Regular Organization of Thick Filaments in Mammalian Smooth Muscle

Abstract

THE thick filaments in vertebrate smooth muscle, demonstrated by recent electron microscopy^1–5, are dispersed among denser random arrays of thin actin filaments^1,4,5 which correspond to those originally detected by X-ray diffraction⁶. The characteristic 144 Å meridional X-ray reflexion of myosin has been detected in smooth muscle and assigned to the thick filaments⁷, and it has been suggested on the basis of electron microscopy that ribbons of uniform width and irregular length are the preferred form of organization of myosin in mammalian smooth muscle and that only occasional thick filaments can be found in well preserved tissues. In our experience, however, such ribbons are shrinkage artefacts (see following article by Somlyo et al.⁸).