Fibrin Clot Structure During Clotting as Observed in Unsupported Films Under Electron Microscope

Abstract

Clots were formed from thin films of solns. under various conditions of fibrinogen and thrombin concn., pH and clotting duration, and viewed directly as unsupported nets under the electron microscope. At a pH of 6.3, the mean fiber width at completion of clotting was independent of the thrombin concn. At shorter clotting durations, the fiber width was roughly a linear function of the logarithm of the trhombin concn. In such partially completed clots the fiber width was a linear function of the pH between 5.0 and 7.0. As a function of time, the fiber width increased steadily until a break with increased slope appeared at the moment of clotting in bulk. In many cases networks were formed by the action of the electron stain-fixative on the thin layer of soln. under conditions which produced no clots in bulk solns, i.e. low pH, low thrombin concn. and short clotting duration. These nets are good evidence of modification at a molecular level since solns. of fibrinogen without thrombin formed only homogeneous films. The unquestioned gross alteration in structure produced here throws some doubt on all structures observed under these conditions and it is concluded that actual sections from bulk clots will be required in order to quantitatively characterize the clotting products in ordinary solns.