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StatPearls . Endowment Island (FL): StatPearls Publishing; 2021 Jan-.



The love is a four-chambered body organ responsible because that pumping throughout the body. That receives deoxygenated blood from the body, sends out it to the lung, receives oxygenated blood from the lungs, and also then distributes the oxygenated blood transparent the body. At the histological level, the cellular functions of the heart play a crucial role in the normal function and adaptations the the heart.

Issues of Concern

The cells that constitute the heart room unique. It have the right to initiate and propagate electricity throughout every cardiac cell. This physiology enables the love to contract synchronously, permitting for optimal function of circulating blood to the lungs and the remainder of the distal organs.


The fibrous skeleton, cardiac muscle, and also impulse conduction mechanism constitute the straightforward framework that the heart. The base of the heart consists of a highly dense structure well-known as the fibrous or cardiac skeleton. Functions of the fibrous skeleton encompass providing together a strong framework because that cardiomyocytes, anchoring the valvular leaflets, and also acting as electric insulation separating the conduction in the atria and also ventricles.<1>

The wall surface of the heart separates into the following layers: epicardium, myocardium, and endocardium. These three layers the the heart are embryologically indistinguishable to the three layers that blood vessels: tunica adventitia, tunica media, and tunica intima, respectively. A double-layer, fluid-filled sac recognized as the pericardium, surrounds the heart. The two layers of the pericardium are called the external fibrous/parietal pericardium and also the inner serous/visceral pericardium. The epicardium constitutes the visceral pericardium, basic fibro-elastic connective tissue, and also adipose tissue.<2> Coronary arteries and also veins, lymphatic vessels and also nerves run listed below the epicardium. The endocardium is written of the endothelium and the subendothelial connective organization layer. The subendocardium is found in between the endocardium and also myocardium and also contains the impulse-conducting system.

The impulse conducting system has committed cardiac cells because that the conduction of electric impulses transparent the heart.  electric impulses initiate at the sinoatrial (SA) node, situated at the junction the the premium vena cava and also right atrium. These impulses travel throughout the atria until it will the atrioventricular (AV) node; located between the interatrial and also interventricular septum. As the fibers take trip inferiorly, that penetrates the main fibrous human body of the cardiac skeleton to type the bundle the His. These fibers room the Purkinje yarn after they division within the interventricular septum and also branch into the ventricles.

Valves are an important component that the heart. Not only do lock act together an departure gate, but they likewise prevent backflow right into the chamber. The aortic valve, separating the aorta indigenous the left ventricle, and the pulmonic valve, separating the pulmonary artery native the best ventricle, are recognized as semilunar valves. The 2 atrioventricular (AV) valves room the tricuspid and also mitral valves. The tricuspid valve clues the separation in between the appropriate atrium and right ventricle when the mitral valve off the left atrium native the left ventricle. A unique facet of the AV valves is their attachments to the ventricles with the assistance of chordae tendinae inserting ~ above the papillary muscle the the ventricles. 


The heart"s main function is come pump blood transparent the body. Cardiac function can be finest represented through cardiac output, the lot of blood pumped out of the heart per minute. Numerous factors determine the cardiac output. The product of punch volume and also heart rate equates to cardiac output. Hence, cardiac output is directly alterable with variations in these 2 factors. Punch volume is the blood volume ejected after ~ ventricular contraction, calculated by acquisition the difference in between end-diastolic volume and end-systolic volume. Contractility, afterload, and also preload can adjust stroke volume.

Preload is the amount of stress placed on cardiomyocytes by the end-diastolic volume before systole. The end-diastolic volume is the best method to measure up preload. Top top the various other hand, afterload is the total tension exerted onto the ventricle that must overcome throughout systole. The law of LaPlace is the foundation for the definition of afterload. Therefore, alters in pressure, radius, or wall thickness directly impact afterload.<3><4>

Tissue Preparation

Histological and cytological research studies of the love are important for diagnostic purposes, assessment of allograft rejection ~ a cardiac transplant, or evaluation of the impact of drug toxicity top top the heart. An endomyocardial biopsy obtains cardiac organization to be analyzed.

During an endomyocardial biopsy procedure, 1 come 2 mm3 the endocardium and myocardium is taken from the appropriate ventricle.<5> Peripheral proximity come the venous entrance of the bioptome and a more thickness wall, family member to the atrium, makes the ideal ventricle perfect location for a biopsy. The cardiac sample is then put into a fixative, such as formalin, to maintain the tissue. These preserved samples are then put into cassettes, installed in paraffin wax, thinly sliced and an installed onto glass slides. Hematoxylin and eosin is an initial, straightforward stain for visualization of the heart tissue under irradiate microscopy.

Depending top top the purpose of the endomyocardial biopsy, the an approach to prepare the tissue may vary. For instance, if viral myocarditis is on the differential, a frozen sample is needed for to know of the virus through a polymerase chain reaction.<6>

Histochemistry and also Cytochemistry

The research of cells and also tissue (histochemistry) and also intracellular tasks (cytochemistry) is valuable for narrowing down the correct diagnosis. Immunohistochemistry supplies antibodies to target details antigens in a specimen. The antibody-antigen complicated can then be stained to appreciate the visibility of the particular antigen. This test can help in the diagnosis the acute allograft rejection, amyloidosis, neoplasms, and also cardiomyopathy.<5> T-lymphocytes, viewed in myocarditis, can additionally be identified with the aid of immunohistochemistry. Immunofluorescence is very comparable to immunohistochemistry. However, the antitoxin contain a fluorescent dye which space visible once the antibody is attached come an antigen.  Immunofluorescence can assist through the diagnosis of allograft rejection and details cardiomyopathies.<7> 

Special stains highlight specific components in a specimen which can be daunting to visualize using hematoxylin and also eosin. The Congo red and also methyl violet stains are useful to finding amyloid deposits in tissue. In myocarditis or allograft rejection, methyl green-pyronine stain can spot if lymphocytes. Masson"s elastic trichrome stains connective tissue, such as elastic fiber and also collagen. In patients v iron-overload cardiomyopathy, possibly because of hemochromatosis, any kind of iron deposition in the tissue can be stained utilizing the Prussian blue stain.<5>

Microscopy Light

Histologically, the heart is mainly composed that cardiomyocytes and also connective tissue. Thick connective tissue with elastic yarn is current in the cardiac/fibrous skeleton. Particular stains such as the Masson"s elastic trichrome stains can help visualize these components. The pericardium subdivides into two layers, a superficial fibrous layer, and also deeper serous layer. The fibrous great is composed of fibrous connective tissue. The serous layer additional divides into two layers, an external layer inseparable indigenous the fibrous pericardium and an inner class overlying the myocardium. Both of these layers are histologically the same; created of a continuous layer that mesothelial cells with microvilli encountering the pericardial cavity. The fibrous pericardium and the outer serous pericardium merged is known as the parietal pericardium. The within serous pericardium, well-known as the visceral pericardium, is also component of the epicardium. In in between the outer and inner serous great is a potential room known together the pericardial cavity containing pericardial fluid, i beg your pardon is produced and reabsorbed through the microvilli on the mesothelial cells.

A huge part of the cardiac wall surface is consisted of of the myocardium. Cardiomyocytes join together to comprise this layer. This cardiomyocytes are striated like myocytes discovered in bones muscle. However, unlike skeleton muscle cells, they are branched, save on computer intercalated disks, and also are commonly mononucleated. Castle are additionally unable come regenerate. After ~ an insult, such as a myocardial infarction, the necrotic area gets replaced by scar tissue. This histological adjust is apparent under light microscopy as the fibrous component of the scar tissue is stained blue through Masson"s elastic trichrome stain.

The endocardium consists of a single layer the endothelial cell lining the chambers the the heart. Occasionally, tiny amounts the smooth muscle can likewise be in the endocardium. Contrasted to the appropriate atrium, the left atrium has a more thick endocardium because of high push from the pulmonary veins. The subendothelial layer, between the myocardium and endocardium, contain loose elastic tissue, collagen bundles, nerves and also occasionally blood vessel.

The conduction system consists of committed myocardial cells and fibers that enable for the initiation and also propagation that impulses. The SA node is created of nodal (P) cells and transitional (T) cells. This cells are looks comparable to myocardial cells but contain under myofibrils. Dense connective tissue insulates and also separates this area indigenous the rest of the atria. The atrioventricular (AV) node, situated beside the fibrous skeleton of the heart, has specialized muscle fibers that obtain impulses from the SA node. Purkinje fibers, branches native the atrioventricular nodes, have the right to be located within the epicardium. This fibers space rich in glycogen and also contain under myofibrils.<8>

Valves have actually three layers: spongiosa, fibrosa, ventricularis. Identifying this layers can aid orient the valves ~ above microscopy. The spongiosa is on the atrial next of atrioventricular valves or arterial side of semilunar valves. Big amounts the proteoglycans, such as glycosaminoglycan, and loosened connective tissue, space characteristic that the spongiosa layer.<9> The fibrosa, an expansion of the cardiac skeleton, contains dense irregular connective tissue. The ventricularis, situated on the ventricular side of the valve, has actually elastic fibers and an endothelial lining. In the AV valves, branches that the ventricularis form the chordae tendineae. The chordae tendineae is predominantly consisted of of dense constant connective tissue, in addition to collagen elastic fibers, to restrains this valves from high pressure.

Microscopy Electron

Electron microscopy permits for the image of the heart"s ultrastructure that are not visible through light microscopy. Fabry disease, cardiac myxoma, cardiomyopathy, and also amyloidosis space a couple of examples of conditions diagnosed through the assistance of electron microscopy.<5>

Individual sarcomeres that the myofibrils are plainly viewable v the assist of a transmission electron microscopy. Critical areas of the sarcomere encompass the Z-lines at the ends, central H-zone, the myosin-rich A-band, and actin-rich I-band.<10> to move structures distinctive to cardiomyocytes are also appreciated. Cardiomyocyte’s thick endomysium, plentiful mitochondria between myofibrils, intercalated disks, and T-tubules (present on the Z-lines) room visible on electron microscopy.

Two unusual attributes that deserve to be current within the cardiac myocytes room lipofuscin granules and also atrial dense-core bodies. Lipofuscin granules are a result of lysosomal digestion. The number of lipofuscin granules boosts with age. On the other hand, atrial dense-core bodies are found in the atrium and are visually opaquer than lipofuscin granules.


In hypertrophic cardiomyopathy, changes at the cellular level have a significant impact ~ above the pistol structure and physiology the the heart. There space several reasons for hypertrophic cardiomyopathy. Some examples include genetic mutation, hypertension, and aortic stenosis. An essential histologic findings space cardiomyocyte disarray (hypertrophied and also disorganized myocytes), and interstitial fibrosis the the left ventricle and interventricular septum.<11> These cellular transforms can be as result of increased afterload placed on the left ventricle or mutations in sarcomere proteins, such together the B-myosin heavy chain (MYH7) and also the cardiac myosin binding protein C (MYBPC3) genes.<12> Under electron microscopy, abnormal myocyte and also sarcomeres room visible in locations of myocyte disarray. Masson"s elastic trichrome stain can assist identify locations of fibrosis. Further hypertrophy the the myocardium reasons diastolic dysfunction and heart failure.

Clinical Significance

A variety of pathologies can influence every area the the heart. The fibrous skeleton is close to the cardiac valves and also conduction fibers. Throughout a valvectomy, sections of the fibrous skeleton might undergo inadvertently removal. Complications, such together arrhythmias, can take place if the conduction fibers room damaged.<1>

Constrictive pericarditis and cardiac tamponade room two severe pathologies entailing the pericardium. Occasionally, a pericardiectomy is essential to allow for the love to duty seamlessly. The pericardium is currently a specimen that should undergo evaluation. Fibrosis and also chronic inflammation existing in constrictive pericarditis. Granulomatous infection and also metastasis room not common yet must be rule out.  Epicardial fat deserve to be an incidental finding in these specimens.

The myocardium occupies the mass of the heart. Restrictive cardiomyopathy, viral myocarditis, cardiac allograft rejection have the right to have unusual histological findings. In border cardiomyopathy, the histology relies on the underlying etiology. Some examples of causes of limit cardiomyopathy incorporate sarcoidosis, amyloidosis, iron-overload cardiomyopathy, and also neoplasms. In famous myocarditis, the virus reasons lymphocytes to infiltrate the myocardium, and also infected cardiomyocytes undergo necrosis. Quickly after the cardiac transplant, cardiac allograft rejection might occur and must be diagnosed v an endomyocardial biopsy. Findings encompass abnormal myocytes and the visibility of lymphocytes.<5>

Aging have the right to cause far-ranging changes come the valves and conduction system. Increased collagen fibers and calcification outcomes in valvular abnormalities. An example of valvular changes in the elderly is aortic stenosis. If the valve is not replaced, severe aortic stenosis can cause syncope, angina, and also eventually, death.

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<13> Fibrosis of the SA node and also bundle branch is additionally a result of aging. Consequently, patient can construct a left bundle branch block or other related arrhythmias.

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