In:
Science, American Association for the Advancement of Science (AAAS), Vol. 381, No. 6659 ( 2023-08-18)
Abstract:
The yolk sac (YS) generates the first blood and immune cells and provides nutritional and metabolic support to the developing embryo. Our current understanding of its functions derives from pivotal studies in model systems, and insights from human studies are limited. Single-cell genomics technologies have facilitated the interrogation of human developmental tissues at unprecedented resolution. Atlases of blood and immune cells from multiple organs have been greatly enhanced by focused, time-resolved analyses of specific tissues. RATIONALE To characterize the functions of the human YS, we performed single-cell RNA sequencing (scRNA-seq) and cellular indexing of transcriptomes and epitopes by sequencing (CITE-seq) on the YS and paired embryonic liver. After integration with external datasets, our reference comprised 169,798 cells from 10 samples spanning 4 to 8 postconception weeks (PCW) or Carnegie stages (CS) 10 to 23. A repertoire of two-dimensional (2D) and 3D imaging techniques provided spatial context and validation. We compared the products of two hematopoietic inducible pluripotent stem cell (iPSC) culture protocols against our reference. RESULTS We determined that YS metabolic and nutritional support originates in the endoderm and that the endoderm produces coagulation proteins and hematopoietic growth factors [erythropoietin (EPO) and thrombopoietin (THPO)]. Although metabolic and coagulation protein production was conserved among humans, mice, and rabbits, EPO and THPO production was observed in humans and rabbits only. We reconstructed trajectories from the YS hemogenic endothelium to early hematopoietic stem and progenitor cells (HSPCs). Using transcriptomic signatures of early and definitive hematopoiesis, we parsed YS HSPCs into myeloid-biased early HSPCs and lymphoid- and megakaryocyte-biased definitive HSPCs. Human embryonic liver remained macroscopically pale before CS14, when hematopoietic cells first emerge from the aorta-gonad-mesonephros (AGM) region. Tracking hemoglobin (Hb) subtypes led us to conclude that initial erythropoiesis is YS restricted. By contrast, in mice, Hb subtypes suggested two waves of pre-AGM erythropoiesis, including maturation in the macroscopically red embryonic liver. Before CS14, monocytes were absent and macrophages originated from HPSCs via a premacrophage cell state. After CS14, monocytes emerged and a second, monocyte-dependent differentiation trajectory was reconstructed. A rare subset of TREM2 + macrophages, with a microglia-like transcriptomic signature, was present after CS14. The iPSC system optimized for macrophage production recapitulated the two routes to macrophage differentiation but did not generate the diversity of macrophages (including TREM2 + macrophages) observed in developing tissues. CONCLUSION Our study illuminates a previously obscure phase of human development, where vital functions are delivered by the YS acting as a transient extraembryonic organ. Our comprehensive single-cell atlas represents a valuable resource for studying the cellular differentiation pathways specific to early life and leveraging these for tissue engineering and cellular therapy. Multiorgan functions of the human YS. We characterized functions of the developing human YS, combining scRNA-seq and CITE-seq with 2D and 3D imaging techniques. Our findings revealed YS contributions to metabolic and nutritional support and to early hematopoiesis. We characterized myeloid bias in early hematopoiesis, distinct myeloid differentiation trajectories, evolutionary divergence in initial erythropoiesis, and YS contributions to developing tissue macrophages. Met., metabolic; Coag., coagulation; Mac, macrophage. [Figure created with Biorender]
Type of Medium:
Online Resource
ISSN:
0036-8075
,
1095-9203
DOI:
10.1126/science.add7564
Language:
English
Publisher:
American Association for the Advancement of Science (AAAS)
Publication Date:
2023
detail.hit.zdb_id:
128410-1
detail.hit.zdb_id:
2066996-3
detail.hit.zdb_id:
2060783-0
SSG:
11
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