Relationships among muscle fiber type composition, fiber diameter and <i><scp>MRF</scp></i> gene expression in different skeletal muscles of naturally grazing Wuzhumuqin sheep during postnatal development

<jats:title>Abstract</jats:title><jats:p>The aim of this study was to determine the relationships among muscle fiber‐type composition, fiber diameter, and myogenic regulatory factor (<jats:italic><jats:styled-content style="fixed-case">MRF)</jats:styled-content></jats:italic> gene expression in different skeletal muscles during development in naturally grazing Wuzhumuqin sheep. Three major muscles (i.e. the Longissimus dorsi (<jats:styled-content style="fixed-case">LD</jats:styled-content>), Biceps femoris (<jats:styled-content style="fixed-case">BF</jats:styled-content>) and Triceps brachii (<jats:styled-content style="fixed-case">TB</jats:styled-content>)) were obtained from 20 Wuzhumuqin sheep and 20 castrated rams at each of the following ages: 1, 3, 6, 9, 12 and 18 months. Muscle fiber‐type composition and fiber diameter were measured using histochemistry and morphological analysis, and <jats:italic><jats:styled-content style="fixed-case">MRF</jats:styled-content></jats:italic> gene expression levels were determined using real‐time <jats:styled-content style="fixed-case">PCR</jats:styled-content>. In the <jats:styled-content style="fixed-case">LD</jats:styled-content> muscle, changes in the proportion of each of different types of fiber (I, <jats:styled-content style="fixed-case">IIA</jats:styled-content> and <jats:styled-content style="fixed-case">IIB</jats:styled-content>) were relatively small. In the <jats:styled-content style="fixed-case">BF</jats:styled-content> muscle, a higher proportion of type I and a 6.19‐fold lower proportion of type <jats:styled-content style="fixed-case">IIA</jats:styled-content> fibers were observed (<jats:italic>P </jats:italic><<jats:italic> </jats:italic>0.05). In addition, the compositions of type I and <jats:styled-content style="fixed-case">IIA</jats:styled-content> fibers continuously changed in the <jats:styled-content style="fixed-case">TB</jats:styled-content> muscle (<jats:italic>P</jats:italic> < 0.05). Moreover, muscle diameter gradually increased throughout development (<jats:italic>P</jats:italic> < 0.05). Almost no significant difference was found in <jats:italic><jats:styled-content style="fixed-case">MRF</jats:styled-content></jats:italic> gene expression patterns, which appeared to be relatively stable. These results suggest that changes in fiber‐type composition and increases in fiber size may be mutually interacting processes during muscle development.</jats:p>