Thrombocytes, or platelets, are non-nucleated cells liberated into the circulation from megakaryocytes in the bone marrow. Thrombocytic cells are derived from a committed stem cell susceptible to the action of thrombopoietin and, in turn, derived from a pluripotential cell.
In contrast to other cell lineages in which multiplication (amplification) is accomplished by the successive duplication of DNA accompanied by cell division, megakaryocytes multiply their DNA (about four-times) without cytoplasmic division.
Amplification thus consists of polyploidization of the cell. The cells enlarge during amplification but maturation takes place almost exclusively after amplification is completed. Maturation includes lobulation of the nucleus, increase in cytoplasm, appearance of granules, and later, of platelet territories.
Four stages of maturation can be distinguished: basophilic, granular and, platelet-producing megakaryocytes, and platelets. The total maturation time is estimated to be 34 h. A 32 N megakaryocyte produces approximately 4,000 platelets.
Platelets measure 2-5 μm in diameter, they contain micro tubules, micro filaments, granules, vacuoles, canaliculi, mitochondria, and inclusions (glycogen). The granules are of several types: azurophilic granules, the content of which is still being debated.
Dense granules are storage sites of serotonin (5-hydroxytryptamine) which is made by the enterochromaffin cells of the intestine and picked up by the circulating platelets from the plasma. The dense granules also contain calcium and nucleotides.
Catalase has been identified in peroxisomes which are also small vesicles. Platelets secrete all of these substances as well as platelet factor 4 and fibrinogen during the “release” reaction. Platelets play a role in adhesion and aggregation.
They are capable of endocytosis. Various stimuli, e.g. contact to foreign surfaces, but also thrombin, proteolytic enzymes, bacterial endotoxin, and collagen, can trigger the release of their granules.