Changes in the myosin heavy chain 2a and 2b isoforms of the anterior belly of the digastric muscle before and after weaning in mice.

During the process of growth and development, the digastric muscle is subjected to marked functional changes, including the change from suckling to mastication. In particular, because the anterior belly of the digastric muscle, which is one of the suprahyoid muscles, plays an important role in mastication. Therefore, this muscle seems to undergo a marked functional change before and after weaning. However, the details remain unknown. Here, to clarify the changes in the muscle fibre characteristics of the anterior belly of the digastric muscle before and after weaning, we examined myosin heavy chain isoforms at the protein (immunohistochemistry) and mRNA (transcription) levels. As a control, the changes in the muscle fibre characteristics of the sternohyoid muscle, which is anatomically aligned in the same direction as the anterior belly of the digastric muscle, were analyzed. The results showed that, in the anterior belly of the digastric muscle that is involved in mandibular movements in mice, the ratio of a fast-contraction isoform with strong contractile force increased after weaning. We believe that this occurred in response to a functional change from suckling to mastication. On the other hand, there was little change in the composition of sternohyoid muscle.