Histomorphometrical analysis of the influence of soft diet on masticatory muscle development in the muscular dystrophic mouse

Alteration of Oral and Perioral Soft Tissue in Mice following Incisor Tooth Extraction

Oral and perioral soft tissues cooperate with other oral and pharyngeal organs to facilitate mastication and swallowing. It is essential for these tissues to maintain their morphology for efficient function. Recently, it was reported that the morphology of oral and perioral soft tissue can be altered by aging or orthodontic treatment. However, it remains unclear whether tooth loss can alter these tissues’ morphology. This study examined whether tooth loss could alter lip morphology. First, an analysis of human anatomy suggested that tooth loss altered lip morphology. Next, a murine model of tooth loss was established by extracting an incisor; micro-computed tomography revealed that a new bone replaced the extraction socket. Body weight was significantly lower in the tooth loss (UH) group than in the non-extraction control (NH) group. The upper lip showed a greater degree of morphological variation in the UH group. Proteomic analysis and immunohistochemical staining of the upper lip illustrated that S100A8/9 expression was higher in the UH group, suggesting that tooth loss induced lip inflammation. Finally, soft-diet feeding improved lip deformity associated with tooth loss, but not inflammation. Therefore, soft-diet feeding is essential for preventing lip morphological changes after tooth loss.

Muscle-Bone Crosstalk in the Masticatory System: From Biomechanical to Molecular Interactions

The masticatory system is a complex and highly organized group of structures, including craniofacial bones (maxillae and mandible), muscles, teeth, joints, and neurovascular elements. While the musculoskeletal structures of the head and neck are known to have a different embryonic origin, morphology, biomechanical demands, and biochemical characteristics than the trunk and limbs, their particular molecular basis and cell biology have been much less explored. In the last decade, the concept of muscle-bone crosstalk has emerged, comprising both the loads generated during muscle contraction and a biochemical component through soluble molecules. Bone cells embedded in the mineralized tissue respond to the biomechanical input by releasing molecular factors that impact the homeostasis of the attaching skeletal muscle. In the same way, muscle-derived factors act as soluble signals that modulate the remodeling process of the underlying bones. This concept of muscle-bone crosstalk at a molecular level is particularly interesting in the mandible, due to its tight anatomical relationship with one of the biggest and strongest masticatory muscles, the masseter. However, despite the close physical and physiological interaction of both tissues for proper functioning, this topic has been poorly addressed. Here we present one of the most detailed reviews of the literature to date regarding the biomechanical and biochemical interaction between muscles and bones of the masticatory system, both during development and in physiological or pathological remodeling processes. Evidence related to how masticatory function shapes the craniofacial bones is discussed, and a proposal presented that the masticatory muscles and craniofacial bones serve as secretory tissues. We furthermore discuss our current findings of myokines-release from masseter muscle in physiological conditions, during functional adaptation or pathology, and their putative role as bone-modulators in the craniofacial system. Finally, we address the physiological implications of the crosstalk between muscles and bones in the masticatory system, analyzing pathologies or clinical procedures in which the alteration of one of them affects the homeostasis of the other. Unveiling the mechanisms of muscle-bone crosstalk in the masticatory system opens broad possibilities for understanding and treating temporomandibular disorders, which severely impair the quality of life, with a high cost for diagnosis and management.

Growth effects of botulinum toxin type A injected into masseter muscle on a developing rat mandible

OBJECTIVES

Botulinum toxin type A (BTX-A) reduces the muscular contractions by temporarily inhibiting the release of acetylcholine at the neuromuscular junction. The purpose of this study was to investigate the effects of the BTX-A injected into the masseter muscle of a developing rat mandible.

MATERIALS AND METHODS

Four-week-old male (no. 80) Sprague-Dawley rats were divided into four groups: control group, saline group, BTX-A group and baseline control group. Rats of baseline group were sacrificed at 0 day to provide baseline values of the mandibular measurements. The masseter muscle of rats in the saline and the BTX-A group were administered with saline and BTX-A solutions respectively. Experimental animals were sacrificed after 4 weeks.

RESULTS

The BTX-A group demonstrated smaller mandibular dimension compared with the other groups (P < 0.05). Their condylar cartilages showed increased apoptosis at the proliferation stage of the reserve zone and masseter muscle fibers demonstrated atrophic changes.

CONCLUSIONS

The result demonstrated BTX-A influence on inhibitory action of the developing mandible because of apoptosis at the proliferation stage of the reserve zone of the condylar cartilage in developing rat mandible.

Developmental expression of vascular endothelial growth factor in the masseter muscle of rats

Developmental changes in vascular endothelial growth factor (VEGF) in rat masseter after birth were investigated. VEGF was extracted efficiently and reproducibly from muscle homogenate with low concentrations of non-ionic detergents, such as Triton X-100, Nonidet P-40, and Tween 20. The amount of VEGF measured by enzyme-linked immunosorbent assay (ELISA) increased markedly by approximately 9-fold, from day 8 to 35 after birth. The increase in VEGF was closely correlated with the development of the capillary network, as shown by the capillary to muscle fibre ratio (C/F ratio). Immunoblotting revealed that the predominant molecular species of VEGF concentrated with heparin-sepharose beads was VEGF(188). These results suggest that VEGF plays an important part in the development and maintenance of the capillary network in the rat masseter.

Effects of assisted feeding on Wobbler mouse motoneuron disease and on serotonergic and peptidergic sprouting in the cervical spinal ventral horn

The Wobbler mouse is used as a model of human motoneuron disease (MND). During the disease progress, the significant loss of motoneurons in cervical spinal cord and cranial motor nuclei leads to the progressive loss of motor function in the forelimb, head, and neck regions. The loss of cutting and chewing ability that results in the inability to feed properly might lead to a lower mean body weight (b. wt.) that is generally one-half that of the normal phenotype littermate controls. Nutritional deficit might also influence neuronal processes sprouting in the cervical spinal ventral horn. To determine whether nutritional deficits contribute to the wt. loss, and influence the progress of MND as well as its sprouting phenomenon, Wobbler and normal phenotype control littermates were dropper-fed three times daily on a regular laboratory diet of Rat Chow. Weight measurements and behavioral tests were taken to monitor the disease. Immunocytochemisty of serotonin, substance P, and leucine enkephalin were conducted in the cervical spinal cord to investigate if any alteration occurred on the previously reported values in ad lib-fed animals. Organ wts. were measured to determine where nutritional benefit was incurred. Although mean wt. loss in Wobblers was reduced, wt. differed significantly from the control values after dropper feeding. However, the progress of the disease or alteration of neurotransmitters containing neuronal processes were not affected by nutritional factors. Therefore, nutritional intake affects wt. gain, but is not a primary consideration in the progress of MND. Behavioral deficits and neurotransmitter alterations are probably directly caused by motoneuron losses.

Effect of the beta 2-adrenergic agonist clenbuterol on the growth of fast- and slow-twitch skeletal muscle of the dystrophic (C57BL6J dy2J/dy2J) mouse

Clenbuterol (4mg/kg in diet for 21 days) had no statistically significant effect on whole body growth. It did cause a significant increase (18.2%) in wet weight of the fast twitch muscle extensor digitorum longus (EDL) and a corresponding 14.9% increase in total muscle protein. In transverse sections through dystrophic muscle fibre sizes were more variable than in normal muscle. Clenbuterol treatment resulted in a reduction in the proportion of small diameter fibres, and therefore an increase in mean fibre diameter, in dystrophic EDL. Clenbuterol had no significant effect upon the slow twitch muscle soleus.