Altered trophoblast functions in implantation-defective mouse embryos

Extracellular proteases, such as plasminogen activator (PA), may play a role in the invasive action of trophoblasts during blastocyst implantation in mice. Detailed analysis of proteases released by trophoblasts must be carried out in culture for practical reasons, but the in vitro results should ideally be related to implantation in utero to test the validity of the conclusions in the model system. The implantation-defective mutant (tw73) allows us to investigate alterations of trophoblasts in culture which will reflect their altered functions in utero. The trophoblasts of mutant embryos attached and proliferated in culture to the same extent as control embryos. Therefore, the mutant effect is not to kill or stop the division of these cells. However, trophoblasts of tw73 homozygotes showed decreased invasive ability when transplanted into testes. To examine the biochemical basis of altered invasive behavior in mutants, PA activity released by blastocysts in culture was measured. Medium conditioned by tw73 homozygotes contained less PA fibrinolytic activity than culture medium of control embryos. The correlation between decreased in vitro PA activity and the invasive defects seen in utero and ectopically, supports the involvement of PA in the invasive phase of the implantation process.