Anatomy of the Heart and Blood Vessels Is Pre- Seven Distinct Septa
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ANATOMY OF THE HEART 1 AND BLOOD VESSELS A fundamental knowledge of cardiac anat- cushion tissue, or primitive connective tissue, omy provides a concrete foundation for diag- begins to invade the cardiac jelly lying between nostic cardiovascular pathology and is requisite the endodermal and mesothelial layers (2). for understanding the clinical manifestations of Over the ensuing weeks, this heart tube will cardiac disease. In this introductory chapter, the form, through active and passive mechanisms, anatomy of the heart and blood vessels is pre- seven distinct septa. These divide it into a four- sented, with special emphasis on development, chamber structure with two pairs of valves, two gross structure, and histology to allow for a com- large arteries, and typically six distinct venous plete understanding of the tumors and tumor- connections. like conditions described in this Fascicle. It follows from this brief overview that the heart and pericardium are derived entirely from CARDIAC EMBRYOLOGY mesodermal structures. All primary tumors of the heart and pericardium are also of mesodermal Formation of the Heart Tube origin. Cardiac tumors can also primarily consist Approximately 2 weeks after fertilization, a of mesodermal elements, such as fat, that become crescentic zone of thickened mesoderm, the entrapped during the process of septation. precursor of the heart and pericardium, appears The exceptions consist of tumors of mis- adjacent to the margin of the embryonic disc placed endodermal rests, and neural tumors, (). A day or two later, this thickened zone of which are of ectodermal origin. Cardiac tumors mesoderm splits into somatic and splanchnic with endodermal elements, such as glandular layers, which surround the pericardial portion structures present in bronchogenic cysts, tumors of the coelomic cavity, or pericardial coelom. of the atrioventricular (AV) node, are presumed Endothelial tubes (cardiac primordial) form as to arise from embryologically misplaced tissues paired, lateral structures from the splanchnic (3). Theoretically, endodermal structures could mesodermal layer of the primitive pericardial be incorporated into cardiac tissue early in em- cavity where it lies close against the develop- bryogenesis when the foregut is adjacent to the ing foregut. By the third week, the endocardial laterally placed cardiogenic plates (4). Neural tubes have fused in the midline, forming the tumors of the heart are rare and are virtually primitive heart tube, consisting of three basic limited to paragangliomas and granular cell layers: the endocardial layer, intervening cardiac tumors of the epicardial surfaces and atria. “jelly,” and a mesothelial layer lining the peri- The histogenesis of some cardiac tumors, cardial coelom. The heart tube has arterial and such as cardiac myxoma, remains somewhat venous poles at the cranial and caudal aspects, controversial. The cardiac jelly, which supports respectively, and begins to beat. the epimyocardial layer and endothelial layer, is present from before 2 weeks of gestation (four- Looping of the Heart Tube and Septation somite embryo), and becomes infiltrated from Rapid bulboventricular growth of the tube, the endothelial surface by endocardial cushion which is tethered to the dorsal aspect of the cells (2). These cells are the putative cells of embryo and contained within the pericardial origin of cardiac myxoma, and are believed cavity, causes an anterior and rightward displace- to persist in some adults near the fossa ovalis ment of the primitive ventricular portion and a (5). The rare occurrence of glandular structures posterior displacement of its primitive atrial and within cardiac myxoma is an incidental curios- venous portions. During this time, endocardial ity for the surgical pathologist, who should not Tumors of the Heart and Great Vessels Figure 1-1 CARDIAC SILHOUETTE The size of the heart, in relation to the thorax, is demonstrated here on chest radiograph. The heart occupies less than 60 percent of the thorax in this view. Figure 1-2 PARIETAL PERICARDIUM mistake them for adenocarcinoma. It is difficult, With the anterior aspect of the parietal pericardium however, to satisfactorily explain their presence removed, the intrapericardial portions of the superior vena cava, aorta, and pulmonary artery are apparent. from an embryologic view. In some cases, glan- dular cells appear to evolve from myxoma cells, casting doubt on the theory that they arise from the atria forming the base and the ventricles entrapped endodermal rests (6). It is also possible forming the apex, which is typically directed that cardiac myxoma is a true neoplasm of plu- leftward, anterior, and inferior. ripotent mesodermal cells (endocardial cushion External Cardiac Anatomy cells) and that the neoplastic alteration is capable of inciting these cells, which are of mesodermal The heart is covered and surrounded by origin, to form mucin-producing glands. the fluid-containing pericardial sac, which consists of both visceral and parietal portions. ANATOMY OF THE HEART The visceral pericardium, or epicardium, covers AND GREAT VESSELS the heart and intrapericardial portions of the great vessels, while the parietal pericardium Cardiac Position surrounds the heart, attaching along the great Consistent with their embryonic origins as vessels at the reflection of the visceral layer midline structures, the heart and great vessels (fig.-2). The space between these two layers are situated in the mid-thorax, within the me- contains serous pericardial fluid (25 mL or less diastinum. The heart, aortic arch, and descend- in adults) providing for friction-free movement ing thoracic aorta are located in the middle, within the chest. superior, and posterior regions, respectively. The AV groove is found at the plane of the Radiologically, the heart occupies less than 60 base of the heart and contains the circumflex percent of the thorax when viewed on a postero- and right coronary arteries, which travel in the anterior chest radiograph (fig. -). The heart is a left and right AV grooves, respectively. The an- four-chambered, roughly conical structure, with terior and inferior interventricular grooves are 2 Anatomy of the Heart and Blood Vessels found at the plane of the ventricular septum and have an endocrine role by releasing natriuretic contain the anterior and posterior descending hormones in response to atrial stretch, helping coronary arteries, respectively. to maintain fluid homeostasis (7). The anterior and inferior free walls of the The atria consist of both septal and free wall right ventricle intersect to form the acute mar- portions. The free wall of the right atrium is gin. The rounded lateral wall of the left ventricle smooth posteriorly and trabeculated anteriorly. forms a more ill-defined obtuse margin. Vessels The smooth portion, derived from the embry- supplying these regions are named accordingly; onic sinus venosus, receives the vena cavae and obtuse marginal branches have the circumflex is bordered anteriorly by the crista terminalis. coronary artery and acute marginal branches The trabeculated portion contains prominent of the right coronary artery. The intersection muscular bands, the pectinate muscles, that of the major lines of division (atrioventricular, arise perpendicular to the crista terminalis and interventricular, and interatrial) is along the dia- extend anteriorly to involve the pyramidal right phragmatic surface of the heart and is referred atrial appendage (fig. -4, left). The free wall of to as the crux of the heart. the left atrium includes a dome-shaped body, Tumor location within the heart is relevant which receives the pulmonary veins, and a to not only its potential hemodynamic conse- worm-like atrial appendage (fig. -4, right). Like quences, but also often provides insights into the right atrium, the left atrial appendage con- the type of tumor (since certain cardiac tumors tains pectinate muscles, although they are not tend to have predilections for various locations prominent and are isolated to the periphery of within the heart). Thus, an understanding of not the appendage where they are radially arranged. only the normal size and shape of the cardiac The left atrium contains no crista terminalis. chambers and great vessels, but also their relative The atrial septum, when viewed from the positions three dimensionally, is required (fig. - right aspect, contains both an interatrial com- 3). The right atrium forms the right lateral border ponent (between the right and left atria) and an of the heart while the right ventricle is situated atrioventricular component (between the right so that it forms the anterior surface of the heart. atrium and the left ventricle). This is because the The left ventricle is a largely posterior structure, tricuspid valve annulus is situated more apically also forming the left lateral heart border. The left than the mitral valve annulus at the septum atrium lies in a midline-posterior position and (fig. -5). The interatrial portion contains the is really not a left-sided structure at all. fossa ovalis, with its two major components: The pulmonary artery arises anterior, supe- a central sheet-like region (the valve of the rior, and to the left of the aorta. The superior fossa ovalis) surrounded by a horseshoe-shaped and inferior vena cavae are continuous with the muscular ridge referred to as the limbus of the right lateral heart border formed by the right fossa ovalis (fig. -6) (8). In some individuals, atrium. The coronary sinus is the major