Simple SummaryThe muscles of mastication, providing the forces allowing us to chew our food properly, may be influenced by what we eat as well as growth. The aim of this study was to evaluate the expression of different molecules that contain the instructions to make the proteins necessary for the muscle fibres, as well as the size of the muscle fibres during growth. In order to do this, we used rats that were evaluated at two different ages corresponding to youth and to young adulthood. The size of the muscle fibres increased with age, as well as the type of proteins made, which favoured muscle fibres of the fast-twitch type with a high fatiguability. These types of fibres are built for short, powerful bursts of energy. These results give us insight into the normal development of the muscles used for chewing, which often consists of short, powerful strokes. This knowledge is important as it will allow us to compare different diets to see how diet can influence muscle characteristics, as well as diseases that may affect muscle characteristics.AbstractDifferent demands on the muscles of mastication may influence their functional profile (size and distribution of muscle fibre types), which may change during growth and maturation, potentially influencing craniofacial growth. The aim of this study was to evaluate mRNA expression and cross-sectional area of masticatory muscle fibres compared with limb muscles in young and adult rats. Twenty-four rats were sacrificed at two different ages, namely 12 at 4 weeks (young) and 12 at 26 weeks (adult). The masseter, digastric, gastrocnemius and soleus muscles were dissected. Gene expression of myosin heavy-chain isoformsMyh7(MyHC-I),Myh2(MyHC-IIa),Myh4(MyHC-IIb) andMyh1(MyHC-IIx) in the muscles was measured using qRT-PCR RNA analysis, and immunofluorescence staining was performed to measure the cross-sectional area of different muscle fibre types. Different muscle types and ages were compared. Significant differences were found in the functional profile between masticatory and limb muscles. For the masticatory muscles, there was an increase inMyh4expression with age, and this change was more intense for the masseter muscles, which also presented an increase inMyh1expression, similarly to limb muscles. The fibre cross-sectional area of the masticatory muscles was generally smaller in young rats; however, this difference was less pronounced than in limb muscles.