction of the muscle permitting trituration for a long time. The middle part has dorso-ventral disposition of fibers, it playing the role in the rapid and strong lifting of the mandible. The pterygoid muscles are the same developed producing the lateral movements of the mandible.

 

Author Contributions: MCS: conceptualization; MCS, CS: methodology; analysis; investigation; resources; data curation; writing, review; supervision; and so on. All authors declare that they have read and approved the publication of the manuscript in this present form.

Funding: There was no external funding for this study.

Conflicts of Interest: No conflicts of interest.

 

 

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