Invasion of the trophoblasts

Mammalian embryonic development and growth require implantation into the uterus. In mammals that form a hemochorial placenta (e.g., humans, mice), embryonic trophoblast cells enable the embryo to invade through the uterine epithelium and deep into the stroma. The penetrative nature of hemochorial placentation so mimics that seen with highly invasive tumors that the normal trophoblast has been called pseudomalignant. Thus, in normal pregnancies the uterus must act to limit implantation; the uncontrolled invasion of the trophoblast, as in choriocarcinoma, results in one of the most metastatic tumors known. Recent work using both naturally occurring and experimentally generated mouse mutations has shed new light on implantation and its regulation. Present knowledge can be considered from the perspective of the two interacting tissues: the trophoblast and the uterus.