- Objective To assess the participation of urokinase-type (uPA) and tissue-type (tPA) plasminogen activators in embryo development and implantation. Design The study was set to compare the activities of PAs in embryos developed in vivo and in vitro and their implantation rates. Endometrial PA activity was studied as well. Setting University laboratories. Patient(s) None. Intervention(s) None. Main outcome measure(s) Plasminogen activator activity in embryos. Result(s) Whereas tPA activity in in vivo-developed embryos decreased from the zygote to the blastocyst stages, it did not change in embryos developed in vitro. The activity of uPA was lowest in two-cell-stage embryos, both in vivo and in vitro, and increased as embryos developed into blastocysts. The activity of uPA in the in vitro-developed embroys was lower in all developmental stages as compared with those developed in vivo. Whereas endometrial tPA activity did not change during the preimplantation period, uPA activity increased gradually toward the time of implantation. The implantation rate of the in vitro-developed embryos was much lower than that of those developed in vivo. Conclusion(s) Taken together, these data indicate that in vitro embryo development leads to lower PA activities as compared with in vivo development. The increased activity of uPA toward the blastocyst stage supports uPA's role in the implantation process. Moreover, the rise in endometrial uPA might also indicate its importance in this process.