Ultrastructural configuration of sarcomeres in passive and contracted frog sartorius muscle

The structural configuration of passive and contracted sarcomeres from frog sartorius muscle was determined for sarcomere lengths between 1.4 and 3.07 microns. Sarcomeres fixed with glutaraldehyde, without detectable contractile activity, were invariably rectangular in shape in longitudinal section regardless of sarcomere length. The structure of sarcomeres excited by osmium tetroxide and contracted depended upon sarcomere length. For lengths greater than 1.6 microns, longitudinally sectioned sarcomeres were convexoconvex (or barrel-shaped). At 1.6 microns, contracted sarcomeres were invariably rectangular in shape. Below 1.6 microns, sarcomeres were compressed and appeared concavoconcave. The results of this study provide models for isovolumetrically contracted sarcomeres which differ significantly from those previously reported.