Epicardium

Epicardium, also called the visceral pericardium, is the thin, transparent layer that covers the surface of the heart. It forms the inner layer of the serous pericardium, with the parietal layer lining the fibrous pericardium and the pericardial cavity between them.

Anatomy and composition: The epicardium consists of a layer of mesothelial cells, beneath which lies connective tissue that may contain adipose tissue, especially along the grooves of the heart. Blood vessels, lymphatics, and autonomic nerves run in the subepicardial space to reach the myocardium. Epicardial fat is common in many individuals and can vary in thickness.

Development: During embryogenesis, epicardial cells arise from the proepicardial organ and migrate onto the developing heart. The epicardium contributes to the formation of the coronary vasculature and to cell lineages such as fibroblasts and smooth muscle cells through epicardial-derived cells, playing a key role in heart development and regeneration.

Function: The epicardium provides a slippery surface that, together with pericardial fluid, reduces friction between the moving heart and the surrounding sac. It serves as a source of cells and signals important for coronary vessel formation, myocardial development, and repair.

Clinical relevance: Inflammation of the epicardium is typically discussed within the context of pericarditis. Epicardial adipose tissue can influence coronary disease through secreted factors. The epicardial surface is also a target in certain cardiac procedures, such as epicardial mapping and ablation for arrhythmias.