The mechanisms that lead to establishment of pregnancy and formation of a functional placenta during early human development are poorly understood due to ethical and technical constraints limiting research, yet this period is when many pregnancies fail and placental pathologies are most likely initiated. We cultured donated human embryos to days 8, 10, and 12 postfertilization and, for each stage, performed single-cell RNA sequencing on the trophoblast cells that compose the early placenta and enclose the embryo proper as it implants. Over the 5 d of culture, 2 cell lineages emerged from a progenitor stem cell population. One was syncytial and produced placental hormones. The second became motile and expressed genes associated with the innate immune system and invasion.
Single-cell RNA sequencing of cells from cultured human blastocysts has enabled us to define the transcriptomic landscape of placental trophoblast (TB) that surrounds the epiblast and associated embryonic tissues during the enigmatic day 8 (D8) to D12 peri-implantation period before the villous placenta forms. We analyzed the transcriptomes of 3 early placental cell types, cytoTB (CTB), syncytioTB (STB), and migratoryTB (MTB), picked manually from cultured embryos dissociated with trypsin and were able to follow sublineages that emerged from proliferating CTB at the periphery of the conceptus. A unique form of CTB with some features of STB was detectable at D8, while mature STB was at its zenith at D10. A form of MTB with a mixed MTB/CTB phenotype arose around D10. By D12, STB generation was in decline, CTB had entered a new phase of proliferation, and mature MTB cells had begun to move from the main body of the conceptus. Notably, the MTB transcriptome at D12 indicated enrichment of transcripts associated with IFN signaling, migration, and invasion and up-regulation of HLA-C, HLA-E, and HLA-G. The STB, which is distinct from the STB of later villous STB, had a phenotype consistent with intense protein export and placental hormone production, as well as migration and invasion. The studies show that TB associated with human embryos is in rapid developmental flux during peri-implantation period when it must invade, signal robustly to the mother to ensure that the pregnancy continues, and make first contact with the maternal immune system.
Author contributions: Z.J. and Y.Y. designed research; R.C.W., H.M., D.M.L, S.K.R., R.A.K., Z.J., and Y.Y. performed research; W.B.S. contributed resources; W.B.S. and R.L.K. contributed project administration; R.C.W., H.M., J.S., S.K.R., Z.J., and Y.Y. analyzed data; and R.C.W., H.M., D.M.L., R.M.R., R.L.K., Z.J., and Y.Y. wrote the paper.
Reviewers: G.J.B., University of Cambridge; S.J.F., University of California, San Francisco; and H.W., Chinese Academy of Sciences.
The authors declare no competing interest.
Published under the PNAS license.
Data deposition: Data have been deposited in the Gene Expression Omnibus (GEO) database,
https://www.ncbi.nlm.nih.gov/geo/ (accession no. GSE 130289).
1 R.C.W., H.M., and D.M.L. contributed equally to this work.
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This article contains supporting information online at www.pnas.org/lookup/suppl/doi:10. 1073/pnas.1911362116/-/DCSupplemental.