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Characteristics of Mastication in the Anodontic Mouse
1 Department of Oral Physiology, Osaka University Graduate School of Dentistry, 1-8, Yamadaoka, Suita, Osaka 565-0871, Japan; Correspondence: * corresponding author, morimoto{at}dent.osaka-u.ac.jp
Teeth and periodontal mechanoreceptors play important roles in regulating jaw movements during mastication. However, little is known concerning how jaw movements develop without tooth eruption. To answer this question, we studied masticatory behavior in the osteopetrotic mouse, where tooth eruption does not occur and periodontal mechanoreceptors are missing. A masticatory sequence of the osteopetrotic mouse was divided into two stages: incision and chewing. Incision is characterized by small amplitude and rapid (7 Hz) open-close jaw movements, while slow (5 Hz) and large amplitude open-close jaw movements characterize chewing. The frequency and properties of jaw movements were comparable with those in the normal mouse, though the osteopetrotic mouse had a higher cycle number during incision than did the normal mouse. These results indicate that conversion from sucking to mastication occurs in the anodontic mouse, and the central pattern generator producing the masticatory rhythm develops almost normally without tooth eruption.
Key Words: anodontia • osteopetrosis • toothless • electromyography • development
Feeding behavior, including sucking and mastication, is accompanied by rhythmic jaw movements that are generated by a certain central neuronal population called the central pattern (or rhythm) generator (CPG; Lund, 1991; Nakamura and Katakura, 1995). Rhythmic jaw movements are modulated by sensory feedback of masticatory force and the direction of the force loaded to the tooth from mechanoreceptors in the periodontal tissues (Appenteng et al., 1982; Lavigne et al., 1987; Inoue et al., 1989; Yamamura and Shimada, 1993). Inoue et al.(1989) show that trigeminal deafferentation modifies jaw-closing muscle activity, the patterns of jaw movements, and the numbers and rates of chewing cycles in the rabbit. In addition, several studies suggest that tooth eruption promotes conversion from sucking to chewing during the early post-natal period (Bosma, 1967; Moyers, 1973). Thus, teeth and periodontal mechanoreceptors play important roles in regulation of oral feeding behaviors; however, it is still unclear whether they play a role in the formation and development of masticatory behavior. To address this issue, we studied the feeding behaviors of the osteopetrotic (op/op) mouse, a mutant with metabolic bone disease that results in the prevention of tooth eruption (Marks and Lane, 1976). The present study aims to compare the feeding behavior in the op/op mouse with that in the normal littermate, by means of our novel jaw movement and EMG analyzing system (Kobayashi et al., 2002).
All experiments were performed in accordance with the National Institute of Health Guide for the Care and Use of Laboratory Animals, and approved by Osaka University Faculty of Dentistry Intramural Animal Care and Use Committee. All efforts were made to minimize suffering and to reduce the number of animals used in the present study. The experiments were performed on 9 adult (> 3 mos) male normal B6C3 (+/+ or +/op) and 6 op/op mice. The techniques for surgical and recording procedures are similar to those described previously (Kobayashi et al., 2002). Only a brief account of the procedures will be given here. With the animals under ketamine anesthesia (50 mg/kg), we inserted EMG electrodes made from enamel-coated copper wires into the middle part of the bilateral deep masseteric muscles and anterior digastric muscles. For jaw movement recording, the magnet was attached to the submental region, and a pair of magnet sensors was mounted on a portion of the skull. After recovery, the mice were fed paste food, a mixture of powdered rat diet (CA-1, Clea Japan, Tokyo, Japan), that was water-shaped into a ball (7 mm diameter). Recordings were made while mice ate these paste balls. EMG signals and jaw movements were stored on magnetic tape by means of a digital tape recorder (PC208A, Sony-Magnescale, Tokyo, Japan), and the following variables were analyzed with the use of Spike 2 software (Cambridge Electronic Design, Cambridge, UK): (1) duration of the masseteric and digastric EMG burst activities, (2) total cycle length (TCL) measured as the time interval between the onset of digastric activity of two consecutive cycles, (3) number of masticatory cycles in different stages of a masticatory sequence, and (4) duration of the stages of the masticatory sequence. The EMG burst was identified as the point when muscle activity exceeded three times the standard deviation (SD) of basal noise for more than 30 ms. Since we found that there was no constant difference between EMG recordings from the right and left muscles, we used the EMG records obtained from the right side in the present study. We excluded the masseteric discharge during the jaw-opening phase and the digastric discharges during the jaw-closing phase from the measurement of the TCL and duration of the EMG activities, referring to the jaw movement recording. The chewing rate was expressed as the reciprocal of the TCL. Since small interindividual variations in the location of the magnet may produce a considerable difference in the magnitude of the recorded jaw movements in mice, the jaw movement recordings were used only for qualitative evaluation of the magnitude of the movements and identification of the jaw-opening and -closing phases. Data are presented as means ± SD. Comparison of data is based on paired and unpaired t tests, with statistical significance accepted at p < 0.05. At the end of the experiments, the mice were deeply anesthetized with an overdose of ketamine hydrochloride and then perfused with 0.1 M phosphate buffer through the heart, followed by 10% formalin. After 24 hrs of post-fixation, the skull was decalcified in 10% formic acid and dehydrated in a graded alcohol series and benzol before being embedded in paraffin wax. Four-µm-thick frontal section slices were made from embedded specimens and then stained with hematoxylin-eosin.
The op/op mice were differentiated from their normal littermates on the basis of their domed heads, small and stubby appearance, and defects of tooth eruption (Marks and Lane, 1976). The skull of the op/op mouse was smaller than that of the normal mouse (Figs. 1A, 1D  ), and the body weight of the op/op mouse (26.7 ± 3.9 g, n = 6) was less than that of the normal mouse (31.4 ± 3.0 g, n = 9; p < 0.01). Histological evaluation of the stomatognathic structures of the normal mouse revealed that the molars erupted into the oral cavity (Fig. 1B), and these roots were surrounded with the periodontal ligament (Fig. 1C). In contrast, the molars remained completely within the maxilla and mandible in the op/op mouse (Fig. 1E), and the periodontal ligament failed to develop. In addition, the severely deformed crown dentin adhered directly to the bone trabeculae (Fig. 1F). The roots of the incisors were observed below the molar roots in the normal mouse, while they were not found in the op/op mouse at the same anterior-posterior level.
Fig. 2A shows an example of jaw movement and EMG activities in the masseter and digastric muscles during paste-food feeding in the normal mouse. As previously reported, the masticatory sequence was divided into two stages based on the rhythm and magnitude of the constituent masticatory cycles: incision and chewing (Kobayashi et al., 2002). In the op/op mouse, we divided the masticatory sequence in the same way as in the normal mouse (Figs. 2B, 2C). Similar to the normal mouse, in the first stage of mastication, incision, the op/op mouse holds the bolus of paste food between the forepaws, then ingests it via the anterior part of the oral cavity, where it is then transported to the posterior part with small-amplitude, open-close jaw movements. Incision was followed by chewing, where the op/op mouse crushed the food with large-amplitude, open-close jaw movements. The amplitude of masseteric activities was often lower, while chewing rates were faster, during incision than in chewing (Figs. 2C, 2D).
As reported in the previous study in the normal mouse (Kobayashi et al., 2002), the TCL and duration of masseteric and digastric activity were significantly shorter during incision than during chewing in the op/op mouse (Table ). These parameters in the op/op mouse were comparable with those in the normal mouse except for the TCL during incision. In contrast, there were striking differences between the normal and op/op mouse in stage duration and masticatory cycle number during incision behavior. Stage duration during incision in the op/op mouse was significantly longer than that in the normal mouse (p < 0.05), though there was no significant difference between stage duration of chewing between the normal and op/op mice. The cycle number of incisions in the op/op mouse was significantly higher than that in the normal mouse (p < 0.05), though the number while chewing was not different between the normal and op/op mice.
There are several types of rhythmic oral behaviors that occur during ingestion in mammals, such as sucking, licking, lapping, and mastication. We considered the feeding pattern of the op/op mouse consuming a bolus of paste food to be mastication for several reasons. The pattern of sucking and licking consists of monotonous, rhythmic jaw movements with fairly constant reciprocal EMG activity in the masseteric and digastric muscles (Yamamoto et al., 1982; Iriki et al., 1988; Kobayashi et al., 2002), and lapping accompanies tonic masseteric activities (Thomas and Peyton, 1983). The op/op mouse, however, possesses two types of rhythmic jaw motions and EMG activity in the masticatory muscles: rapid (7 Hz), small-amplitude open-close jaw movements with small-amplitude EMGs; and slow (5 Hz), large-amplitude open-close jaw movements with large EMG activity. These features and rhythms are quite similar to those of mastication in the normal mouse (Kobayashi et al., 2002). Although several parameters of mastication in the op/op mouse are different from those in the normal mouse, the basic rhythm and pattern of mastication in the op/op mouse are similar to those of the normal mouse. These results suggest that the CPG for mastication can develop in the absence of teeth and periodontal mechanoreceptors, structures that play an important role in the regulation of feeding pattern and rhythm. Other receptors, such as the muscle spindle in the jaw-closing muscles and/or mechanoreceptors in the oral mucosa, could compensate for the lack of periodontal mechanoreceptors (Lavigne et al., 1987; Morimoto et al., 1989). In addition, formation of the CPG may be genetically pre-programmed, needing no feedback from peripheral receptors to develop.
The differences in stage duration, cycle number, and TCL during incision—but not in chewing—between the op/op and normal mice may be due to their morphological features. The op/op mouse has difficulty in biting the bolus of paste food because of the wide intermaxillar space owing to the lack of incisors. Compensatory movements would be more complicated and less efficient than those of the normal mouse. On the other hand, the maxilla and mandible of the op/op mouse come into contact with each other at the posterior part of the mouth during jaw closure (Fig. 1 The present study demonstrates that oral feeding behavior develops almost normally without tooth eruption. Conversion from sucking to mastication occurs in the anodontic op/op mouse, and the masticatory rhythm produced by CPG in the anodontic mouse is similar to that in the normal mouse.
We thank A. Komuro, T. Kato, J. Mizuno, and K. Kojima for their assistance. We also thank J.E. Austin and Dr. D. Thomas for critical reading of this manuscript. This work was supported by Grants-in-Aid for Scientific Research (Nos. 11357017 and 13307056) from the Ministry of Education, Science, Culture and Sports of Japan. Received for publication February 11, 2002. Revision received July 8, 2002. Accepted for publication July 9, 2002.
Journal of Dental Research, Vol. 81, No. 9,
594-597 (2002)
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