PATTERN OF MUSCLE-FIBER TYPE FORMATION IN THE PIG

The aim of this study was to analyze the temporal sequence of expressi on of the myosin isoforms in the populations of muscle fibers in the p ig and to bring more information on the origin of the strikingly diffe rent pattern of fiber composition and distribution between the deep me dial red (oxido-glycolytic) and superficial white (glycolytic) portion s of semitendinosus (ST) muscle. Muscle samples were taken from 49-, 5 5-, 75-, 90-, 103-, and 113- (birth) day-old fetuses, from 6-, 11-, 21 -, 35-, 50-, and 80-day-old piglets, and from a 3-year-old pig. Our re sults confirm the sequential formation of primary and secondary genera tion fibers. The use of immunohistochemistry and heterologous monoclon al antibodies (mAb) directed against specific myosin heavy chain (MHC) isoforms revealed a different pattern of gene expression between the two portions of the ST muscle for both generations of fibers. By 75 da ys of gestation (dg), primary myotubes from the deep medial portion st ained positively for the anti-slow MHC mAb and negatively for the adul t anti-fast MHC, whereas the opposite was observed in the superficial portion. Secondary fibers never expressed slow MHC until late gestatio n. Instead, they expressed an adult fast MHC isoform as soon as they f ormed in the deep medial portion and later on in the superficial porti on. From late gestation to the first 3 postnatal weeks, slow MHC began to be expressed in a subpopulation of secondary fibers. These fibers were in the direct vicinity of primary myotubes in the deep medial por tion, whereas their location could not be established in the superfici al portion. The remaining secondary fibers matured to type IIA in the direct vicinity of these type I fibers and to type IIB at the peripher y of the islets. In both portions of the muscle, a subpopulation of se condary fibers, the first ones to express slow MHC, also transitorily expressed a MHC that was identical or closely related to the cu-cardia c MHC during the early postnatal period. A third generation of small d iameter fibers was observed shortly after birth and reacted with the a nti-fetal MHC mAb; their destiny remains to be established. The presen t work reveals a remarkable pattern of MHC gene expression in the pig and raises many questions on the real nature of these isoforms. In ord er to answer these questions, we have undertaken to make a cDNA librar y of pig skeletal muscle and to screen this library with the same mAbs used in the present study. (C) 1995 Wiley-Liss, Inc.