|
|
|
|
Chapter 12: The Heart by John F. NeasCardiopericardial Primordium Pericardial Cavity The heart and the lining of the primitive pericardial region develop during the primitive streak stage from mesoderm anterior and lateral to the embryonic disc. The lateral plate mesoderm splits into somatic and splanchnic layers that will form parietal and visceral layers. Vesicular spaces between the two layers coalesce to form a cavity that becomes the pericardial portion of the coelom. The coelom extends posteriorly on both sides of the foregut as the mesoderm continues to split. The right and left mesodermal layers become continuous ventral to the developing heart, forming a median pericardial cavity from the original right and left coelomic spaces. Endocardial Heart Tubes The development of the heart is first apparent at the eighteenth or nineteenth day in the cardiogenic area of the mesoderm layer beneath the foregut. The primordial heart consists of two layers derived from splanchnic mesenchyme—an inner tube of endocardium surrounded by epimyocardium. The endocardium appears first as scattered clusters and cords of cells (heart cords) derived from splanchnic mesoderm and situated cephalic to the oropharyngeal membrane and neural plate. The strands soon acquire a lumen and form a plexus of endothelial vessels. In the cardiac region, the endothelial vessels fuse to form thin-walled, muscular, right and left endocardial heart tubes beneath the floor of the pharynx. Anterior to the endocardial tubes, the endothelial vessels become the primitive aortae; posteriorly, they become the veins that enter the heart. Splanchnic mesoderm thickens markedly over the endocardial tubes, forming the epimyocardium (myoepicardium). The cells of this layer differentiate into the muscle cells of the myocardium and the mesothelial cells that invest the heart, or the epicardium (visceral pericardium). (The parietal layer of the pericardium develops from somatic mesoderm.) The paired endocardial heart tubes begin to migrate toward each other during the twenty-first day and soon fuse to form a single median endocardial heart tube. Thus, a heart with a single central chamber is present at the end of the third week. The heart elongates as the embryo grows, and it acquires dilatations and constrictions. When fusion is completed during the fourth week, the heart has five distinct regions. These regional divisions, in the order followed by circulating blood, are the sinus venosus, atrium, ventricle, bulbus cordis, and truncus arteriosus. (The great veins of the heart, the superior vena cava and inferior vena cava, develop from the venous end of the primitive heart tube; the trunks arteriosus carries blood to the general circulation.) The bulbus cordis and ventricle grow more rapidly than the other regions, and the heart grows more rapidly than its superior and inferior attachments. Thus, the originally straight endocardial heart tube forms a U-shaped bulboventricular loop. The heart tube later curves back upon itself, forming an S-curve that gradually becomes more pronounced. The flexures of the heart reorient the regions so that the atrium and sinus venosus eventually lie superior to the bulbus cordis, ventricle, and truncus arteriosus. The head fold causes the pericardial cavity and the endocardial tube to rotate approximately 180º so that they lie ventral to the foregut and caudal to the oropharyngeal membrane. This brings the septum transversum to a position between the pericardial cavity and yolk stalk. The heart tube then bulges dorsally into the pericardial cavity. The heart tube sinks further into the pericardial cavity and becomes dorsomedially suspended by the dorsal mesocardium, formed from the right and left epimyocardial layers. The dorsal mesocardium later disappears, except at its cephalic and caudal ends. Circulation of blood starts by the beginning of the third week when the heart begins to contract. Peristaltic waves of contraction begin in the sinus venosus and force blood through the tubular heart. Partition of the Atrioventricular Canal Partitioning of the primitive heart, with its single atrium and ventricle into the typical four-chambered structure occurs between the fourth and seventh weeks by formation of interatrial and interventricular septa. Many congenital heart problems develop during this crucial time. The opening between the primitive atrium and the primitive ventricle is at first a single channel, the common atrioventricular canal. Toward the end of the fourth week, dorsal and ventral endocardial cushions develop in the walls of the common atrioventricular canal. These grow toward each other and, during the sixth week, meet and fuse, dividing the common atrioventricular canal into right (tricuspid) and left (mitral, or bicuspid) atrioventricular canals. Partition of the Atrium The septum primum first appears during the fourth week as a partition in the dorsocephalic wall of the primitive atrium. During the fifth and sixth weeks, the septum primum grows rapidly toward the endocardial cushions, partially dividing the atrium, but leaving the foramen primum. The foramen primum obliterates when the septum primum meets the fused endocardial cushions. As this contact occurs, a second opening, the foramen secundum, appears cranially in the septum primum. The foramen secundum rapidly enlarges, permitting most blood in the right atrium to pass to the left. The septum secundum appears to the right of the septum primum during the seventh or eighth week. Eventually, its free margin grows toward the endocardial cushions but fails to divide the common atrium, leaving the foramen ovale as a communication between the two atria that persists throughout fetal development. The septum primum eventually covers the foramen ovale and serves as a one-way valve allowing blood to flow from the right atrium through the foramen ovale and foramen secundum into the left atrium. Until birth, this atrial short circuit diverts blood from the pulmonary circulation. At birth, pressure in the left atrium increases, bringing the septum primum and the septum secundum together. Fusion eliminates the foramen ovale and completes the interatrial septum. A depression, called the fossa ovalis, indicates the former foramen ovale. Sinus Venosus and Formation of the Definitive Atrium The sinus venosus consists of a small transverse portion and right and left sinus horns. The sinus venosus develops from the common cardinal, umbilical, and vitelline veins. The valves of the inferior vena cava (right and left sinal valves) guard the orifice between the sinus venosus and the right atrium. The sinus venosus incorporates into the dorsal heart wall. The right sinus horn and veins enlarge greatly and become the only communication between the original sinus venosus and the atrium. The right auricle and the rough portion of the right atrium that contain pectinate muscles represent remnants of the original embryonic right atrium. The original embryonic left atrium becomes the trabeculated left auricle. The smooth left atrium develops from the primitive pulmonary vein as its main branches incorporate into the wall. The left horn of the sinus venosus and the proximal part of the left common cardinal vein become the coronary sinus and oblique vein of the left atrium. Semilunar Valves Two very short pairs of tubercles alternate with the main truncus swellings in the portion of the bulbus cordis that contributes to the truncus arteriosus. The pulmonary and aortic channels each have one pair. A third swelling appears in both channels opposite the fused truncus swellings. These three guard the orifices of both the aorta and pulmonary artery. Gradually, they canalize to form the three cusps of the semilunar valves. Atrioventricular Valves The tricuspid and bicuspid valves arise by proliferation of subendocardial connective tissue around the atrioventricular openings and from the fused endocardial cushions. The proliferations canalize on their ventricular sides, but the newly formed valves remain connected to the trabeculae carnae of the ventricular wall by chordae tendineae attached to papillary muscles. Partition of the Ventricle The bulbar ridges (conus swellings) grow distally toward each other and unite with the aorticopulmonary septum. After fusion, they form the muscular interventricular septum. The muscular interventricular septum divides the bulbus cordis into the outflow tracts of the right and left ventricles. The two ventricular chambers openly communicate above this septum through the interventricular foramen. Connective tissue from the interventricular septum, the right side of the fused endocardial cushions, and the bulbar ridges (right dorsal and left ventral conus swellings) closes the interventricular foramen. This mass of connective tissue later becomes the membranous part of the interventricular septum. After closure, all blood from the right ventricle passes into the pulmonary trunk and all blood from the left ventricle passes to the aorta. Partition of Bulbus Cordis and Truncus Arteriosus The bulbus cordis has proximal, middle and distal divisions. The truncus arteriosus will form the roots and proximal portion of the aorta and pulmonary trunk. The conus arteriosus forms the outflow tracts of the pulmonary trunk and aortic vestibule. The narrow proximal third becomes the trabeculated portion of the right ventricle. During the fifth week, bulbar ridges develop along the right dorsal and left ventral walls of the bulbus cordis. A pair of opposing truncal ridges develops on the right superior and left inferior walls of the truncus arteriosus continuous with the bulbar ridges. The right superior and left inferior truncal ridges twist around each other while growing distally to opposite sides. Fusion creates the aorticopulmonary septum. The aorticopulmonary septum divides the bulbus cordis and truncus arteriosus into the aorta (arising from the left ventricle) and pulmonary trunk (arising from the right ventricle). The ductus arteriosus is a temporary vessel between the aorta and pulmonary trunk that functions until birth. Formation of the Conducting System The conductive tissue (sinoatrial node, atrioventricular node and bundle, and Purkinje fibers) develops from myocardial cells that acquire special conducting powers. Development of Blood and Nerve Supply The two coronary arteries rapidly arise as branches of the ascending aorta. The cardiac veins that drain the superficial myocardium are tributaries of the left horn of the sinus venosus. Later, they become the coronary sinus. Visceral afferent neurons from both vagus nerves and both sympathetic chains innervate the heart, especially the sinoatrial node. Histology of the Fetal Heart The heart assumes its adult type of gross anatomic structure by the end of the second month of gestation. The microscopic appearance between the 4th and the 5th months of gestation is rather unlike the adult heart. The endocardium (E) during this age period is always very thick and covered by a single layer of endothelial cells. The epicardium (Epi) is thicker than the endocardium and covered on the free surface by a single layer of flattened mesothelial cells. The myocardium (M) consists of a network of branching and anastomosing cylindrical or prismatic muscle fibers, approximately parallel with each other. The cross-striations of the myocardium begin to be apparent during the 3rd month, but they are not usually very obvious in H & E sections of autopsy specimens until after birth. A = a branch of the coronary artery; P = papillary muscle. Partition of the Atrioventricular Canal Sinus Venosus and Formation of the Definitive Atrium Partition of Bulbus Cordis and Truncus Arteriosus
|