3107 – MULTI-ORGAN FUNCTIONS OF YOLK SAC DURING HUMAN EARLY DEVELOPMENT

Abstract

Background Model organisms have provided valuable insights into the functions of yolk sac (YS), a conserved extraembryonic structure which provides haematopoietic and metabolic support, but species-specific differences demand a systematic examination of human YS. Methods We profiled n=6 human YS and matched liver samples (CS14-CS23) by droplet-based scRNAseq and leveraged additional data sets on YS at gastrulation (CS7) and CS10-11, aorta gonad mesonephros (AGM) at CS14-17, fetal liver and bone marrow (BM). We incorporated data from a time course of induced pluripotent stem cells (iPSCs) undergoing macrophage differentiation. Results The proportional representation of YS cell states varied over gestational age. Progenitors and megakaryocytes were over-represented early, while dendritic cells and microglia-like cells emerged later. B-lineage cells (including B1 cells) were exclusive to matched liver, while NFIL3, ACY3-expressing innate lymphoid progenitors and PTGS2, MSL1-expressing pre-macrophages were exclusive to YS. CS7-11 YS progenitors expressed SPINK2 and HLF in common with later YS (CS14-23), AGM, liver and BM samples. Liver and BM HSPCs expressed HOXA7, PROM1 and HLA-DRA. Monocytes emerged in YS only after CS14. In force-directed graph embedding, there were two routes to macrophages; a monocyte-independent route prior to CS14 and a monocyte-dependent trajectory after CS14. These pathways were recapitulated during macrophage differentiation from iPSCs. YS endoderm and matched liver shared functions in lipid and glucose metabolism and coagulation. YS endoderm cells also expressed EPO and THPO, critical for erythropoiesis and megakaryopoiesis, and F3-activated coagulation pathway components. Conclusions These findings support the concept of the human YS providing multiorgan functions, which are subsequently subsumed by the liver, kidney and BM. Model organisms have provided valuable insights into the functions of yolk sac (YS), a conserved extraembryonic structure which provides haematopoietic and metabolic support, but species-specific differences demand a systematic examination of human YS. We profiled n=6 human YS and matched liver samples (CS14-CS23) by droplet-based scRNAseq and leveraged additional data sets on YS at gastrulation (CS7) and CS10-11, aorta gonad mesonephros (AGM) at CS14-17, fetal liver and bone marrow (BM). We incorporated data from a time course of induced pluripotent stem cells (iPSCs) undergoing macrophage differentiation. The proportional representation of YS cell states varied over gestational age. Progenitors and megakaryocytes were over-represented early, while dendritic cells and microglia-like cells emerged later. B-lineage cells (including B1 cells) were exclusive to matched liver, while NFIL3, ACY3-expressing innate lymphoid progenitors and PTGS2, MSL1-expressing pre-macrophages were exclusive to YS. CS7-11 YS progenitors expressed SPINK2 and HLF in common with later YS (CS14-23), AGM, liver and BM samples. Liver and BM HSPCs expressed HOXA7, PROM1 and HLA-DRA. Monocytes emerged in YS only after CS14. In force-directed graph embedding, there were two routes to macrophages; a monocyte-independent route prior to CS14 and a monocyte-dependent trajectory after CS14. These pathways were recapitulated during macrophage differentiation from iPSCs. YS endoderm and matched liver shared functions in lipid and glucose metabolism and coagulation. YS endoderm cells also expressed EPO and THPO, critical for erythropoiesis and megakaryopoiesis, and F3-activated coagulation pathway components. These findings support the concept of the human YS providing multiorgan functions, which are subsequently subsumed by the liver, kidney and BM.