Aortic valve: anatomy and structure and the role of vasculature in the degenerative process

Aortic valve stenosis is a degenerative disease affecting increasing number of individuals and characterised by thickening, calcification and fibrosis of the valve resulting in restricted valve motion. Degeneration of the aortic valve is no longer considered a passive deposition of calcium, but an active process that involves certain mechanisms, that is endothelial dysfunction, inflammation, increased oxidative stress, calcification, bone formation, lipid deposition, extracellular matrix (ECM) remodelling and neoangiogenesis. Accumulating evidence indicates an important role for neoangiogenesis (i.e. formation of new vessels) in the pathogenesis of aortic valve stenosis. The normal aortic valve is generally an avascular tissue supplied with oxygen and nutrients via diffusion from the circulating blood. In contrast, presence of intrinsic micro-vasculature has been demonstrated in stenotic and calcified valves. Importantly, presence and density of neovessels have been associated with inflammation, calcification and bone formation. It remains unclear whether neoangiogenesis is a compensatory mechanism aiming to counteract hypoxia and increased metabolic demands of the thickened tissue or represents an active contributor to disease progression. Data extracted mainly from animal studies are supportive of a direct detrimental effect of neoangiogenesis, however, robust evidence from human studies is lacking. Thus, there is inadequate knowledge to assess whether neoangiogenesis could serve as a future therapeutic target for a disease that no effective medical therapy exists. In this review, we present basic aspects of anatomy and structure of the normal and stenotic aortic valve and we focus on the role of valve vasculature in the natural course of valve calcification and stenosis.