Megakaryocyte and platelet structure in thrombocytopoiesis: the effect of cytokines

This paper presents an overview of selected data which the author considers crucial to an understanding of structure/function relationships of megakaryocytes (MK) and platelets. The observation that platelet territories form within the MK cytoplasm and that, therefore, MK and platelet plasma membranes need not be structurally or antigenically identical is substantiated on the basis of results obtained with a variety of experiments. While the predominant site of MK fragmentation is still debated, it is generally accepted that such terms as "proplatelets," "giant platelets" or "megathrombocytes" refer to MK fragments consisting of more than one platelet territory. It is suggested that such fragments be called "compound" platelets to convey a unifying concept. The terms "young" or "immature" could be reserved for platelets which still contain ribosomes, rough endoplasmic reticulum or other organelles not usually seen in circulating platelets. Finally, the structure changes induced by cytokines, such as interleukin 3 (IL-3), IL-6, IL-11 and thrombopoietin have been illustrated.