The heart is a remarkable muscular organ that continuously pumps blood throughout the body, delivering oxygen and nutrients while removing waste products. This ceaseless activity relies on a complex and organized structure. The heart is enveloped by several layers, each contributing to its overall function. The epicardium is the outermost covering of the heart. This article explores its structure, physiological roles, developmental contributions, and clinical importance.
Anatomy and Location of the Epicardium
The epicardium forms the outermost boundary of the heart wall, serving as the visceral layer of the serous pericardium. It directly adheres to the myocardium, the muscular middle layer of the heart. This layer is composed of a superficial layer of simple squamous epithelial cells, known as mesothelial cells, supported by underlying loose connective tissue and fat.
The epicardium also contains the heart’s blood vessels, lymphatic vessels, and nerves. It is distinct from the parietal pericardium, the outer fibrous sac enclosing the entire heart. The space between them is the pericardial cavity, which contains a small amount of pericardial fluid.
Physiological Roles of the Epicardium
The epicardium performs several functions in the healthy adult heart, contributing to its efficient operation. It acts as a protective outer covering for the heart and reduces friction during its continuous contractions.
The mesothelial cells of the epicardium secrete pericardial fluid into the pericardial cavity. This fluid acts as a lubricant, allowing the heart to move smoothly within the pericardial sac. The epicardium also houses the coronary blood vessels and nerves that supply the heart muscle, supporting these structures.
Epicardium’s Contribution to Heart Development and Regeneration
The epicardium plays a significant role during embryonic heart development, originating from a transient embryonic cell cluster called the proepicardial organ (PEO). This PEO is a highly conserved structure across vertebrates. Epicardial cells exhibit extensive developmental plasticity and give rise to various cell types through a process called epithelial-to-mesenchymal transition (EMT).
These epicardium-derived cells (EPDCs) migrate into the developing heart and differentiate into cardiac fibroblasts, vascular smooth muscle cells, and pericytes, which are essential for forming the coronary vasculature and supporting the heart’s structure. The epicardium also secretes paracrine factors that promote the growth and maturation of cardiomyocytes, the heart muscle cells. In the adult heart, the epicardium becomes quiescent, but it can reactivate in response to injury, initiating an embryonic-like response that contributes to repair processes. This reactivation suggests its potential in adult heart regeneration.
Clinical Relevance of the Epicardium
The epicardium is involved in various heart conditions. Pericarditis, an inflammation of the pericardium, can affect the epicardium, leading to chest pain and other symptoms. The proximity of the epicardium to the heart muscle means that inflammation in this layer can directly impact cardiac function.
The accumulation of epicardial adipose tissue (EAT), a fat depot located between the myocardium and the visceral pericardium, is significant. EAT is intimately associated with the coronary arteries and can influence cardiac function and vascular health. Under pathological conditions, EAT can secrete pro-inflammatory cytokines, locally affecting the heart and coronary arteries.
Research suggests that EAT volume is linked to higher cardiometabolic indices and subclinical left ventricular deterioration. Understanding the role of EAT in conditions like coronary artery disease and heart failure has opened avenues for potential therapeutic interventions. Lifestyle management, including weight loss and exercise, has shown promise in reducing epicardial fat deposition, offering a potential target for improving cardiovascular health.