The action of thrombin on blood platelets has been studied under the electron microscope. In control experiments with preparations treated with normal plasma or Tyrode fluid, the normal structure of platelets in their different forms of extension and flattening on the film has been observed.

Thrombin (Parke-Davis Co.) in concentrations of 25 and 50 units per ml. has a lytic effect on platelets. The number of platelets affected by this treatment increases in the period of 5 to 30 minutes.

Disintegration proceeds in the hyalomere from the periphery to the center with the formation of long fibrillar processes constituted by microvesicles. These processes resemble myelin figures. The granulomere turns into an amorphous dense body which also undergoes lysis. The final residue of disintegration is constituted by microvesicular material and dense amorphous bodies.

The purified bovine thrombin of Seegers has exactly the same action. The lytic effect is observed with concentrations of thrombin as low as 2.5 units per ml. The number of platelets undergoing disintegration increases proportionally to the logarithm of the thrombin concentration. The probable role of thrombin and the lysis of platelets in normal blood clotting is discussed.

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