Stress fibers in the splenic sinus endothelium in situ: molecular structure, relationship to the extracellular matrix, and contractility.

In the present study, we investigated structural and functional aspects of stress fibers in a cell type in situ, i.e., the sinus endothelium of the human spleen. In this cell type, stress fibers extend underneath the basal plasma membrane and are arranged parallel to the cellular long axis. Ultrastructurally, the stress fibers were found to be composed of thin actin-like filaments (5-8 nm) and thick myosin-like filaments (10-15 nm X 300 nm). Actin filaments displayed changes in polarity (determined by S-1-myosin subfragment decoration), which may allow a sliding filament mechanism. At their plasmalemmal attachment sites, actin filaments exhibited uniform polarity with the S-1-arrowhead complexes pointing away from the plasma membrane. Fluorescence microscopy showed that the stress fibers have a high affinity for phalloidin and antibodies to actin, myosin, tropomyosin, and alpha-actinin. Vinculin was confined to the cytoplasmic aspect of the plasmalemmal termination sites of stress fibers, while laminin, fibronectin, and collagens were located at the extracellular aspect of these stress fiber-membrane associations. Western blot analysis revealed polypeptide bands that contained actin, myosin, and alpha-actinin to be major components of isolated cells. Exposure of permeabilized cells to MgATP results in prominent changes in cellular shape caused by stress fiber contraction. It is concluded that the stress fibers in situ anchored to cell-to-extracellular matrix contacts can create tension that might allow the endothelium to resist the fluid shear forces of blood flow.