Characterizing a model of early human placenta

Abstract

In the few weeks following fertilization of a human ovum, there is little known about the events of early human development. What we do know we've learned from a few collections of fixed sections of implantation sites acquired in the 1950s. Much has been learned about the structures of the embryo formed during this time, but any experimentation to determine function is impossible. This is not only due to ethical restrictions but also due to the fact that women are unaware of when implantation occurs. What has been observed from these collections is that few changes occur within the inner cell mass, or embryo compartment, compared to the placental lineage or trophoblast which undergoes extreme morphological changes that take place during these crucial few weeks. The function behind these morphological changes probably involves establishing an anchoring and nutrient and gas transfer system very early. Recurrent implantation failure is a common source of infertility and the cause is often unknown. The causes of diseases of pregnancy such as preeclampsia or intrauterine growth restriction are to this date unknown and it is possible that it begins early during the implantation window. It is for the above reasons that a suitable in vitro model of early human placental development be created to study this important time period. Classically, it is thought that primed-type human embryonic stem cells resemble a post-implantation time point when the trophectoderm has already differentiated and therefore these cells cannot give rise to the trophoblast lineage. However, as shown here and in much work done beyond what is contained here, it does appear that treatment of hESCs with BMP4, an ACTIVIN/NODAL inhibitor (A83-01), and an FGF receptor inhibitor (PD173074) gives rise to cells entirely made up of cells positive for common trophoblast markers (KRT7, CGA, CGB, HLAG, GATA3, TEAD4, CDX2, to name a few) and markers of mesoderm, a common result of stem cell treatment with BMP4, or other lineages do not occur. Growing evidence is showing that the type of trophoblast cells that result from BAP treatment are not perfectly analogous to the villus type placenta. In chapter II, I show that an invasive extravillous trophoblast marker ITGA1 is most highly expressed by syncytiotrophoblast in our model, indicating that the syncytiotrophoblast in this model is invasive. The goal of my work is to understand what stage of placental development this model replicates. Contained in chapter III, we show that the BAP model does appear to replicate a post-implantation early first trimester timepoint. We also utilized this model as a discovery tool to identify highly expressed genes and identified that these genes (GABRP, WFDC2, VTCN1, and ACTC1) are also expressed in the first trimester and specifically expressed by cytotrophoblast, syncytiotrophoblast, and extravillous trophoblast. Finally, in chapter IV, I delve deeper into one of the genes of interest, ACTC1. Preliminary results demonstrate that this structural actin protein may play an important role in syncytialization of cytotrophoblasts in our model. Overall, many more studies are required to confirm these results and the prospects of developing a model of early placental development in vitro from primed type embryonic stem cells is an exciting avenue of science that will hopefully result in discoveries that help us understand more about this black box in human placental development.