Epicardium
Epicardium
Also known as: visceral pericardium, visceral layer of serous pericardium
The epicardium is the outermost layer of the heart wall and the visceral layer of the serous pericardium. It consists of mesothelium over connective tissue and fat, and it carries the main coronary vessels and nerves.
Overview
The epicardium is a single layer of mesothelium (simple squamous epithelium) lying on a subepicardial layer of areolar connective tissue and adipose tissue. The fat is most abundant in the coronary and interventricular sulci, around the coronary arteries and over the right ventricle.
The major coronary arteries, cardiac veins, lymphatics and autonomic nerves run in this subepicardial layer before they send branches into the myocardium. For this reason, the main coronary arteries are called the epicardial coronary arteries. They are the vessels affected by clinically significant atherosclerosis.
Epicardial adipose tissue is metabolically active visceral fat that releases cytokines and free fatty acids. A larger volume of it is associated with coronary artery disease and atrial fibrillation.
- Location
Covers the whole external surface of the heart. It is continuous with the parietal serous pericardium where the layers reflect around the great vessels.
Anatomy
- Histology
The surface is simple squamous mesothelium on a basement membrane. Beneath it is a thin layer of fibroelastic connective tissue and a variable subepicardial layer of adipose tissue that contains the coronary vessels, nerves and ganglia.
- Development
The epicardium comes from the proepicardial organ near the septum transversum. Its cells migrate over the heart tube. Some then undergo epithelial-to-mesenchymal transition and become cardiac fibroblasts and coronary vascular smooth muscle.
Function
- Provides a smooth, lubricated outer surface for the heart within the pericardial cavity.
- Houses and protects the epicardial coronary arteries, cardiac veins, lymphatics and nerves.
- Its mesothelium contributes to the production of pericardial fluid.
Clinical significance
Coronary bypass grafting
Because the coronary arteries run in the subepicardial fat, surgeons can reach them directly. In coronary artery bypass grafting (CABG), grafts are sewn onto the epicardial segments of the arteries beyond the stenosis.
Myocardial bridging
In myocardial bridging, a segment of an epicardial coronary artery (most often the LAD) dips into the myocardium and is squeezed during systole. It is usually harmless but can cause ischaemia.
Compiled from standard anatomy and physiology references — see the Cardiovascular System sources. For education only; not medical advice.