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To clarify changes in the muscle fibre properties of the temporal muscle related to the start of masticatory movement, we immunohistochemically investigated myosin heavy chain (MyHC) isoform protein expression using pre-weaning and post-weaning mice. In addition, we examined the expression of a gene coding for those MyHC proteins. Immediately after weaning, isoforms with fast and potent contractility were frequent. This suggests that the temporal muscle plays an important role in a marked functional change in the oral cavity from lactation to mastication, contributing to oral function in cooperation with other masticatory muscles.

Parafunctional habits, such as prolonged clenching and bruxism, have been associated with dysfunctional overloading in the temporomandibular joint ( TMJ). In this study, stress distributions in the TMJ were analysed during prolonged clenching, using three-dimensional finite element ( FE) models of the TMJ with and without disc displacement. The aim of this study was to investigate stress distribution of the cartilaginous tissues in the TMJ with and without disc displacement. Finite element models were developed on the basis of magnetic resonance images from two subjects with and without anterior disc displacement. Condylar movements recorded during a 5-min clenching were used as the loading condition for stress analysis. In the asymptomatic model, the highest von Mises stresses were located in the lateral area (4·91 MPa) of the disc surfaces, and after 5-min clenching, the higher stresses were still located at the lateral area (3·65 MPa). In all the cartilaginous tissues, 30-50% of stress reduction occurred during 5-min clenching. In contrast, the von Mises stress in the cartilaginous tissues of the symptomatic model with disc displacement was markedly lower, compared with the asymptomatic model. However, in the condylar cartilage, stress relaxation during clenching was not recognised. Furthermore, relatively high stresses were observed in the retrodiscal tissues throughout clenching. The present results indicate that disc position could be involved in the stress distribution of the TMJ components during prolonged clenching. [ABSTRACT FROM AUTHOR]

Summary Masticatory muscles that are derived from the branchial arches express different compositions of myosin heavy chain (MHC) isoforms during the transitional phase from suckling to mastication. To clarify the developmental changes of murine masseter muscle, the composition of MHC isoforms was examined using immunohistochemical staining and competitive reverse transcription PCR. We found that MHC1 was expressed transiently in the pre and post-natal stages. In the compositional change of isoforms, the embryonic type MHCp was expressed consistently, whereas the adult isoforms increased with the developmental process. In particular, a significant change was observed between embryonic days 14 and 16, a stage when murine facial development is conspicuous. This suggests that the development of murine masseter muscle is closely associated with facial development. [ABSTRACT FROM AUTHOR]