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Fig. 7 | BMC Developmental Biology

Fig. 7

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

Fig. 7

Formation of the primary circulatory system, overview. A = anterior, P = posterior, BI = blood islands, DA = dorsal aortae, VV = vitelline veins, VA = vitelline arteries. (a): section through an E6.5 embryo (as in [28]), showing that cells of the epiblast, at the border of the extra-embryonic region and the epiblast, become mesoderm and form the primitive streak. Cells at the top of the primitive streak migrate upwards into the extra-embryonic region and the primitive streak expands downwards into the embryonic region. Mesodermal cells also migrate all around, just underneath the outer (endoderm) layer, in the embryonic and extra-embryonic regions; the former become the mesodermal wings (MW). (b): between E7 and E7.25, a fraction of the mesodermal cells in the extra-embryonic region acquires haemato-endothelial cell fate in the region where the BI are developing and start expressing Tal1. (c): at E7.5, Tal1 positive cells spread throughout the YS and the bud of the allantois (interior, not shown), and are also seen in the embryo near the anterior part of the neural tube. They may be attracted by a chemo-attractant secreted at the anterior part of the midline, and repulsed by a signal with a more limited range, coming from the notochord, leaving the observed symmetrical Tal1 positive-cell-free zone on either side of the midline. (d): at E7.75, cells have also migrated out of the yolk sac into the middle and posterior part of the embryo. Again, cells arrest at a distance from the midline on either side. The cells that previously migrated towards the anterior part of the neural tube are now localised just above the anterior intestinal portal, where they start forming the endocardium, while the head folds (interior, not shown) are developing towards the inside of the embryo. Some cells have started to ingress into the head folds. (e): at E8, cords of endothelial cells start forming the dorsal aortae at either side of the midline, where they stopped migrating. At the anterior part of the embryo, the dorsal aortae extend into the head folds and cells move from the dorsal aortae underneath the endocardium straight into the head folds. Cells from the yolk sac keep contributing to the endocardium, which is now a pronounced crescent. In the yolk sac, the omphalomesenteric system develops, connecting the back of the dorsal aortae with the blood islands via the vitelline arteries at the posterior part of the embryo, and the venous poles of the heart with the blood islands via the vitelline veins at the anterior part of the embryo. Tal1 positive cells remain in the rest of the yolk sac and the blood islands but have been omitted here for clarity

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