L'implantation placentaire humaine

Like malignant tumor cells, cells of the trophoblast, of the blastocyst and of the hemochorial placenta are highly invasive. This invasiveness is essential for implantation and penetration of the epithelial and connective tissues of the endometrium. According to Hertig's data and the observations (henceforth classical) of the Carnegie Foundation, it is commonly admitted that trophoblastic cells penetrate rapidly the blood vessels of the maternal endometrium, open them and allow the establishment of a communication between maternal circulation and fetal tissues from the 20th day after fertilization. As suggested in the classical textbooks of Embryology, the hemochorial placentation would be established very early in human pregnancy. These observations (essentially histological) are actually contradicted by clinical observations performed by Doppler contact echography and by dynamic observations with a microvideo camera implanted in the intervillous placental chamber. Echographic observations coupled with the study of Doppler signals suggest the absence of placental maternal vascularization during the first trimester of pregnancy. It is possible to mount an optical system between the membranes and the uterine wall up to the placental tissue to observe the placental circulation. This direct observation concludes to the absence of maternal vascularization in the intervillous chamber during the first trimester of pregnancy. At the beginning of the 13th week of gestation, uterine arteries open directly into the intervillous chamber ensuring the direct perfusion of placental villi. These informations allow to suggest that the placenta becomes hemochorial at the beginning of the second trimester of pregnancy.