Organized thrombus, femoral vein, H/E, x25
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If the thrombus is too large to be lysed by the fibrinolytic system, then organization takes place, analogous to the process of wound healing. The thrombus is often tightly adherent to the vessel wall, and fibroblasts and macrophages are mobilized to the area. Eventually, the vessel becomes recanalized due to the action of angiogenic factors, and small endothelial-lined channels are created throughout the collagenous scar. Collateral circulation begins to develop over this period of time, sometimes forming varicose veins. The process takes several weeks.