Progenitor cells confer plasticity to cardiac valve endothelium

The endothelium covering the aortic, pulmonary, mitral, and tricuspid valves looks much like the endothelium throughout the vasculature, in terms of general morphology and expression of many endothelial markers. Closer examination, however, reveals important differences and hints of a unique phenotype that reflects the valvular endothelium's embryonic history, and potentially, its ability to maintain integrity and function over a life span of dynamic mechanical stress. A well-studied property that sets the cardiac valvular endothelium apart is the ability to transition from an endothelial to a mesenchymal phenotype-an event known as epithelial to mesenchymal transition (EMT). EMT is a critical step during embryonic valvulogenesis, it can occur in post-natal valves and has recently been implicated in the adaptive response of mitral valve leaflets exposed to a controlled in vivo setting designed to mimic the leaflet tethering that occurs in ischemic mitral regurgitation. In this review, we will discuss what is known about valvular endothelial cells, with a particular focus on post-natal, adult valves. We will put forth the idea that at subset of valvular endothelial cells are progenitor cells, which may serve to replenish valvular cells during normal cellular turnover and in response to injury and disease.