The organization of thrombi and emboli may be performed exclusively by mononuclear blood cells which represent precursors of various mesenchymal cell types. Between the 10th and 20th day after the onset of blood clotting, mononuclear cells within the fibrinous matrix differentiate into (1) macrophages responsible for hematoclasia and hematophagocytosis, (2) endothelial cells lining autolytic slits in the fibrinous matrix and forming new capillaries, and (3) fibroblasts and even smooth muscle cells building up a young mesenchymal connective tissue.