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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)-β₂, individual clones adopted a mesenchymal phenotype to varying degrees and expressed markers of endothelial to mesenchymal transformation (EMT). Both VEGF- and TGFβ₂–induced phenotypic changes were partially reversible, indicating the plasticity of these cells. When challenged with VEGF or TGFβ₂, a hierarchy of endothelial/mesenchymal potential could be seen among the clonal …