Demonstration of Fibrin in Early Stages of Experimental Metastases

Tumor emboli were produced in lungs of Sprague-Dawley rats by i.v. injection of Walker 256 tumor cells into the tail vein. Tissues were examined by electron microscopy at periods from 30 sec to 72 hr after tumor injection. Two methods of conventional staining were used, in addition to immunoperoxidase techniques, with antifibrin antibodies produced in rabbits. Tumor cells accompanied by a platelet mass were seen in pulmonary arterioles at the earliest time period (30 sec). By conventional staining, small amounts of fibrin were detected within the platelet clumps by 5 min after inoculation. Periodicity indicating stable fibrin was not seen by this technique until 15 to 45 min. When peroxidase-labeled antibody was applied to tissue, sections showed fibrin-positive material at 30 sec, and periodicity of fibrin was detected by 5 min. Fibrin reached a maximum by both techniques at about 1 hr and disappeared, along with the platelets, at about 9 hr. When fibrinolysin was injected prior to the tumor cell inoculation, platelets and fibrin were either absent or present only in traces, and no stable fibrin was detected. These observations show that fibrin occurs very early in small amounts in association with tumor cell emboli, and is removed while the cells are still intravascular.