How Arteries and Veins Self-Organize In Parallel

What is the reason that arteries and veins run in parallel? An interdisciplinary team of biologists and physicists from France and Germany just might have an answer. These scientists have published a paper in the latest Physical Review E showing that ” during vertebrate development, arteries exert a morphological control over the venous pattern through physical factors.”
Centre National de la Recherche Scientifique elaborates:

Using physical measurements, theoretical models and numerical simulations, the researchers showed how the growth of the arteries directly controls that of the veins through a process that depends solely on the mechanical forces present.
An amazingly complex vascular network, made up of arteries, capillaries and veins, runs through the bodies of vertebrates. It carries the necessary oxygen and nutrients to each cell and is used to remove the metabolic waste produced. The network has such a huge number of branches that the positions of each vessel cannot possibly be coded for genetically. However, genetics is often called upon to explain the fact that, in adults, arteries and veins are very frequently found in parallel pairs, with an artery even often being flanked by two veins that are exactly parallel to it. During the development of the embryo, a "genetic conversation" between arteries and veins could provide an explanation for this phenomenon.
In their article published in Physical Review E, the researchers show how physical (mechanical, hydrodynamic and elastic) phenomena lead to parallel development of the arteries and veins.
A detailed study of the spatial and temporal development of the arteries and veins at the embryonic stage shows that a transformation of vascular branching takes place spontaneously during growth. At the early embryonic stage,a spatial organization is observed where the arteries and veins are located in different regions of space. Then rapidly, after several days of embryonic development, new veins develop parallel to the existing arteries and the vascular regions intertwine.
By using images of the vascular network and measurements of local mechanical parameters carried out in situ, the researchers showed that this transformation is triggered by the growth of the arteries. In their vicinity a viscoelastic response of the living tissue is observed, causing swelling. This response leads in turn to an increase in the permeability of the capillary bed which is highly localized in areas that are perfectly parallel to the previously formed arteries. Blood flows preferentially into these zones of high conductivity since it is easier for it to do so, and they are then remodeled as veins as soon as the tissue reaches a critical size, which has been predicted theoretically. Numerical simulations of blood flow carried out in idealized vascular networks in organs, at different stages of growth, have confirmed these results.
This work sheds new light on the importance of mechanics in embryonic development. Within embryos there is a landscape of mechanical forces forming a filigree of hard and soft regions, which develops spontaneously due to the pressure exerted by the cells. By analyzing the physical component of the various episodes in the scenario of embryonic development it will become possible to understand the cause of aberrations in development or pathologies caused by defective genes, which alter the physical properties of the tissue.

Full story: Arteries and veins : a forced marriage…
Abstract: During vertebrate development, arteries exert a morphological control over the venous pattern through physical factors Phys. Rev. E 77, 051912 (2008)