part of endocardium development endothelium development endocardial endothelium development 2010-06-03T09:15:31Z dph The progression of the endocardial endothelium over time, from its initial formation to the mature structure. 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:0061147 biological_process