Endocardium

Overview
In the heart, the endocardium is the innermost layer of tissue that lines the chambers of the heart. Its cells, embryologically and biologically, are similar to the endothelial cells that line blood vessels.

The endocardium overlies the much more voluminous myocardium, the muscular tissue responsible for the contraction of the heart. The outer layer of the heart is termed epicardium and the heart is surrounded by a small amount of fluid enclosed by a fibrous sac called the pericardium.

Function
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 (heart muscle) capillaries, which is also closely appositioned to the cardiomyocytes (heart muscle cells) are involved in this modulatory role. Thus, the cardiac endothelium (both the endocardial endothelium and the endothelium of the myocardial capillaries) controls the development of the heart in the embryo as well as in the adult, for example 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 infective endocarditis, the endocardium (especially the endocardium lining the heart valves) may be affected by bacteria.