Histochemical Study of Rabbit Masseter Muscle: The effect of the alteration of food on the muscle fibers

Changes in masseter muscle fibers by liquid diet rearing in rabbits and recovery by chewing of solid diet

OBJECTIVE

To investigate the effects of liquid diet on the development of masseter muscle fibers and whether the changes in the masseter muscle can be recovered by chewing of solid diet.

DESIGN

Masseter muscles from 40 rabbits (solid- and liquid-diet groups, n = 30; unweaned group, n = 5; recovery group, n = 5) were histochemically examined at 4, 12, 18, and 33 weeks after birth. Six fiber types (I, IC, IIC, IIA, IIAB, and IIB) were distinguished via mATPase staining. Muscle fiber diameter and fiber type composition were measured and compared between groups.

RESULTS

In the liquid diet group, the diameter of types IIAB (solid group: 81.7 μm, liquid group: 60.9 μm) and IIB (solid group: 89.3 μm, liquid group: 68.8 μm) and the fiber type composition of type I (solid group: 18.4%, liquid group: 9.6%) decreased significantly at 33 weeks of age. In the recovery group, the fiber type composition of type I fibers recovered to 16.5%, while no recovery of type IIAB (56.6 μm) and IIB (64.6 μm) fiber diameter was observed.

CONCLUSIONS

Liquid diet caused atrophy of muscle fibers and an increase in the proportion of fast-twitch fibers. Although the diameter and ratio of slow-twitch fibers were recovered by chewing of solid diet, recovery was not observed for fast-twitch fibers. Our findings are relevant for dental medicine as it explored the possibility of masticatory muscle function recovery by hard food.

Development of medial pterygoid muscle fibers in rabbits fed with a liquid diet

OBJECTIVE

This study aimed to investigate the influence of decreased functional load on the medial pterygoid muscle during mastication in rabbits fed with a liquid-diet.

MATERIALS AND METHODS

Medial pterygoid muscles from 54 rabbits (solid- and liquid-diet groups, n=48; unweaned group, n=6) were histochemically examined at 4, 9, 12, 18, and 33 weeks after birth. Six fiber types (I, IC, IIC, IIA, IIAB, and IIB) were distinguished via mATPase staining.

RESULTS

Significant increases in the diameters of all fiber types were seen up to 33 weeks of age in the solid-diet group; however, no significant increase was noted in fiber types I and IC, from 4 to 33 weeks of age, in the liquid-diet group. The proportion of slow fibers increased up to 12 weeks followed by an increase in the number of fast fibers in the solid-diet group, whereas in the liquid-diet group, the number of slow fiber declined after weaning.

CONCLUSIONS

Liquid-diet consumption caused muscle fiber atrophy and an increase in the number of fast fibers during early developmental stages after weaning. Furthermore, the growth pattern of the medial pterygoid muscle in the liquid-diet group was different from that in the solid-diet group.

The masticatory system under varying functional load. Part 2: Effect of reduced masticatory load on the degree and distribution of mineralization in the rabbit mandible

A reduction in mechanical loading of the mandible brought about by mastication of soft food is assumed to decrease the remodelling rate of bone, which, in turn, might increase the degree of bone mineralization. The effect of a reduction in masticatory functional load on the degree and distribution of mineralization of mandibular bone was investigated in male juvenile New Zealand White rabbits. The experimental animals (n=8) had been raised on a diet of soft pellets from 8 to 20 weeks of age, while the controls (n=8) had been fed pellets of normal hardness. The degree of mineralization of bone (DMB) was assessed at the attachment sites of various jaw muscles, the condylar head, and the alveolar process. Differences between groups and among sites were tested for statistical significance using a Student's t-test and one-way analysis of variance, respectively. The DMB did not differ significantly between the experimental and control animals at any of the sites assessed. However, in the rabbits that had been fed soft pellets, both cortical bone at the attachment sites of the temporalis and digastric muscles and cortical bone in the alveolar process had a significantly higher DMB than cortical bone at the attachment site of the masseter muscle, while there were no significant differences among these sites in the control animals. The results suggest that a moderate reduction in masticatory functional load does not significantly affect the remodelling rate and the DMB in areas of the mandible that are loaded during mastication but might induce a more heterogeneous mineral distribution.

Myofiber properties of mouse mylohyoid muscle in the growth period

The mouse mylohyoid muscle belongs to the mastication-related suprahyoid muscle group. It shows a plate-like morphology and forms the mouth floor. There have been no reports on the characteristics of the mouse mylohyoid muscle fibers, and especially on their functional role during ingestion action, and many points remain unclear. We examined the mouse mylohyoid muscle at both the transcriptional and protein levels by RT-PCR, immunohistochemistry, and Western Blotting. MyHC-2b, which is expressed in almost all head and neck muscles and is thought to play a role in rapid mastication movement, was not detected in the mouse mylohyoid muscle. This result suggests that the mouse mylohyoid muscle has a special function and does not directly function during ingestion.

Changes in Muscle-fiber Properties of the Murine Digastric Muscle Before and After Weaning

The digastric muscle is one of the suprahyoid muscles and consists of the anterior and posterior bellies. Because muscle fiber alignments in these two bellies are different, the functional roles are said to be different. Since the digastric muscle relates to mastication, its functions may change markedly before and after weaning, but many details remain unknown. The aim of this study was to clarify changes in muscle fiber properties of the anterior and posterior bellies of the digastric muscle in mice before and after weaning. Expressions of myosin heavy chain (MyHC) isoforms were assessed at the protein and transcriptional levels. Expression of the MyHC-2b isoform, an isoform displaying fast, strong contraction, was greater in the anterior belly than in the posterior belly after weaning. This suggests that, in mice, the anterior belly of the digastric muscle needs to move rapidly anteroposteriorly for mastication, compared with the posterior belly.

Alterations in enzyme histochemical characteristics of the masseter muscle caused by long-term soft diet in growing rabbits

OBJECTIVE

Recently young people have an increasing tendency to intake an easily chewable diet and spend less time on mastication. The aim of the present study was to investigate the histochemical effects of long-term soft diet on the masseter muscle in growing rabbits.

MATERIALS AND METHODS

Twelve young male Japanese white rabbits were divided into two groups (n = 6 each) at weaning (1 month after birth) and fed a solid diet (control group) or a powder diet (soft-diet group). The duration of the experimental period was 6 months. Masseter fibers from the superficial and the deep portions were histochemically defined as type 1, 2A, 2B, or 2C fibers.

RESULTS

As compared with that of the control, the deep masseter of the soft-diet group showed a significantly lower ratio of type 1 fiber cross-sectional area to total area (6.3 and 10.1% for the soft-diet and control group, respectively), significantly more type 2A fibers (74.0%vs 50.3%) and significantly fewer type 2B fibers (4.3%vs 12.5%). However, fiber size did not differ between the two groups. NADH-tetrazolium-reductase (NADH-TR) of the masseter was less reactive in the soft-diet group, reflecting a lower oxidative capacity.

CONCLUSIONS

These findings indicate that the alteration of the functional activities contributed to selective disuse influences on the type 1 and type 2B fibers, and a resultant increase in type 2A fibers. This study suggests that long-term alteration of jaw function induced by a soft diet can lead to adaptations of the masseter muscle.