Syncytial nuclear aggregates as markers for villous maturation in placentas from preterm birth

Abstract

The syncytiotrophoblast is the multinucleated syncytial layer lining the placental villi. In some areas nuclei form aggregates termed true knots by the process of trophoblast turnover. Thereby, the underlying cytotrophoblast cells proliferate, differentiate and fuse with the overlying synctiotrophoblast cells which have no own generative potency. The nuclei in the syncytiotrophoblast are undergoing apoptosis and form true knots which are shed into the maternal circulation as syncytiotrophoblast microparticles (STBMs). This process ensures that the diffusion distance between maternal and foetal blood is decreased to nourish the growing foetus. Excessive formation of true knots has been reported in placentas of pregnancies complicated by pre-eclampsia. This is a complication of pregnancy characterized by increased maternal blood pressure and induced preterm labour is the only treatment. Pregnancies complicated by chorioamnionitis, a severe inflammation of the foetal membranes due to a bacterial infection, are also at increased risk of preterm labour. The aim of this study is to establish an objective scoring system for placental villous maturation based on true knots, STBMs and syncytial knotting frequency. Our assumption is that the frequency of syncytial knots in placentas complicated by pre-eclampsia are increased. True knots, STBMs and syncytial knotting are assessed by histological examination of placental tissue using a light microscope. In pre-eclamptic placentas the highest numbers of true knots, syncytial knotting and STBMs compared to an idiopathic preterm control group were found. Placentas complicated by chorioamnionitis show a slight decrease of true knots, syncytial knotting and STBMs compared to the idiopathic preterm group and a significant decrease compared to pre-eclamptic placentas. The increase of true knots and syncytial knotting could be a sign for an increased trophoblast turnover during pre-eclampsia. This implies enhanced proliferation, differentiation and apoptosis of the placental epithelium. Chorioamnionitis placentas display lower number of true knots indicating a lower trophoblast turnover which may be part of another anomaly of the placental development. For our scoring system are true knots, STBMs and syncytial knotting convincing markers for analysis of villous maturation in pre-eclampsia. In case of chorioamnionitis other markers may be more useful.