Flow regulates arterial-venous differentiation in the chick embryo yolk sac

Abstract

Formation of the yolk sac vascular system and its connection to the embryonic circulation is crucial for embryo survival in both mammals and birds. Most mice with mutations in genes involved in vascular development die because of a failure to establish this circulatory loop. Surprisingly,formation of yolk sac arteries and veins has not been well described in the recent literature. Using time-lapse video-microscopy, we have studied arterial-venous differentiation in the yolk sac of chick embryos. Immediately after the onset of perfusion, the yolk sac exhibits a posterior arterial and an anterior venous pole, which are connected to each other by cis-cis endothelial interactions. To form the paired and interlaced arterial-venous pattern characteristic of mature yolk sac vessels, small caliber vessels of the arterial domain are selectively disconnected from the growing arterial tree and subsequently reconnected to the venous system, implying that endothelial plasticity is needed to fashion normal growth of veins. Arterial-venous differentiation and patterning are controlled by hemodynamic forces, as shown by flow manipulation and in situ hybridization with arterial markers ephrinB2 and neuropilin 1, which show that expression of both mRNAs is not genetically determined but plastic and regulated by flow. In vivo application of ephrinB2 or EphB4 in the developing yolk sac failed to produce any morphological effects. By contrast, ephrinB2 and EphB4 application in the allantois of older embryos resulted in the rapid formation of arterial-venous shunts. In conclusion, we show that flow shapes the global patterning of the arterial tree and regulates the activation of the arterial markers ephrinB2 and neuropilin 1.

Definitive erythropoiesis in chicken yolk sac

“…6A,B) or section (data not shown) examination. A similar observation, in addition to the lack of prominent apoptosis during this period, has also been reported previously (le Noble et al, 2004). This suggested that these definitive erythropoietic cells are derived either from blood-borne hematopoietic cells or from pre-existing vascular cells.…”

“…The first phase, in which yolk sac vascular plexus is remodeled to form well-arborized arterial and venous trees, takes place between E2 and E3, soon after the initiation of embryonic circulation. This phase of vascular remodeling has been reported to be controlled by hemodynamics in the yolk sac (le Noble et al, 2004). Our observations suggest that yolk sac definitive erythropoiesis does not coincide with, although it may be a consequence of, this remodeling.…”

“…During the same period, intraembryonic hemogenic endothelial cells were reported to appear in dorsal aorta. It is also during this period that the yolk sac vasculature undergoes a major remodel from primary (apposing) to secondary (paired) arterial/venous connection (le Noble et al, 2004). We, therefore, investigated spatial and temporal relationships between yolk sac definitive erythropoiesis and vascular remodeling.…”

Différenciation artérioveineuse : génétique ou hémodynamique ?

“…L'apparente homogénéité des vaisseaux primitifs avant leur remodelage pouvait suggérer une implication importante des forces hémodynamiques dans ce remodelage [3]. Nous avons récemment réalisé une étude détaillée du remodelage du plexus vasculaire primitif en artères et en veines chez l'embryon de poulet [4]. Dans ce modèle, on peut visualiser directement, in vivo, la circulation sanguine et le sens du flux par vidéo-microscopie.…”

“…Ce processus est contrôlé par le flux sanguin qui croît dans les tubes vitellins majeurs, ce qui entraîne une diminution du diamètre des tubes annexes suivie de leur obturation. Ces vaisseaux déconnec-tés sont, par la suite, réutilisés pour façonner le réseau veineux secondaire qui se met en place dorsalement et parallèlement au réseau artériel [4]. Pour se reconnecter à la circulation veineuse, les branches annexes bourgeonnent dorsalement et perpendiculairement aux artères.…”