In the heart, the endocardium is the innermost layer of cells, embryologically and biologically similar to the endothelium that lines blood vessels.
The endocardium overlies the much more voluminous myocardium, the muscular tissue responsible for the contaction of the heart. The outer layer of the heart is termed pericardium.
Recently, it has become evident that the endocardium, which is primarily made up of endothelial cells, controls myocardial function. This modulating role is separate from the homeometric and heterometric regulatory mechanisms that control myocardial contractility. Moreover, the endothelium of the myocardial capillaries, which is also closely appositioned to the cardiomyocytes is involved in this modulatory role. Thus, the cardiac endothelium (endocardial endothelium and the endothelium of the myocardial capillaries) controls the development of the heart in the embryo, as well as in the adult, i.e. during hypertrophy. Additionally, the contractility and electrophysiological environment of the cardiomyocyte are regulated by the cardiac endothelium.
The endocardial endothelium may also act as a kind of blood-heart barrier (analogous to the blood-brain barrier), thus controlling the ionic composition of the extracellular fluid in which the cardiomyocytes bathe.
Role in disease
In myocardial infarction, ischemia of the myocardium can extend to the endocardium, disrupting the inner lining of the heart. Less extensive (but no less dangerous) infarctions are often "subendocardial" and do not affect the endocardium.
In infective endocarditis, the endocardium (especially the endocardium lining the heart valves) is affected by bacteria.