AXIAL ARRANGEMENT OF CROSSBRIDGES IN THICK FILAMENTS OF VERTEBRATE SKELETAL-MUSCLE

Antibodies to subfragment-1 were used to label glycerinated rabbit psoas muscle in order to determine the axial arrangement of the crossbridges. In the electron microscope dense labeling is observed throughout the A-band except at the center and near the ends. There is a region in the center of the filament devoid of crossbridges as Huxley proposed; this bare zone is about 1500 .ANG. wide, and the crossbridges extend in a continuous array from the edge of the bare zone to the end of the filament except for a single gap between the 2nd and 3rd crossbridges from the end. The models of the thick filament proposed by Pepe and Squire are inconsistent with these observations.