A histochemical study of myofiber types in muscle of the growing pig.

The conversion of myofiber types was studied in the masseter, trapezius, longissimus, rectus femoris and vastus intermedius muscles of pigs at birth and 2, 4, 8 and 16 weeks of age. The histochemical reaction for ATPase was used to classify myofibers as type I (acid-stable, alkali-labile) or type II (alkali-stable, acid labile). Type II myofibers were divided further into categories IIA and IIB. Subtypes SM, SS and MS were intermediate to types I and II and were classified on the basis of pH sensitivity of the ATPase reaction. In the longissimus, rectus femoris and masseter muscles, the proportion of type II myofibers decreased and the proportion of type I myofibers increased from birth to 8 weeks of age, and little change occurred thereafter. These three muscles had more type II than type I myofibers, regardless of age. At birth the trapezius and vastus intermedius muscles both and more type II than type I myofibers, but during development, the proportion of type I myofibers increased greatly while that of type II decreased markedly. At 16 weeks, the trapezius had about equal proportions of type II and type l myofibers, but the vastus intermedius had a much higher proportion of type I than type II myofibers. A flow diagram was developed to illustrate the proposed conversion of type II to type I myofibers via the intermediate stages identified as subtypes MS, SS and SM. Development of the type I grouped pattern and the association of myofiber proportions with functions in the masseter muscle are discussed.