Advancing human trophoblast biology : transcriptomic insights into progenitor cells, sex-related differences, and human trophoblast organoids

The human placenta is a specialized organ that develops during pregnancy. Forming from cells of the blastocyst, the placenta predominantly shares the genetic identity of the fetus and serves multiple functions that promote fetal health and development. Trophoblasts are specialized cells of the placenta that execute the majority of the placenta’s functions. These include the transfer of nutrients and gases between maternal and fetal circulations, remodelling components of the uterus to promote blood supply to the placenta, and the modulation of the maternal immune system to establish a level of immune tolerance to fetal and placental tissues. To perform these diverse functions, progenitor trophoblasts must differentiate into highly specialized cell types. These cellular processes are controlled by important spatial and cell-intrinsic cues that are only now partially understood. Applying single-cell transcriptomics to first trimester human placentas and human trophoblast organoids, my dissertation sets out to identify key regulatory processes controlling trophoblast progenitor cell maintenance and cell differentiation. Further, I aim to explore similarities between state-of-the-art organoid systems and placental trophoblasts. With these novel datasets, I generate a nearly comprehensive and publicly-available trophoblast-specific atlas of the first trimester human placenta. Using this resource, I identify Basal Cell Adhesion Molecule (BCAM) as a novel gene marker of progenitor trophoblasts in vivo and demonstrate that BCAM confers an organoid growth advantage in vitro. I also examine the contribution of chromosomal sex to gene signature differences in trophoblasts differentiating along the invasive extravillous trophoblast pathway. In doing this, I identify week 10 of gestation as a developmental timepoint in pregnancy where distinct patterns of XY and XX autosome gene expression initiate. Lastly, I execute a thorough evaluation of two state-of-the-art human trophoblast organoid cultures. This latter work identifies a unique progenitor cell state that is highly-enriched in trophoblast stem cell-based organoids and absent in primary trophoblast-derived organoids. Together, my dissertation advances our knowledge of human trophoblast biology by providing important transcriptomic insights into human trophoblast progenitor cells, sex-related differences in extravillous lineage trophoblasts, and state-of-the-art human trophoblast organoids.