Vascular remodelling is a process which occurs when an immature heart begins contracting, pushing fluid through the early vasculature. The process typically begins at day 22, and continues to the tenth week of human embryogenesis. 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.
Physical cues such as pressure, velocity, flow patterns, and shear stress are known to act on the vascular network in a number of ways, including branching morphogenesis, enlargement of vessels in high-flow areas, angiogenesis, and the development of vein valves. The mechanotransduction of these physical cues to endothelial and smooth muscle cells in the vascular wall can also trigger the promotion or repression of certain genes which are responsible for vasodilation, cell alignment, and other shear stress-mitigating factors. This relationship between genetics and environment is not clearly understood, but researchers are attempting to clarify it by combining reliable genetic techniques, such as genetically ablated model organisms and tissues, with new technologies developed to measure and track flow patterns, velocity profiles, and pressure fluctuations in vivo.
Both in vivo study and modelling are necessary tools to understand this complex process. Vascular remodelling is pertinent to wound healing and proper integration of tissue grafting and organ donations. Promoting an active remodelling process in some cases could help patients recover faster and retain functional use of donated tissues. However, outside of wound healing, chronic vascular remodelling in the adult is often symptomatic of cardiovascular disease. Thus, increased understanding of this biomedical phenomenon could aid in the development of therapeutics or preventative measures to combat diseases such as atherosclerosis.
Historical view
Over 100 years ago, Thoma observed that increases in local blood flow cause widening of the vessel diameter and he even went so far as to postulate that blood flow might be responsible for the growth and development of blood vessels.Thoma, R. (1893). Untersuchungen ü ber die Histogenese und Histo- mechanik des 1186 Gefä ßsystems. Stuttgart, Germany: Ferdinand Enke Subsequently, Chapman in 1918 discovered that removing a chick embryo's heart disrupted the remodelling process, but the initial vessel patterns laid down by vasculogenesis remained undisturbed. Next, in 1926 Murray proposed that vessel diameter was proportional to the amount of shear stress at the vessel wall; that is, that vessels actively adapted to flow patterns based on physical cues from the environment, such as shear stress.
The chemical basis of morphogenesis," written in 1952 by mathematician and computer scientist Alan Turing advocated for various biological models based on molecular diffusion of nutrients. However, a diffusive model of vascular development would seem to fall short of the complexity of capillary beds and the interwoven network of arteries and veins. In 2000, Fleury proposed that instead of diffusive molecules bearing responsibility for the branching morphogenesis of the vascular tree, a long-range morphogen may be implicated. In this model, a traveling pressure wave would act upon the vasculature via shear stress to rearrange branches into the lowest-energy configuration by widening vessels carrying increased blood flow and rearranging networks upon the initiation of fluid flow. It is known that mechanical forces can have a dramatic impact on the morphology and complexity of the vascular tree. However, these forces have comparably little impact on the diffusion of nutrients, and it therefore seems unlikely that acquisition of nutrients and oxygen plays a significant role in embryonic vascular remodelling.
It is now widely accepted that vascular remodelling in the embryo is a process distinct from vasculogenesis; however these two processes are inextricably linked. Vasculogenesis occurs prior to vascular remodelling, but is a necessary step in the development of the blood vessel network and has implications on the identification of vessels as either arterial or venous. Once contraction of the heart begins, vascular remodelling progresses via the interplay of forces resulting from biomechanical cues and fluid dynamics, which are translated by mechanotransduction to changes at cellular and genetic levels.
Vasculogenesis
Vasculogenesis is the formation of early vasculature, which is laid down by genetic factors. Structures called blood islands form in the mesoderm layer of the yolk sac by cellular differentiation of hemangioblasts into endothelial and red blood cells. Next, the capillary plexus forms as endothelial cells migrate outward from blood islands and form a random network of continuous strands. These strands then undergo a process called lumenization, the spontaneous rearrangement of endothelial cells from a solid cord into a hollow tube.Forgacs, G. and Newman, S.A. (2005). Biological Physics of the Developing Embryo. Cambridge, UK: Cambridge University Press.
Inside the embryo, the dorsal aorta forms and eventually connect the heart to the capillary plexus of the yolk sac. This forms a closed-loop system of rigid endothelial tubing. Even this early in the process of vasculogenesis, before the onset of blood flow, sections of the tube system may express ephrins or neuropilins, genetic markers of arterial or venous identities, respectively. These identities are still somewhat flexible, but the initial characterization is important to the embryonic remodelling process.
Angiogenesis also contributes to the complexity of the initial network; spouting endothelial buds form by an extrusion-like process which is prompted by the expression of vascular endothelial growth factor (VEGF). These endothelial buds grow away from the parent vessel to form smaller, daughter vessels reaching into new territory. Intussusception, the phenomenon of a single tube splitting to form two branching tubes, also contributes to angiogenesis. Angiogenesis is generally responsible for colonizing individual organ systems with blood vessels, whereas vasculogenesis lays down the initial pipelines of the network. Angiogenesis is also known to occur during vascular remodelling.
Arterial-venous identity
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.
Biomechanics
Once the heart begins to beat, mechanical forces start acting upon the early vascular system, which rapidly expands and reorganizes to serve tissue metabolism. In embryos devoid of blood flow, endothelial cells retain an undifferentiated morphology similar to angioblasts (compared to flattened epithelial cells found in mature vasculature). Once the heart begins beating, the morphology and behaviour of endothelial cells change. By changing the heart rate, the heart can also control perfusion or pressure acting upon the system in order to trigger sprouting of new vessels. In turn, new vessel sprouting is balanced by the expansion of other embryo tissues, which compress blood vessels as they grow. The equilibrium of these forces plays a major role in vascular remodelling, but although the angiogenic mechanisms required to trigger the sprouting of new vessels have been studied, little is known about the remodelling processes required to curb the growth of unnecessary branches.
As blood perfuses the system, it exerts shear and pressure forces on the vessel walls. At the same time, tissue growth outside the cardiovascular system pushes back on the outside of the vessel walls. These forces must be balanced to obtain an efficient energy state for low-cost delivery of nutrients and oxygen to all tissues of the embryo body. When growth of the yolk sac (external tissue) is constrained, the balance between vascular forces and tissue forces is shifted and some vascular branches may be disconnected or diminished during the remodelling process because they are unable to forge new paths through the compressed tissue. In general, the stiffness and resistance of these tissues dictates the degree to which they can be deformed and the way in which biomechanical forces can affect them.
The development of the vascular network is self-organized at each point in the tissue due to the balance between compressive forces of tissue expansion and circumferential stretch of the vessel walls. Over time, this means that migrating lines become straight rather than curving; this is akin to imagining two moving boundaries pushing on each other. Straight vessels are usually parallel to isopressure lines because the boundaries have acted to equilibriate pressure gradients. In addition, vessel direction tends to follow the direction of the normal to the steepest stress gradient.