An autocrine function for transforming growth factor beta 3 in the atrioventricular endocardial cushion tissue formation during chick heart development

Importance of myocyte-nonmyocyte interactions in cardiac development and disease

Emerging data in the field of cardiac development as well as repair and regeneration indicate a complex and important interplay between endocardial, epicardial, and myofibroblast populations that is critical for cardiomyocyte differentiation and postnatal function. For example, epicardial cells have been shown to generate cardiac myofibroblasts and may be one of the primary sources for this cell lineage during development. Moreover, paracrine signaling from the epicardium and endocardium is critical for proper development of the heart and pathways such as Wnt, fibroblast growth factor, and retinoic acid signaling have been shown to be key players in this process. Despite this progress, interactions between nonmyocyte cells and cardiomyocytes in the heart are still poorly understood. We review the various nonmyocyte-myocyte interactions that occur in the heart and how these interactions, primarily through signaling networks, help direct cardiomyocyte differentiation and regulate postnatal cardiac function.

Human pulmonary valve progenitor cells exhibit endothelial/mesenchymal plasticity in response to vascular endothelial growth factor-A and transforming growth factor-beta2

In situ analysis of fetal semilunar valve leaflets has revealed cells coexpressing endothelial and mesenchymal markers along the endothelium, with diminished frequency seen in adult valves. To determine whether such cells are progenitor cells, we isolated clonal populations from human pulmonary valves. The clones expressed endothelial markers but showed potential to further differentiate into endothelium in response to vascular endothelial growth factor (VEGF)-A. When exposed to transforming growth factor (TGF)-beta2, individual clones adopted a mesenchymal phenotype to varying degrees and expressed markers of endothelial to mesenchymal transformation (EMT). Both VEGF- and TGFbeta2-induced phenotypic changes were partially reversible, indicating the plasticity of these cells. When challenged with VEGF or TGFbeta2, a hierarchy of endothelial/mesenchymal potential could be seen among the clonal populations: cells initially closer to an endothelial phenotype showed a strong response to TGFbeta2 that could be inhibited by VEGF, whereas cells closer to a mesenchymal phenotype responded to TGFbeta2 but were resistant to endothelial-inducing effects of VEGF. These findings suggest the presence of bipotential valve progenitor cells with ability to differentiate into either endothelial or interstitial cells of the valve leaflet. Understanding the differentiation potential and function of these cells may be important for understanding heart valve disease and may also be applied to current paradigms for creating tissue-engineered heart valves.

Cell-cell interactions in vascular development

Research into areas as divergent as hemangiopoiesis and cardiogenesis as well as investigations of diseases such as cancer and diabetic retinopathy have converged to form the face of research in vascular development today. This convergence of disparate topics has resulted in rapid advances in many areas of vascular research. The focus of this review has been the role of cell-cell interactions in the development of the vascular system, but we have included discussions of pathology where the mechanism of disease progression may have parallels with developmental processes. A number of intriguing questions remain unanswered. For example, what triggers abnormal angiogenesis in the disease state? Are the mechanisms similar to those that control developmental neovascularization? Perhaps the difference in development in angiogenesis versus in disease is context driven, that is, an adult versus an embryonic organism. If this is the case, can the controls that curtail developmental vessel formation be applied in pathologies? Can cell-cell interactions be targeted as a control point for new vessel formation? For instance, can perivascular cells be stimulated or eliminated to result in increased vessel stability or instability, respectively? If the hypothesis that mural cell association is required for vessel stabilization is accurate, are there mechanisms to promote or inhibit mural cell recruitment and differentiation as needed? These and other questions lie in wait for the next generation of approaches to discern the mechanisms and the nature of the cell-cell interactions and the influence of the microenvironment on vascular development.