[An experimental study on muscle regeneration--formation of myotubes and environment]

The repair of injured muscle is completed by a proliferation and differentiation of myogenic cells and myotubes. However, little is known about the microenvironment in which the formation of myotubes can proceed in vivo. We employed the polyvinyl alcohol (PVA) sponge model to investigate the milieu in which myotubes could be differentiated. Small pieces of PVA sponge were implanted after immersion in physiological saline into the gastrocnemius of adult Wistar rats as controls. In the experimental groups, before implantation, PVA were treated with basic fibroblast growth factor (FGF), type IV collagen, laminin and hydrocortisone. Newly-formed tissues within the PVA were examined immuno- and histochemically under light- and electronmicroscopy after 7 to 10 days implantation. Particular attention was focused on the formation of myofilaments in the migrating cells. From this study, it became clear that FGF, collagen and laminin accelerated the migration of mesenchymal cells into the PVA compared with controls. However, myotube formation could not be detected in either the experimental or control groups. But in the specimen treated with 10mg/ml hydrocortisone, myotubes appeared frequently in the migrating cells of PVA. A small amount of fibroblasts, macrophages and eosinophils were scattered around the myotubes and not clustered. They were provided with undefined basal lamina. Fibronectin and collagen fibers were also detected surrounding the myotubes. These findings suggested that, although the migration of satellite cells and the appearance of fibronectin are prerequisites for myotube formation, the most promising condition for myotube formation involved a suppression in the migration of the fibroblasts, macrophages and eosinophils. A certain concentration of steroids can therefore prepare a favorable microenvironment for this process.