Fine Structure of Connectin Nets in Cardiac Myofibrils

A net-like structure extending through Z lines of myofibrils in frog cardiac muscle was clearly demonstrated under an electron microscope after the removal of myosin and actin. The diameter of the very thin filaments forming the nets was approximately 2 nm; this width was thinner than that of the actin filament. The net structure is ascribed to connectin, an elastic protein of muscle.