Blood clotting or coagulability and the opposite reaction, fibrinolysis, are in a subtle balance in the blood and can be shown to be present only at the end of the embryonic period.
Thrombocytes (platelets) play a central role in the clotting of blood. They are formed through the disintegration of megakaryocytes that can be found in the liver and spleen from the 12th week of pregnancy onwards. Later, the bone marrow takes over the production of megacaryocytes. After the middle of the pregnancy, the level of thrombocytes amounts to 250'000/mm3, which corresponds to an average normal value for adults. Up to birth, though, the thrombocytes remain still very immature in their functioning.
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