Some Topographical Observations on Fibrinolysis.

The distribution of fibrinolytic activity in tissues may be studied by the technique of fibrinolysis autography (Todd, 1958, 1959, 1960). This is a histological method in which a section of fresh tissue is incubated in contact with a thin layer of bovine fibrin. During incubation discrete zones of digestion appear in the fibrin, related presumably to concentrations of fibrinolytic enzymes in the overlying tissue. If the fibrin and tissue, still in contact, be fixed, stained and mounted for microscopy, the areas of digestion are seen as pale gaps in a uniform background of stained fibrin and can be correlated topographically with the structures in the overlying section (Fig. 1). The digestion must be due to plasminogen activator in the tissues since no fibrinolytic zones are seen if the fibrin is previously rendered plasminogen-free by heating according to the method of Lassen (1953). This technique reveals that most of the plasminogen activator in tissues is concentrated in the