has_part has part has part develops_from develops from develops from immediate transformation of endocardium endocardial lining The endocardium is an anatomical structure comprised of an endothelium and an extracellular matrix that forms the innermost layer of tissue of the heart, and lines the heart chambers[GO]. heart endocardium endocardial tissue heart endocardial tissue tunica intima endocardial endothelium heart layer presumptive endocardium