Organization of contractile units in smooth muscle based on stereological analysis of dense body distribution
Abstract
It is generally assumed that smooth muscle (SM) cells contract by a sliding filament mechanism based on the structure of a contractile unit (cu) comprised of thick myosin filaments (mfs) and thin actin filaments (afs), which are anchored to cytoplasmic dense bodies (dbs) or membrane dense plaques (dps). Since the structure of the contractile apparatus is not readily apparent in SM, as it is in striated muscle, stereological estimates of structural parameters have been developed based on specific assumptions. The feasibility of measurements was tested on electron micrographs (EMs) of SM cells from sequential rabbit ear arteries. It is tentatively concluded that the number and length of cus aligned across the cell can be determined by measurements of relative areas of dps and dbs, and the orientation of dbs and filaments.