In vivo clonal tracking reveals evidence of haemangioblast and haematomesoblast contribution to yolk sac haematopoiesis

During embryogenesis, haematopoietic and endothelial lineages emerge closely in time and space. It is thought that the first blood and endothelium derive from a common clonal ancestor, the haemangioblast. However, investigation of candidate haemangioblasts in vitro revealed the capacity for mesenchymal differentiation, a feature more compatible with an earlier mesodermal precursor. To date, no evidence for an in vivo haemangioblast has been discovered. Using single cell RNA-Sequencing and in vivo cellular barcoding, we have unravelled the ancestral relationships that give rise to the haematopoietic lineages of the yolk sac, the endothelium, and the mesenchyme. We show that the mesodermal derivatives of the yolk sac are produced by three distinct precursors with dual-lineage outcomes: the haemangioblast, the mesenchymoangioblast, and a previously undescribed cell type: the haematomesoblast. Between E5.5 and E7.5, this trio of precursors seeds haematopoietic, endothelial, and mesenchymal trajectories. The lineage relationship between blood and endothelial cells has been difficult to examine due to the multiphasic timing of hematopoiesis in the embryo. Here the authors use using in vivo barcoding technology to assess cell ancestry and show that blood and endothelial cells emerge through common (haemangioblast) or separate (mesenchymoangioblasts and haematomesoblasts) progenitors in the yolk sac.

Automated summary

By using single cell RNA-Sequencing and in vivo cellular barcoding, researchers have unraveled the ancestral relationships that give rise to the haematopoietic lineages of the yolk sac, the endothelium, and the mesenchyme. They found that blood and endothelial cells emerge from three distinct precursors: the haemangioblast, the mesenchymoangioblast, and a previously undescribed cell type called the haematomesoblast. This study is of great significance for a better understanding of embryonic development and the formation of the hematopoietic system.