Ultrastructure of clots during isometric contraction.

Abstract

We explored the retraction or contraction of platelet-fibrin clots under isometric conditions . In the presence of micromolar calcium clots of normal platelet-rich plasma developed tension at an initial rate of 0 .1 to 0.2 g/min per cm2 (initial cross-sectional area) . Electron microscopy of clots fixed after attaining a force of 1 .6 g/cm 2 revealed platelets with elongated bodies and pseudopods in close apposition to fibrin strands which were oriented in cablelike fashion in the direction of tension . The development of tension could not be explained simply on the basis of platelet-platelet association and interaction alone. First, factor XIII-dependent cross-linking of fibrin fibers was critical to normal isometric contraction . Second, tension decreased linearly, rather than exponentially, when the platelet count in the platelet-fibrin clot was decreased, suggesting that platelets must be interacting with another component (i .e . fibrin) . Thrombasthenic platelets, deficient in fibrinogen receptors, failed to develop tension or to align fibrin strands or pseudopods in the clot. Platelet-fibrin clots treated with vincristine to disassemble microtubules or cytochalasin B to disrupt microfilaments failed to develop tension and relaxed if these agents were added after tension had developed . Relaxation under these conditions, however, was not associated with loss of orientation of fibrin strands . Our findings suggest that platelet-fibrin interaction in clots under isometric conditions leads to orientation of fibrin strands and platelets in the direction of force generation . Tension develops as platelets simultaneously attach to and spread along fibrin strands, and contract . The contraction draws some fibrin into platelet-fibrin clumps and aligns other strands in the long axis of tension . The achievement and maintenance of maximum tension appears to depend on the development of platelet-fibrin attachments and extension of platelet bodies and long pseudopods containing bundles of microfilaments and microtubules along the oriented fibrin fibers .The phenomenon of clot retraction has been recognized for more than two hundred years (20). Physiologically, clot retraction may enhance clot lysis and facilitate recanalization of blood vessels (6) . Mechanistically, clot contraction remains poorly understood, though in platelet-rich plasma it has been clearly shown to result from the interaction of a static, nonmotile fibrin mesh and dynamic, actively motile blood platelets (5,12,13,29,39) . Concepts concerning interaction of these two components can be classed into three general proposals . The first suggests that platelets reach out with their pseudopods to combine with and attach to strands of fibrin. Pseudopods then contract, pulling the fibrin mesh toward the platelets . According to one variation of this concept, the center of the platelet may be an anchoring point for pulling in the fibers (35,43) . Another modification proposes that contraction is related to THE JOURNAL OF CELL BIOLOGY " VOLUME 93 JU...

Fibrin Clot Retraction by Human Platelets Correlates with αIIbβ3Integrin-dependent Protein Tyrosine Dephosphorylation

“…Cellular matrix retraction corresponds macroscopically to the reduction of the matrix volume, due to the activity of cells that actively reorganize the extracellular matrix by shortening and thickening matrix fibers (3). Matrix contraction is exhibited by numerous cell types, including smooth muscle cells, fibroblasts, monocytes, endothelial cells, or platelets.…”

Functional significance of mobile receptors on human platelets.

“…42 Under these conditions, actin polymers are generating isometric tension to hold the spread cell to the surface and do not shorten. 43 The findings suggest existence of a force-generating mechanism within the plasma membrane and membrane cytoskeleton capable of driving mobile receptor/ligand complexes across the surface membrane separate from and largely independent of cytoplasmic actin. 43 Next page, Figs 13 through 18 Platelets in suspension cleared small latex spherules (0.09 to 3.13 fim) with relative ease.…”

“…43 The findings suggest existence of a force-generating mechanism within the plasma membrane and membrane cytoskeleton capable of driving mobile receptor/ligand complexes across the surface membrane separate from and largely independent of cytoplasmic actin. 43 Next page, Figs 13 through 18 Platelets in suspension cleared small latex spherules (0.09 to 3.13 fim) with relative ease. The discoid cells translocated latex across the membrane to channels of the OCS without undergoing significant shape change.…”

The Platelet Integrin αIIbβ3 Differentially Interacts with Fibrin Versus Fibrinogen

“…Thus, much less is known about the former interaction. However, differences in the ability of ␣IIb␤3 antagonists to inhibit clot contraction versus platelet aggregation suggest that the interactions of ␣IIb␤3 with fibrinogen and fibrin are different (13)(14)(15)(16)(17)(18). The sites at which ␣IIb␤3 interacts with fibrin and fibrinogen also appear to be substantially different.…”