Morphological Signs of Intravital Contraction (Retraction) of Pulmonary Thrombotic Emboli

The goal of the study was to establish whether contraction (retraction) of thrombi and/or thrombotic emboli occurs in vivo using morphological signs of blood clot compression, such as characteristic deformation of erythrocytes and redistribution of fibrin toward the surface of a thrombus/embolus. Three postmortem human pulmonary thrombotic emboli were examined by scanning electron microscopy and light microscopy after staining with hematoxylin and eosin as well as with Mallory’s stain. In two pulmonary emboli, which were extracted at 7 and 15 h after death of the patients, compressed polyhedral erythrocytes (polyhedrocytes) were revealed that were formed due to mechanical deformation under the action of contractile forces generated by activated platelets. In addition, the uneven distribution of fibrin within the emboli was found with accumulation of fibrin at the periphery of the emboli, which is another structural characteristic of a contracted blood clot. In one of the three emboli analyzed, which was extracted 38 h after the patient’s death (the “oldest” embolus), the morphological signs of contraction were absent, which was likely related to the partial postmortem autolysis of the embolus or intravital impairment of contraction. The ex vivo thrombotic emboli have morphological signs of clot contraction, suggesting intravital compression of the primary thrombi and/or thrombotic emboli. The in vivo contraction of thrombi and emboli may be an important pathogenic mechanism for modulation of blood flow past otherwise obstructive clots at the sites of thrombotic occlusion of a vessel. The presence of compressed erythrocytes inside and the predominant location of fibrin around the periphery of a thrombus or embolus can potentially serve as additional pathomorphological criteria for the intravital contraction of thrombi and thrombotic emboli.