Cellular Systems Biology of Trophoblast Heterogeneity in Human Early Placenta Development

Chorionic villi, the functional units of the human placenta, form the only inter- face between fetal and maternal systems facilitating exchange of nutrients, metabolic waste, and signaling factors. Complex and branched villi structures composed of cy- totrophoblast, extravillous trophoblasts, syncytiotrophoblasts, and stromal cells arise from a single epithelial layer of a blastocyst. How a single cell monolayer specifies and differentiates into the three trophoblast lineages during early development re- mains uncharacterized. Additionally, heterogeneity within each trophoblast lineage has described on the basis of varying cell morphologies but a wide scale investigation to characterize functional differences has not been conducted. In this thesis, I inves- tigate the cellular composition of the human placenta in order to characterize hetero- geneity within trophoblast lineages and how population dynamics change during the first trimester. Firstly, I identified dynamic trophoblast subpopulations through cell surface proteomics and single cell transcriptomics. A novel EpCAM positive popula- tion identified with greater expression before gestational week 6 is further explored as candidate progenitor marker. This population is more mitotically active than the respective negative population and does not express markers of terminally differentiated trophoblasts. Temporal information is required to investigate the role of these cell populations in the development of a placenta. This motivated me to create a cell atlas of first trimester development by integrating four individual single cell sequencing datasets. Single cell transcriptomics of over 65 000 cells covering placenta develop- ment from gestational week 2 to 12 suggests the existence of a transient bipotential progenitor during early first trimester compared to two separate developmental cen- ters for EVT and STB lineages in late first trimester. Work in this thesis highlights first trimester trophoblasts are much more heterogeneous than perviously believed and due to their rapidly dynamic nature, gestational age is an important consideration for future studies.Ph.D.2021-11-15 00:00:0