Desmin and vimentin are intermediate filaments that play crucial roles in maturation, maintenance and recovery of muscle fibers and mesenchymal cells. The expression of these proteins has not been investigated extensively in human fetuses. In the present study, we examined the immunohistochemical expression of intermediate filaments in skeletal muscles of the head, neck and thorax in 12 mid-term human fetuses at 9-18 weeks of gestation. We also used immunohistochemistry to localise expression of the myosin heavy chain, and silver-impregnation to identify the fetal endomysium.Expression of desmin and vimentin was already detectable in intercostal muscle at 9weeks, especially at sites of muscle attachment to the perichondrium. At this stage, myosin heavy chain was expressed throughout the muscle fibers, and the endomysium had already developed. Beginning with punctate expression, the positive areas became diffusely distributed in the muscle fibers. At 15-18 weeks, intermediate filament proteinswere extensively expressed in all of the muscles examined. Expression at the bone-muscle interface was continuous with expression along the intramuscular tendon fibres. These results suggest that the development of intermediate filaments begins in areas of mechanical stress due to early muscle contraction. Their initially punctate distribution, as observed here, probably corresponds to the earliest stage of fetal enthesis formation.

Mammals exhibit marked morphological differences in the muscles surrounding the jaw bone due to differences in eating habits. Furthermore, the myofiber properties of the muscles differ with function. Since the muscles in the oral region have various functions such as eating, swallowing, and speech, it is believed that the functional role of each muscle differs. Therefore, to clarify the functional role of each masticatory muscle, the myofiber properties of the adult mouse masticatory muscles were investigated at the transcriptional level. Expression of MyHC-2b with a fast contraction rate and strong force was frequently noted in the temporal and masseter muscles. This suggests that the temporal and masseter muscles are closely involved in rapid antero-posterior masticatory movement, which is characteristic in mice. Furthermore, expression of MyHC-1 with a low contraction rate and weak continuous force was frequently detected in the lateral pterygoid muscle. This suggests that, in contrast to other masticatory muscles, mouse lateral pterygoid muscle is not involved in fast masticatory movement, but is involved in functions requiring continuous force such as retention of jaw position. This study revealed that muscles with different roles function comprehensively during complicated masticatory movement.

Craniofacial growth is influenced by the interaction of muscle and bone tissues. The medial pterygoid (MP) is one of the muscles of mastication attached to the mandible. The purpose of the study was to investigate the relation between the MP and mandible during embryogenesis. Specimens were prepared from fifty fetal mice at embryonic day (ED) 12.5, 13.5, 14.5 and 15.5. Slides were stained with hematoxylin and eosin (H&E) and observed under the light microscope. Immunohistochemistry using desmin, a muscle specific marker, as well as vimentin, a mesenchymal specific marker, were also carried out. Desmin and vimentin mRNA expressions were analyzed by quantitative polymerase chain reaction. Results showed that at ED 12.5, the MP and lateral pterygoid (LP) and tensor veli palatine (TVP) were adjacent to one another. Over time, the LP moved more externally while the TVP moved more medially. At ED 13.5, the mandible started to form while the MP moved towards the developing mandible. At ED 14.5, the palatine shelves were also seen in a horizontal position. Over time, desmin localization was observed at myotendinous junctions in between the MP and Meckel's cartilage (MC) as well as in between the MP and mandible and finally in the center of the muscle. The increase in the expression was supported by desmin mRNA expression. Vimentin was widely distributed in mesenchymal cells with no significant difference among the stages. The results suggest that although the muscles of mastication were still immature, the premature contraction of the MP and the positional relationship provide a dynamic change between the development growth of the mandible and the start of the fusion of the secondary palate.