Summary
Haemostasis in primitive organisms relies on simple mechanisms of wound re-pair. During evolution, haemostasis developed in parallel with rising metabolic activity, more complex circulatory systems, and increasing size of organisms. Thrombocytes are first found in lower vertebrates. However, these cells are nucleated and derive directly from haematopoietic progenitor cells, without an interposed megakaryocyte system. Megakaryocytes are an evolutionary innovation in mammalians and human beings. Their unique system of proliferation and differentiation provided the basis for an enormous increase in the production of the haemostatically active elements, the platelets. In contrast to nucleated thrombocytes, these fragments of megakaryocyte cytoplasm have improved haemostatic functions and show favourable rheologic features that facilitate their interaction with the vessel wall. The megakaryocyte system is susceptible to a variety of regulatory factors, in particular thrombopoietin and other haematopoietic and inflammatory cytokines. On the one hand, this enables the stem cell -megakaryocyte - platelet system to adapt its platelet production to increased haemostatic demand. On the other hand, acute or chronic overstimulation of megakaryocytopoiesis by infection, inflammation or malignant disease, can cause severe thromboembolic or thrombohaemorrhagic complications. Because these problems usually manifest themselves relatively late in life, namely after reproductive ages, they could not be eliminated during evolution.
Keywords
Nucleated thrombocytes - blood platelets - megakaryocytopoiesis - adhesion molecules - stem cells
