Histology of the Heart wall - Shotgun Histology

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Histology of the Heart wall - Shotgun Histology

The heart is a four-chambered organ responsible for pumping throughout the body. It receives deoxygenated blood from the body, sends it to the lung, receives oxygenated blood from the lungs, and then distributes the oxygenated blood throughout the body. At the histological level, the cellular features of the heart play a vital role in the normal function and adaptations of the heart.

The fibrous skeleton, cardiac muscle, and impulse conduction system constitute the basic framework of the heart. The base of the heart contains a highly dense structure known as the fibrous or cardiac skeleton. Functions of the fibrous skeleton include providing as a strong framework for cardiomyocytes, anchoring the valvular leaflets, and acting as electrical insulation separating the conduction in the atria and ventricles.

The wall of the heart separates into the following layers: epicardium, myocardium, and endocardium. These three layers of the heart are embryologically equivalent to the three layers of blood vessels: tunica adventitia, tunica media, and tunica intima, respectively. A double-layer, fluid-filled sac known as the pericardium, surrounds the heart. The two layers of the pericardium are called the outer fibrous/parietal pericardium and the inner serous/visceral pericardium. The epicardium constitutes the visceral pericardium, underlying fibro-elastic connective tissue, and adipose tissue.Coronary arteries and veins, lymphatic vessels and nerves run below the epicardium. The endocardium is composed of the endothelium and the subendothelial connective tissue layer. The subendocardium is found between the endocardium and myocardium and contains the impulse-conducting system.

The impulse conducting system has specialized cardiac cells for the conduction of electrical impulses throughout the heart. Electrical impulses initiate at the sinoatrial (SA) node, situated at the junction of the superior vena cava and right atrium. These impulses travel throughout the atria until it reaches the atrioventricular (AV) node; located between the interatrial and interventricular septum. As the fibers travel inferiorly, it penetrates the central fibrous body of the cardiac skeleton to form the bundle of His. These fibers are the Purkinje fibers after they divide within the interventricular septum and branch into the ventricles.

Valves are an important component of the heart. Not only do they act as an exit gate, but they also prevent backflow into the chamber. The aortic valve, separating the aorta from the left ventricle, and the pulmonic valve, separating the pulmonary artery from the right ventricle, are known as semilunar valves. The two atrioventricular (AV) valves are the tricuspid and mitral valves. The tricuspid valve marks the separation between the right atrium and right ventricle while the mitral valve separates the left atrium from the left ventricle. A unique aspect of the AV valves is their attachments to the ventricles with the assistance of chordae tendinae inserting onto the papillary muscle of the ventricles.

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