Expression of fiber type specific proteins during ontogeny of canine temporalis muscle

The canine masticatory muscles contain a unique adult fiber type composition and different contractile protein isoforms than do adult limb muscles. To determine when these characteristic proteins are expressed during development, samples from canine temporalis (masticatory) and pectineus (limb) muscles were compared between 55 days gestation and 60 days postpartum by histochemical, biochemical, and immunocytochemical analysis. At 55 days gestation and 3 days postpartum, both muscles contained identical histochemical type 2C fibers, native myosin isozymes, and myosin light and heavy chains. By 14 days postpartum, fiber-type expression in these muscles diverged, with resultant formation of type 1 and type 2M fibers in the temporalis muscle and type 1 and 2A fibers in the pectineus muscle. The distinctive myosin isoforms, light chains, and heavy chain of the temporalis muscle were also expressed 2 weeks postpartum. Based on the methods used in this study, we conclude that (1) the temporalis muscle develops from embryonic fibers that initially contain a myosin indistinguishable from embryonic limb muscle fibers, suggesting they have a common precursor, and (2) the myosin light chains and heavy chain unique to the temporalis muscle are initially expressed 2 weeks postpartum.