[The characteristic phenotype of the masseter muscle fibers is established after birth].

Muscle biopsies were taken from the superficial portion of the M. masseter in 10 foetuses (aged between 12 and 38 weeks), in a child of 18 months and in an adult without any neuromuscular disease. Serial frozen sections were processed for immunocytochemistry using antibodies specific for the embryonic, foetal, slow and fast myosin heavy chains (MHCs). Diameter of the different types of fibers were measured with a Leitz ASM 68 K; the results have been expressed as average diameters and histograms. During foetal development, the masseter is formed from two successive generations of muscle fibers in a manner very similar to that which has been previously described for other skeletal muscles. After birth, a particular phenotype appears. This phenotype is characterised by the persistence of embryonic and foetal MHCs and by the presence of two distinct populations of fibers: small diameter fibers which coexpress embryonic, foetal and fast myosin isoforms but never express the slow MHC; large diameter fibers which express slow myosin either exclusively or in variable associations with the other isoforms.