IN-SITU LOCALIZATION OF MUSCLE INSULIN-LIKE GROWTH FACTOR-II MESSENGER-RNA IN DEVELOPING BOVINE FETUSES

Insulin-like growth factor-II (IGF-II) modulates myogenesis in muscle cell cultures, in utero. IGF-II gene expression is developmentally reg ulated in several tissues including muscle. Determining whether IGF-II is expressed by developing muscle cells or by neighbouring cells in d eveloping muscle tissue is crucial for determining whether IGF-II exer ts a paracrine or an autocrine affect on myogenesis. Semitendinosus mu scle samples from 12 bovine fetuses ranging from 60 to 274 days post c onception (pc) were analysed for the amount and localization of muscle IGF-II mRNA using Northern, dot blot and in situ hybridization analys es. Northern blot analysis revealed multiple IGF-II transcripts of 5.1 , 3.7, 2.6, 2.0, 1.7 and 1.1 kb in developing bovine muscle tissue. Th e relative amount of muscle IGF-II mRNA increased (P<0.05) until 162 d ays pc, then decreased (P<0.01) to near undetectable levels by the end of gestation (approximately 284 days pc). Between 60 and 162 days pc, in situ hybridization revealed that the majority of the IGF-II transc ripts were localized to developing muscle cells rather than connective tissue. After 162 days pc the IGF-II hybridization signal shifted awa y from muscle cells and greater accumulation was observed in the conne ctive tissue at 274 days pc. These data confirm that the expression of IGF-II in developing bovine muscle tissue is primarily localized in m uscle cells and support the claim that IGF-II acts as an autocrine-act ing growth factor during myogenesis in vivo.