CLONAL DYNAMICS OF EMBRYONIC AND EARLY-POST-BIRTH BLOOD PRECURSORS

Abstract

In the mouse the first Hematopoietic Stem Cell (HSC) is detected by transplantation at embryonic day E10.5. HSC emerge in the Aorta-Gonad-Mesonephros Region and migrate to the Fetal Liver (FL) where they supposedly expand dramatically (about 20-fold) between day E12.5-E15.5. HSC then migrate to the Bone Marrow (BM), where they predominantly reside in adult life. We recently reported that lifelong hematopoiesis is established by hundreds of blood precursors throughout mouse ontogeny (Ganuza et al., NCB 2017). Our novel non-invasive approach (based on the inducible multi-color ROSA26Confetti reporter) has allowed us to re-interrogate the clonal dynamics of blood precursors during steady-state without perturbing embryogenesis or using transplantation as a read-out for HSC potential. Here, we applied this approach to interrogate the clonal dynamics of embryonic (E14-E19) and neonatal (P1-P21) hematopoietic precursors. Towards this, Confetti labeling of blood progenitors at discrete stages of development was induced by treating ROSA26(+/Confetti) Ubiquitin(+/ERT2-Cre) (UbiqERT2-Cre) embryos or neonates with tamoxifen at E12-E14, E15-E17, P1, P8-P9, P14-15 or P21-P22. In contrast to previous dogma, we failed to detect a 20-fold expansion in the number of blood precursors during the presumed FL expansion period. Rather, between E11.5 and E15.5 we observed a three-fold expansion in blood precursors that contribute to adult peripheral blood. This was followed by a two-fold expansion in these same precursors between E15.5 and P1 and a 1.5-fold expansion between P1 and P8. These results contribute to our understanding of the precise developmental windows (and presumably niches) that support the symmetric cell division of HSCs.