FUNCTIONAL HUMAN PLATELET GENERATION IN-V ITRO AND REGULATION OF CYTOPLASMIC PROCESS FORMATION

A recent report from this laboratory described an in vitro system in w hich CD34(+) cells are stimulated to form mature megakaryocytes and, s ubsequently, cytoplasmic processes also known as proplatelets that giv e rise to functional platelets. Thrombin, an important regulator of he mostasis, has been demonstrated to have an inhibitory role in cytoplas mic process formation from both human and guinea pig megakaryocytes. T his inhibition can be reversed by antithrombin III (ATIII), an inhibit or of thrombin, in combination with heparin, a cofactor of ATIII and a glycosaminoglycan. Using the described human in vitro system, the rol e of thrombin and of glycosaminoglycans are investigated Thrombin rece ptors are expressed on megakaryocytes, suggesting that the inhibition by thrombin may be direct. Matrigel, a basement membrane matrix contai ning glycosaminoglycans, is used to compare the frequency and the rate of cytoplasmic process formation. A possible role of glycosaminoglyca ns in platelet production is discussed.