Background: Through their ability to invade endometrium, remodel the uterine spiral arteries, and sustain placental blood fluidity, trophoblast cells play a central role in establishing and maintaining the integrity of the uteroplacental vasculature. The expression of urokinase receptors by trophoblast may facilitate these processes by focusing plasminogen activator activity to discrete sites on the cell surface and promoting the activation of cell-bound plasminogen. However, although urokinase receptors are expressed by cultured trophoblast, the expression of these receptors by trophoblast in vivo has not been examined.
Experimental design: Immunohistochemistry and immunoelectron microscopy were used to characterize the expression of urokinase receptors by villous and extravillous trophoblast at several points in gestation.
Results: Urokinase receptors were expressed in a polarized fashion at the leading edge of migrating extravillous trophoblast cells. Receptors were also abundantly expressed during the first and second trimesters of gestation by villous trophoblast, where they were located on apical villous projections and within intracellular vacuoles, a subset of which were lysosomes.
Conclusions: The polarized expression of urokinase receptors by invasive extravillous trophoblast cells is consistent with a role for these receptors in mediating the extent and directionality of trophoblast migration. In contrast, the expression of urokinase receptors by villous trophoblast, which are not actively invasive in vivo, may serve to facilitate the generation of plasmin at the interface of these cells with maternal plasma, thereby limiting the deposition of fibrin within the placental intervillous spaces. Diminished urokinase receptor expression by villous trophoblast at term may represent a physiologic adaptation to diminish local fibrinolysis and limit hemorrhage at parturition.