PRESENCE OF FUNCTIONALLY ACTIVE TISSUE PLASMINOGEN-ACTIVATOR IN HUMAN CFU-M DERIVED MEGAKARYOCYTES INVITRO

Human megakaryocytes directly obtained from bone marrow have been shown by fibrinolytic and immunoperoxidase studies to express tissue plasminogen activator (t-PA) activity and antigen. In the present study, megakaryocytes (MKs) derived from human megakaryocytic stem cells in vitro are also shown to have t-PA antigen at the ultrastructural level. MK colonies grown in human plasma and methylcellulose, picked on day 16, 17 or 18 of culture, were analysed for fibrinolytic activity and antigen localisation. When applied to fibrin films, these cells developed clear zones of fibrinolysis, which were totally inhibited in the presence of specific anti-t-PA antibodies. t-PA antigen was identified by immunoperoxidase in native cells as diffuse cytoplasmic staining. At the ultrastructural level, t-PA antigen was detectable in the perinuclear space, smooth reticulum or small vesicles near the Golgi complex (however Golgi cisternae or rough endoplasmic reticulum were not labelled). We have demonstrated, therefore, that as soon as t-PA antigen is detectable during megakaryocytic maturation, t-PA is capable of fibrinolytic activity. Our observations are also compatible with the synthesis of t-PA by MKs. © 1990.