Defining the Emerging Blood System During Development at Single-Cell Resolution

Abstract

Developmental hematopoiesis differs from adult and is far less described. In the developing embryo, waves of lineage-restricted blood precede the ultimate emergence of definitive hematopoietic stem cells (dHSCs) capable of maintaining hematopoiesis throughout life. During the last two decades, the advent of single-cell genomics has provided tools to circumvent previously impeding characteristics of embryonic hematopoiesis, such as cell heterogeneity and rare cell states, allowing for definition of lineage trajectories, cellular hierarchies, and cell-type specification. The field has rapidly advanced from microfluidic platforms and targeted gene expression analysis, to high throughput unbiased single-cell transcriptomic profiling, single-cell chromatin analysis, and cell tracing—offering a plethora of tools to resolve important questions within hematopoietic development. Here, we describe how these technologies have been implemented to address a wide range of aspects of embryonic hematopoiesis ranging from the gene regulatory network of dHSC formation via endothelial to hematopoietic transition (EHT) and how EHT can be recapitulated in vitro, to hematopoietic trajectories and cell fate decisions. Together, these studies have important relevance for regenerative medicine and for our understanding of genetic blood disorders and childhood leukemias.

Increasing Complexity of Molecular Landscapes in Human Hematopoietic Stem and Progenitor Cells during Development and Aging

“…It has been reported that HSC-independent HPCs also arise for example from distinct HE or intra-aortic clusters in the yolk sac and P-Sp/AGM region of the embryo proper at approximately E9.5 [ 13 ], with multipotential, lymphoid or lympho-myeloid biased progenitors preceding or emerging simultaneously with pre-HSCs [ 70 , 71 , 72 ]. Expansion and differentiation of second- and third-wave HPCs and definitive HSCs occur in the murine fetal liver before HSC colonization of the fetal spleen and fetal bone marrow [ 73 ]. MMP-3 (multi-potent progenitors-3) and lesser numbers of HSCs have also been reported to originate from HE in murine fetal/young adult bone marrow [ 74 ], perhaps constituting a fourth wave of hematopoiesis.…”

Recent Advances in Developmental Hematopoiesis: Diving Deeper With New Technologies

“…Bulk RNA-seq, scRNA-seq and microarray analyses have then been performed to analyze the intrinsic regulation (e.g. by transcription factors) of HSPC formation by sequencing phenotypically enriched populations for arterial endothelial cells, HE cells, cells undergoing EHT, pre-HSCs and/or HSCs sorted from mouse, chicken, human and/or zebrafish embryos (78,86,97,(112)(113)(114)(115)(116)(117)(118)(119)(120)(121)(122)(123). Overall, these studies highlighted important features.…”

“…Whilst much attention goes out to transcriptomic approaches ( 122 ), recent advances in protein-based techniques should not be overlooked. Measuring proteins present in or on the cell by fluorescence-based flow cytometry has proven to be a rapid and powerful tool for isolating, sub-typing and phenotyping the cells of the immune system, including HSCs ( 143 , 144 ).…”