Concealed expansion of immature precursors underpins acute burst of adult HSC activity in foetal liver.

Stanislav Rybtsov,Alexander Medvinsky,Andrejs Ivanovs,Suling Zhao

doi:10.1242/dev.131193

Abstract

One day prior to mass emergence of haematopoietic stem cells (HSCs) in the foetal liver at E12.5, the embryo contains only a few definitive HSCs. It is thought that the burst of HSC activity in the foetal liver is underpinned by rapid maturation of immature embryonic precursors of definitive HSCs, termed pre-HSCs. However, because pre-HSCs are not detectable by direct transplantations into adult irradiated recipients, the size and growth of this population, which represents the embryonic rudiment of the adult haematopoietic system, remains uncertain. Using a novel quantitative assay, we demonstrate that from E9.5 the pre-HSC pool undergoes dramatic growth in the aorta-gonad-mesonephros region and by E11.5 reaches the size that matches the number of definitive HSCs in the E12.5 foetal liver. Thus, this study provides for the first time a quantitative basis for our understanding of how the large population of definitive HSCs emerges in the foetal liver.

Highlights

The first signs of haematopoietic differentiation in mouse development are observed in the embryonic day (E) 7.5 yolk sac of the mouse embryo, definitive haematopoietic stem cells (HSCs), which generate the adult haematopoietic system and are detectable by direct transplantation into adult irradiated recipients, emerge only by E10.5-E11.5 in very small numbers (Kumaravelu et al, 2002; Medvinsky and Dzierzak, 1996; Müller et al, 1994)
Limiting dilution approach to quantify pre-HSCs definitive HSCs (dHSCs) have previously been quantitatively mapped in the embryo using limiting dilution analysis (Fig. 1A) (Ema and Nakauchi, 2000; Kumaravelu et al, 2002; Sutherland et al, 1990), the numbers of pre-HSCs, which produce dHSCs in vivo in these tissues, remained unknown
Cell suspensions obtained from dissected embryonic tissues were divided into portions, each separately co-aggregated with OP9 cells and cultured in conditions supporting pre-HSC maturation

Summary

Introduction

The first signs of haematopoietic differentiation in mouse development are observed in the embryonic day (E) 7.5 yolk sac of the mouse embryo, definitive HSCs (dHSCs), which generate the adult haematopoietic system and are detectable by direct transplantation into adult irradiated recipients, emerge only by E10.5-E11.5 in very small numbers (Kumaravelu et al, 2002; Medvinsky and Dzierzak, 1996; Müller et al, 1994). Given that dHSCs that repopulate adult irradiated recipients appear only by the end of E10, it is broadly understood that dHSCs develop from immature HSC precursors ( pre-HSCs), which are not detectable by direct transplantation (Medvinsky and Dzierzak, 1996).

Methods

Results

Conclusion