Functional human platelet generation in vitro and regulation of cytoplasmic process formation.

A recent report from this laboratory described an in vitro system in which CD34+ cells are stimulated to form mature megakaryocytes and, subsequently, cytoplasmic processes also known as pro-platelets that give rise to functional platelets. Thrombin, an important regulator of hemostasis, has been demonstrated to have an inhibitory role in cytoplasmic process formation from both human and guinea pig megakaryocytes. This inhibition can be reversed by antithrombin III (ATIII), an inhibitor of thrombin, in combination with heparin, a cofactor of ATIII and a glycosaminoglycan. Using the described human in vitro system, the role of thrombin and of glycosaminoglycans are investigated. Thrombin receptors are expressed on megakaryocytes, suggesting that the inhibition by thrombin may be direct. Matrigel, a basement membrane matrix containing glycosaminoglycans, is used to compare the frequency and the rate of cytoplasmic process formation. A possible role of glycosaminoglycans in platelet production is discussed.