Like all other blood cells, thrombocytes originate in the bone marrow. A healthy adult aged between 30 and 70, weighing 70 kg, has about 1,300-1,500 g of thrombocytes. The stem cell is megakaryoblast, 21-50 µm in diameter. It gives rise to the promegakaryocyte and then the megakaryocyte - the largest cell in the bone marrow, 70-100 µm in diameter.

 The development of the megakaryocyte from the megakaryoblast takes 4-5 days, and their life-span is 7-10 days. There are 1/3 immature and 2/3 mature elements of megakaryocyte lineage in the bone marrow. Under normal conditions in the bloodstream, only thrombocytes exists. The number of thrombocytes in the peripheral blood is 150-350 x 10⁹/L. The organism can tolerate a large decrease in the thrombocyte number because spontaneous bleeding appears when their number falls below 20-30 x 10⁹/L. The states of transiently or permanently increased number of thrombocytes are called thrombocytosis. Apart from thrombocitopenia and thrombocytosis the following alterations can be found:

 anisothrombocytosis,
 aggregates of thrombocytes,
 megakaryocyte nuclei and
 megathrombocyte