Smooth muscle is a machine consisting of working and supporting elements whose structure and 3D organization must be elucidated for the mechanics of shortening and tension generation to be understood. Based on longitudinal and serial transverse sections of rabbit portal vein it was suggested that the contractile elements of smooth muscle formed “mini-sarcomeres”, analogous to skeletal muscle, containing parallel arrays of 3-5 myosin filaments 1.6-2.2 um long. Observations at the light microscopic level were consistent with this idea. The past decade has seen little further investigation into the in situ ultrastructure of this or other smooth muscles, and the general applicability of these findings remains unknown. We have taken advantage of recent methodological advances, which can provide full 3D computer representations of cellular organization based on EM data, using guinea pig taenia coli muscle as a model system.

Serial transverse sections (Fig 1) were used to generate 3D reconstructions of the organization of the myosin filaments and their relation to dense bodies, actin bundles, mitochondria and other organelles.