Developmental transitions in the myosin heavy chain phenotype of human respiratory muscle

We studied the expression of myosin heavy chain (MHC) isoforms in the costal diaphragm (DIA) and the genioglossus (GG) muscles from 16 to 42 weeks gestation in the human using Western blotting techniques. Embryonic/neonatal MHC (MHC(emb/neo)) was the predominant isoform expressed in the DIA and GG at 16-24 weeks gestation. Subsequently, MHC(emb/neo) expression declined and the expression of MHC(slow) and MHC(ZA) increased. At term, the DIA MHC phenotype was a composite of MHC(emb/neo) (15 % of the total MHC complement), MHC(slow) (32%), MHC(2A) (47%), and MHC(2B) (6%); whereas, the GG was largely comprised of MHC(2A) (74%). We conclude that human DIA and GG demonstrate temporally dependent changes in MHC expression during gestation- and muscle-specific MHC phenotypes as they approach term.