[The formation, metabolism and the evolution of blood platelets]

Platelets are the smallest, depleted of nucleus blood cells which contain a typical cellular organelles including the mitochondria, so that have active metabolism. Platelets possess the highly organized cytoskeleton, specific secretory granules and unique membrane receptors system responsible for their high reactivity. The key role of blood platelets is to maintain normal hemostasis, but they also play important roles in inflammation, immune processes and the cancer progression. The anucleated, small platelets occur in representatives of all clusters of mammals, so it seems to be an adaptation feature. In other vertebrates similar hemostatic functions are played by large nucleated platelets, which are much more weakly reactive. Small, reactive platelets, appearing in the evolution of mammals, allowed the formation of clots faster and slower blood loss in case of injury, but also increased the risk of thromboembolic and cardiovascular diseases. Daily the human body forms about 1x10¹¹ platelets, which are produced by a process of differentiation, maturation and fragmentation of the cytoplasm of mature megakaryocytes. The emergence of platelets is the final stage of megakaryocyte differentiation and is followed by formation of the direct precursors called proplatelets. The anucleated platelets are regarded as terminally differentiated cells, which are not capable of further cell division. However, despite the absence of a nucleus, in blood platelets the synthesis and transcription of mitochondrial DNA and protein synthesis occurring on the basis of mRNA from megakaryocytes has been confirmed. However, recent studies published in 2012 show that the platelets are capable not only of the process of protein synthesis, but also of generation of new cells, which are functionally and structurally similar to the parent platelets.