Transition of myosin isozymes during development of human masseter muscle. Persistence of developmental isoforms during postnatal stage.

Previous results have shown that the adult human masseter muscle contains myosin isoforms that are specific to early stages of development in trunk and limb muscles, i.e. embryonic and fetal (neonatal) myosin heavy chains (MHC) and embryonic myosin light chain (MLC1emb). We wanted to know if this specific pattern is the result of a late maturation or of a distinct evolution during development. We show here that the embryonic and the fetal MHC and the MLC1emb are expressed throughout perinatal and postnatal masseter development. Our results also demonstrate that MLC1emb accumulation increases considerably during the postnatal period. In addition, both the slow MLCs and the slow isoform of tropomyosin are expressed later in the masseter than quadriceps and the fast skeletal muscle isoform MLC3 is not detected during fetal and early postnatal development in the masseter whereas it is expressed throughout fetal development in the quadriceps. Our results thus confirm previous histochemical data and demonstrate that the masseter muscle displays a pattern of myosin and tropomyosin isoform transitions different to that previously described in trunk and limb muscles. This suggests that control of masseter muscle development involves mechanisms distinct from other body muscles, possibly as a result of either its craniofacial innervation or of a possibly different embryonic origin.