Fetal myosin heavy chain increases in human masseter muscle during aging.

Biochemical, immunohistochemical and molecular biological methods were used to detect fetal myosin heavy chain (MyHC) in the human masseter of elderly and young subjects. Samples from the elderly subjects contained larger amounts of fetal MyHC than those of young adults. Only a very small amount of embryonic MyHC could be detected in both age groups. Embryonic and fetal MyHCs were never detected in the control adult orofacial, limb and trunk muscles. Polymerase chain reaction (PCR) analysis revealed the presence of fetal mRNA sequences in elderly and young masseter muscles. We conclude that fetal MyHC is present in the human masseter throughout the life span and that there is an increase in the relative amount of this protein with age.