page_title: Vascular remodelling in the embryo
section_title: Arterial-venous identity
hierarchical_section_title: Vascular remodelling in the embryo / Arterial-venous identity
context_page_description: Vascular remodelling is a process which begins usually at day 22, and continues to around the tenth week of human embryogenesis, when, on day 22, an immature heart begins contracting, pushing fluid through the early vasculature. This first passage of fluid initiates a signal cascade and cell movement based on physical cues including shear stress and circumferential stress, which is necessary for the remodelling of the vascular network, arterial-venous identity, angiogenesis, and the regulation of genes through mechanotransduction. This embryonic process is necessary for the future stability of the mature vascular network.
  Vasculogenesis is the initial establishment of the components of the blood vessel network, or vascular tree. This is dictated by genetic factors and has no inherent function other than to lay down the preliminary outline of the circulatory system. Once fluid flow begins, biomechanical and hemodynamic inputs are applied to the system set up by vasculogenesis, and the active remodelling process can begin.
context_section_description: The classification of angioblasts into arterial- or venous-identified cells is essential to form the proper branching morphology. Arterial segments of the early vasculature express ephrinB2 and DLL4 whereas venous segments express neuropilin-2 and EPHB4; this is believed to assist in guidance of flow from arterial-venous sections of the loop. However, mechanical cues provided by the heart's first contractions are still necessary for complete remodelling.
  The first event of biomechanical-driven hierarchal remodelling occurs just after the onset of heart beat, when the vitelline artery forms by the fusion of several smaller capillaries. Subsequently, side branches may disconnect from the main artery and reattach to the venous network, effectively changing their identity. This is thought to be due to the high luminal pressure in the arterial lines, which prevents reattachment of the branches back onto arterial vessels. This also prevents the formation of shunts between the two components of the network. Moyon et al. showed that arterial endothelial cells could become venous and vice versa.  They grafted sections of quail endothelial tubing which had previously expressed arterial markers onto chick veins (or vice versa), showcasing the plasticity of the system. Reversing flow patterns in arteries and/or veins can also have the same effect, although it is unclear whether this is due to differences in physical or chemical properties of venous vs. arterial flow (i.e. pressure profile and oxygen tension).
  Another example of the fluidity of arterial-venous identity is that of the intersomitic vessel. At early stages, this vessel is connected to the aorta, making it part of the arterial network. However, sprouts from the cardiac vein may fuse with the intersomitic vessel, which slowly disconnects from the aorta and becomes a vein. This process is not fully understood, but may occur out of a need to balance mechanical forces such as pressure and perfusion.
  Arterial-venous identity in the early stages of embryonic vascular remodelling is flexible, with arterial segments often being recycled to venous lines and the physical structure and genetic markers of segments being actively remodelled along with the network itself.  This indicates that the system as a whole exhibits a degree of plasticity which allows it to be shaped by transitory flow patterns and hemodynamic signals, however genetic factors do play a role in the initial specification of vessel identity.
caption: In this anatomical image from Gray's Anatomy, it can be seen that the cardiac vein develops in close proximity to the arterial network, and in early embryos, sections may switch identities from arterial to venous vessels.
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