# Sclerotomal hematopoiesis in vertebrate embryos contributes to robustness of the blood system

## TL;DR

This study shows that bone-forming cells in embryos can also become blood stem cells, providing a backup system for blood production in vertebrates.

## Contribution

The discovery that sclerotomal cells can generate hematopoietic stem cells independently of the aortic endothelium is novel and expands our understanding of blood system development.

## Key findings

- Sclerotomal cells directly become hematopoietic stem and progenitor cells in zebrafish and mouse embryos.
- Sclerotome-derived HSPCs contribute to blood cell production throughout life.
- The proportion of these cells increases when standard hematopoiesis is impaired.

## Abstract

During vertebrate embryogenesis, hematopoietic stem cells (HSCs) are believed to almost exclusively arise from hemogenic endothelial cells (ECs) of the dorsal aorta through endothelial-to-hematopoietic transition (EHT). It remains elusive whether HSCs can be generated by other tissues. Sclerotomal cells (SCs) in embryonic somites predominantly form vertebrae and ribs, and some SCs may also insert into aortic endothelia. Here we show that a subset of SCs directly enter the vascular lumen to become hematopoietic stem and progenitor cells (HSPCs) in an EHT-independent way in both zebrafish and mouse embryos. The sclerotome-derived HSPCs (scHSPCs) contribute to various blood cells throughout the lifetime. The proportion of scHSPCs increases dramatically when endothelium hematopoiesis is defective. Thus, the sclerotomal hematopoiesis is evolutionarily conserved and would ensure the robustness of hematopoiesis.