Changes in the properties of mouse tongue muscle fibres before and after weaning.

The purpose of this study was to clarify any changes in muscle fibre properties in different regions of murine tongue during development, and to assess the effects of functional changes including weaning on these muscle fibres. The tongue was divided into upper and lower regions at the lateral margin, and the expression of myosin heavy chain (MHC) isoforms at different ages was investigated. Expression of genes encoding MHC proteins was quantified at the transcription level by quantitative reverse transcriptase polymerase chain reaction, and the protein expression of MHC isoforms was assessed by immunostaining. No difference was found in isoform expression between the upper and lower regions of the tongue before weaning. However, the expression of MHC-2b increased markedly in both regions after weaning, while that of MHC-2a decreased. At the age of 16 weeks, the expression of MHC-2b in the lower region was greater than that in the upper region. These findings show that during weaning, when there is a shift from sucking behaviour to mastication, the expression of MHC-2b increases along with an increase in the speed and strength of muscle contraction. Also, contraction force becomes stronger in the lower region of the tongue than the upper region at the age of 16 weeks.