Murine tongue muscle displays a distinct developmental profile of MRF and contractile gene expression

Few studies have addressed the molecular differences that exist between mus cles of the body and those of the craniofacial apparatus. In this study, we characterize the molecular events associated with determination and differ entiation of the tongue musculature. We assess the expression of myogenic r egulatory factors as well as the developmentally regulated myosin heavy cha in, (MHC), genes which serve as markers of differentiation. These results s uggest that tongue and limb muscle form by distinct molecular pathways. The myoblasts that contribute to the formation of the tongue preferentially ex press Myf-5 during myoblast determination rather than MyoD. Subsequently, i solated regions of myogenin expression mark the differentiation of first, t he small primary myofibers and later, the larger secondary myofibers. Analy sis of differentiation markers demonstrates that the tongue muscle also ass umes a unique profile of MHC expression as compared to that of the muscles of the body. Unlike the myoblasts of the developing limb, which express emb ryonic and neonatal forms of MHC and later express MHC-slow, the tongue myo blasts co-express MHC-embryonic, MHC-slow and MHC-fast isoforms from gestat ional age E12. Proteins for MHC embryonic and MHC fast isoforms are detecte d almost simultaneously. Interestingly, MHC-slow transcripts do not appear to be translated into a detectable MHC slow protein at any developmental st age assayed. These results provide further evidence to suggest that skeleta l tongue muscle represents a myoblast lineage that develops differently tha n the limb.