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The migratory pathways of the cells that form the endocardium, dorsal aortae, and head vasculature in the mouse embryo

ABSTRACT: Background Vasculogenesis in amniotes is often viewed as two spatially and temporally distinct processes, occurring in the yolk sac and in the embryo. However, the spatial origins of the cells that form the primary intra-embryonic vasculature remain uncertain. In particular, do they obtain their haemato-endothelial cell fate in situ, or do they migrate from elsewhere? Recently developed imaging techniques, together with new Tal1 and existing Flk1 reporter mouse lines, have allowed us to investigate this question directly, by visualising cell trajectories live and in three dimensions. Results We describe the pathways that cells follow to form the primary embryonic circulatory system in the mouse embryo. In particular, we show that Tal1-positive cells migrate from within the yolk sac, at its distal border, to contribute to the endocardium, dorsal aortae and head vasculature. Other Tal1 positive cells, similarly activated within the yolk sac, contribute to the yolk sac vasculature. Using single-cell transcriptomics and our imaging, we identify VEGF and Apela as potential chemo-attractants that may regulate the migration into the embryo. The dorsal aortae and head vasculature are known sites of secondary haematopoiesis; given the common origins that we observe, we investigate whether this is also the case for the endocardium. We discover cells budding from the wall of the endocardium with high Tal1 expression and diminished Flk1 expression, indicative of an endothelial to haematopoietic transition. Conclusions In contrast to the view that the yolk sac and embryonic circulatory systems form by two separate processes, our results indicate that Tal1-positive cells from the yolk sac contribute to both vascular systems. It may be that initial Tal1 activation in these cells is through a common mechanism.

ORGANISM(S): Mus musculus

PROVIDER: GSE165402 | GEO | 2021/03/23

REPOSITORIES: GEO

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Project description:The signaling cascades that direct the morphological differentiation of the vascular system during early embryogenesis are not well defined. To further understand the role of Notch signaling during endothelial differentiation, this study uses both an in vivo gain-of-function and in vivo loss-of-function approach. At embryonic day 9.5, embryos with activated Notch1 signaling in the endothelia display a variety of growth and cardiovascular defects, and die soon after E10.5. Most notably, the extra-embryonic vasculature of the yolk sac displays remodeling differentiation defects. In the wild-type yolk sac, the primary vascular network has begun to reorganize, forming the large primary vessels and the smaller capillaries. In the activated Notch1 embryos remodeling is defective; the vasculature have an enlarged surface with decreased inter-vessel space. Embryos with ablated Notch signaling also display growth and vascular defects at E9.5 similar to the activated Notch1 embryos, however they exhibit a lack of vascular remodeling in the yolk sac, retaining the simple vascular plexus seen at E8.5. These results indicate that Notch signaling plays a critical role in the remodeling of the vasculature in the early embryo, particularly in the extra embryonic region. A conditional transgenic system was used in this study to activate Notch signaling. The ubiquitous ROSA26Notch transgene with a Neo/stop cassette flanked by loxP sites, followed by the N1-ICD cDNA, was recombined with a Tie2-CRE mouse, resulting in the removal of the STOP cassette and the subsequent activation of the Notch1-intracellular domain. This allowed for the overexpression and expansion of Notch signaling in all endothelial cells. Male Tie2-Cre mice were mated with female ROSA26Notch mice and resulting embryos were dissected at embryonic day 9.5. To ablate Notch signaling, Tie2-Cre mice were used in a two generation cross to obtain Tie2-Cre; Rbpj flox/flox embryos. These embryos lack RBPJ binding activity in the endothelia. In both instances embryos were used for genotyping and the yolk sac were separated and used to isolate total RNA with an RNeasy mini kit. The RNA was analyzed with the Mouse Genome 430A Array from Affymetrix. Samples were performed in duplicate, and RNA from wild type yolk sac tissues was compared to activated Notch and RBPJ loss-of-function yolk sac tissues.

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The migratory pathways of the cells that form the endocardium, dorsal aortae, and head vasculature in the mouse embryo

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