Sulcus Of The Heart

Figure 11.1 A. Dorsal view of a late presomite embryo (approximately 18 days) after removal of the amnion. Prospective myoblasts and hemangioblasts reside in the splanchnic mesoderm in front of the neural plate and on each side of the embryo. B. Transverse section through a similar-staged embryo to show the position of the blood islands in the splanchnic mesoderm layer. C. Cephalocaudal section through a similar-staged embryo showing the position of the pericardial cavity and cardiogenic field. D. Scanning electron micrograph of a mouse embryo equivalent to 19 days in the human, showing coalescence of the blood islands by vasculogenesis into a horseshoe-shaped heart tube (arrows) lying in the primitive pericardial cavity under the cranial neural folds (asterisks).

Formation and Position of the Heart Tube

Initially, the central portion of the cardiogenic area is anterior to the buccopharyngeal membrane and the neural plate (Fig. 11.2 A). With closure of the neural tube and formation of the brain vesicles, however, the central nervous system grows cephalad so rapidly that it extends over the central cardiogenic area and the future pericardial cavity (Fig. 11.2). As a result of growth of the brain and cephalic folding of the embryo, the buccopharyngeal membrane is pulled forward, while the heart and pericardial cavity move first to the cervical region and finally to the thorax (Fig. 11.2).

As the embryo folds cephalocaudally, it also folds laterally (Fig. 11.3). As a result, the caudal regions of the paired cardiac primordia merge except at their caudalmost ends. Simultaneously, the crescent part of the horseshoe-shaped area expands to form the future outflow tract and ventricular regions. Thus, the heart becomes a continuous expanded tube consisting of an inner endothelial lining and an outer myocardial layer. It receives venous drainage at its caudal pole and begins to pump blood out of the first aortic arch into the dorsal aorta at its cranial pole (Figs. 11.4 and 11.5).

The developing heart tube bulges more and more into the pericardial cavity. Initially, however, the tube remains attached to the dorsal side of the pericardial cavity by a fold of mesodermal tissue, the dorsal mesocardium (Figs. 11.3 and 11.5). No ventral mesocardium is ever formed. With further development, the dorsal mesocardium disappears, creating the transverse pericardial sinus, which connects both sides of the pericardial cavity. The heart is now suspended in the cavity by blood vessels at its cranial and caudal poles (Fig. 11.5).

During these events, the myocardium thickens and secretes a thick layer of extracellular matrix, rich in hyaluronic acid, that separates it from the en-dothelium (Figs. 11.3 and 11.5). In addition, mesothelial cells from the region of the sinus venosus migrate over the heart to form the epicardium. Thus the heart tube consists of three layers: (a) the endocardium, forming the internal endothelial lining of the heart; (b) the myocardium, forming the muscular wall; and (c) the epicardium or visceral pericardium, covering the outside of the tube. This outer layer is responsible for formation of the coronary arteries, including their endothelial lining and smooth muscle.

Formation of the Cardiac Loop

The heart tube continues to elongate and bend on day 23. The cephalic portion of the tube bends ventrally, caudally, and to the right (Fig. 11.6, B and C), and the atrial (caudal) portion shifts dorsocranially and to the left (Figs. 11.6 and 11.7A). This bending, which may be due to cell shape changes, creates the cardiac loop. It is complete by day 28.

While the cardiac loop is forming, local expansions become visible throughout the length of the tube. The atrial portion, initially a paired structure outside

Hindgut

Hindgut

Cardiogenic Area

Figure 11.2 Figures showing effects of the rapid growth of the brain on positioning of the heart. Initially the cardiogenic area and the pericardial cavity are in front of the buccopharyngeal membrane. A. 18 days. B. 20 days. C. 21 days. D. 22 days. E. Scanning electron micrograph of a mouse embryo at a stage similar to that shown in C. The amnion, yolk sac, and caudal half of the embryo have been removed. The head folds (HF) are expanding and curving over the heart (H) and pericardial cavity (asterisk). The intestinal opening (arrow) of the gut into the primitive pharynx and the endoderm (E) of the open region of the gut tube are shown.

Figure 11.2 Figures showing effects of the rapid growth of the brain on positioning of the heart. Initially the cardiogenic area and the pericardial cavity are in front of the buccopharyngeal membrane. A. 18 days. B. 20 days. C. 21 days. D. 22 days. E. Scanning electron micrograph of a mouse embryo at a stage similar to that shown in C. The amnion, yolk sac, and caudal half of the embryo have been removed. The head folds (HF) are expanding and curving over the heart (H) and pericardial cavity (asterisk). The intestinal opening (arrow) of the gut into the primitive pharynx and the endoderm (E) of the open region of the gut tube are shown.

Cardiac Tube Formation Splanchnic

Figure 11.3 Transverse sections through embryos at different stages of development, showing formation of a single heart tube from paired primordia. A. Early presomite embryo (17 days). B. Late presomite embryo (18 days). C. Eight-somite stage (22 days). Fusion occurs only in the caudal region of the horseshoe-shaped tube (see Fig. 12.4). The outflow tract and most of the ventricular region form by expansion and growth of the crescent portion of the horseshoe.

Figure 11.3 Transverse sections through embryos at different stages of development, showing formation of a single heart tube from paired primordia. A. Early presomite embryo (17 days). B. Late presomite embryo (18 days). C. Eight-somite stage (22 days). Fusion occurs only in the caudal region of the horseshoe-shaped tube (see Fig. 12.4). The outflow tract and most of the ventricular region form by expansion and growth of the crescent portion of the horseshoe.

the pericardial cavity, forms a common atrium and is incorporated into the pericardial cavity (Figs. 11.7A). The atrioventricular junction remains narrow and forms the atrioventricular canal, which connects the common atrium and the early embryonic ventricle (Fig. 11.8). The bulbus cordis is narrow except for its proximal third. This portion will form the trabeculated part of the right ventricle (Figs. 11.7B and 11.8). The midportion, the conus cordis, will form the outflow tracts of both ventricles. The distal part of the bulbus, the truncus arteriosus, will form the roots and proximal portion of the aorta and pulmonary artery (Fig. 11.8). The junction between the ventricle and the bulbus cordis, externally indicated by the bulboventricular sulcus (Fig. 11.6C), remains narrow. It is called the primary interventricular foramen (Fig. 11.8). Thus, the cardiac tube is organized by regions along its craniocaudal axis from the conotrun-cus to the right ventricle to the left ventricle to the atrial region, respectively (Fig. 11.6, A - C). Evidence suggests that organization of these segments is regulated by homeobox genes in a manner similar to that for the craniocaudal axis of the embryo (see Chapter 5).

Heart Tube Mouse Embryo

Figure 11.4 Formation of the heart tube on days 19, 20, 21, and 22 in scanning electron micrographs of mouse embryos at equivalent stages of human development. A. The heart tube (arrows) is horseshoe shaped in the pericardial cavity beneath the neural folds (stars). B. The crescent portion of the horseshoe expands to form the ventricular and outflow tract regions, while lateral folding brings the caudal (venous) poles of the horseshoe together (see Fig. 12.3). C. The caudal regions begin to fuse. D. Fusion of the caudal regions is complete, leaving the caudal poles embedded in the septum transversum (arrowheads). Cardiac looping has also been initiated. Asterisk,pericardial cavity; large arrow, anterior intestinal portal.

Figure 11.4 Formation of the heart tube on days 19, 20, 21, and 22 in scanning electron micrographs of mouse embryos at equivalent stages of human development. A. The heart tube (arrows) is horseshoe shaped in the pericardial cavity beneath the neural folds (stars). B. The crescent portion of the horseshoe expands to form the ventricular and outflow tract regions, while lateral folding brings the caudal (venous) poles of the horseshoe together (see Fig. 12.3). C. The caudal regions begin to fuse. D. Fusion of the caudal regions is complete, leaving the caudal poles embedded in the septum transversum (arrowheads). Cardiac looping has also been initiated. Asterisk,pericardial cavity; large arrow, anterior intestinal portal.

Foregut

Foregut

Foregut And Heart Weeks Human

Endocardial heart tube

Figure 11.5 Cephalic end of an early somite embryo. The developing endocardial heart tube and its investing layer bulge into the pericardial cavity. The dorsal mesocardium is breaking down.

Endocardial heart tube

Figure 11.5 Cephalic end of an early somite embryo. The developing endocardial heart tube and its investing layer bulge into the pericardial cavity. The dorsal mesocardium is breaking down.

At the end of loop formation, the smooth-walled heart tube begins to form primitive trabeculae in two sharply defined areas just proximal and distal to the primary interventricular foramen (Fig. 11.8). The bulbus temporarily remains smooth walled. The primitive ventricle, which is now trabeculated, is called the primitive left ventricle. Likewise, the trabeculated proximal third of the bulbus cordis may be called the primitive right ventricle (Fig. 11.8).

The conotruncal portion of the heart tube, initially on the right side of the pericardial cavity, shifts gradually to a more medial position. This change in position is the result of formation of two transverse dilations of the atrium, bulging on each side of the bulbus cordis (Figs. 11.7B and 11.8).

CLINICAL CORRELATES Abnormalities of Cardiac Looping

Dextrocardia, in which the heart lies on the right side of the thorax instead of the left, is caused because the heart loops to the left instead of the right. Dextrocardia may coincide with situs inversus, a complete reversal of asymmetry in all organs. Situs inversus, which occurs in 1/7000 individuals, usually is associated with normal physiology, although there is a slight risk of heart defects. In other cases sidedness is random, such that some organs are reversed and others are not; this is heterotaxy. These cases are classified as

Diagram Mouse Embryo

Figure 11.6 Formation of the cardiac loop. A. 22 days. B. 23 days. C. 24 days. Broken line, pericardium. D and E. Scanning electron micrographs of mouse embryos showing frontal views of the process shown in the diagrams. Initially the cardiac tube is short and relatively straight (D), but as it lengthens, it bends (loops), bringing the atrial region cranial and dorsal to the ventricular region (E). The tube is organized in segments, illustrated by the different colors, from the outflow region to the right ventricle to the left ventricle to the atrial region. These segments represent a craniocaudal axis that appears to be regulated by homeobox gene expression. A, primitive atrium; arrow, septum transversum; S, sinus venosus; V, ventricle.

Figure 11.6 Formation of the cardiac loop. A. 22 days. B. 23 days. C. 24 days. Broken line, pericardium. D and E. Scanning electron micrographs of mouse embryos showing frontal views of the process shown in the diagrams. Initially the cardiac tube is short and relatively straight (D), but as it lengthens, it bends (loops), bringing the atrial region cranial and dorsal to the ventricular region (E). The tube is organized in segments, illustrated by the different colors, from the outflow region to the right ventricle to the left ventricle to the atrial region. These segments represent a craniocaudal axis that appears to be regulated by homeobox gene expression. A, primitive atrium; arrow, septum transversum; S, sinus venosus; V, ventricle.

Conus Cordis And Truncus Arteriosus

Figure 11.7 Heart of a 5-mm embryo (28 days). A. Viewed from the left. B. Frontal view. The bulbus cordis is divided into the truncus arteriosus, conus cordis, and trabeculated part of the right ventricle. Broken line, pericardium. C. Scanning electron micrograph of the heart of a similar-staged mouse embryo showing a view similar to B.

Figure 11.7 Heart of a 5-mm embryo (28 days). A. Viewed from the left. B. Frontal view. The bulbus cordis is divided into the truncus arteriosus, conus cordis, and trabeculated part of the right ventricle. Broken line, pericardium. C. Scanning electron micrograph of the heart of a similar-staged mouse embryo showing a view similar to B.

laterality sequences. Patients with these conditions appear to be predominantly left sided bilaterally or right sided bilaterally The spleen reflects the differences: those with left-sided bilaterality have polysplenia; those with right-sided bilaterality have asplenia or hypoplastic spleen. Patients with laterality sequences also have increased incidences of other malformations, especially heart defects. Genes regulating sidedness are expressed during gastrulation (see Chapter 4).

Molecular Regulation of Cardiac Development

Signals from anterior (cranial) endoderm induce a heart-forming region in overlying splanchnic mesoderm by turning on the transcription factor NKX2.5. The

Bulboventricular Sulcus
Figure 11.8 Frontal section through the heart of a 30-day embryo showing the primary interventricular foramen and entrance of the atrium into the primitive left ventricle. Note the bulboventricular flange. Arrows, direction of blood flow.

signals require secretion of bone morphogenetic proteins (BMPs) 2 and 4 and inhibitors (crescent) of WNT genes in the endoderm and lateral plate meso-derm (Fig. 11.9). This combination is responsible for inducing expression of NKX2.5 that specifies the cardiogenic field and, later, plays a role in septation and in development of the conduction system.

NKX2.5 contains a homeodomain and is a homologue of the gene tinman, which regulates heart development in Drosophila. TBX5is another transcription factor that contains a DNA-binding motif known as the T-box. Expressed later than NKX2.5, it plays a role in septation.

Development of the Sinus Venosus

In the middle of the fourth week, the sinus venosus receives venous blood from the right and left sinus horns (Fig. 11.10A). Each horn receives blood

Primitive Streak Induction

Figure 11.9 Heart induction. BMPs secreted in the posterior portion of the primitive streak and periphery of the embryo, in combination with inhibition of WNT expression by crescent in the anterior half of the embryo, induce expression of NKX2.5 in the heart forming region of the lateral plate mesoderm (splanchnic layer). NKX2.5 is then responsible for heart induction.

(crescent)

Figure 11.9 Heart induction. BMPs secreted in the posterior portion of the primitive streak and periphery of the embryo, in combination with inhibition of WNT expression by crescent in the anterior half of the embryo, induce expression of NKX2.5 in the heart forming region of the lateral plate mesoderm (splanchnic layer). NKX2.5 is then responsible for heart induction.

from three important veins: (a) the vitelline or omphalomesenteric vein, (b) the umbilical vein, and (c) the common cardinal vein. At first communication between the sinus and the atrium is wide. Soon, however, the entrance of the sinus shifts to the right (Fig. 11.10B). This shift is caused primarily by left-to-right shunts of blood, which occur in the venous system during the fourth and fifth weeks of development.

With obliteration of the right umbilical vein and the left vitelline vein during the fifth week, the left sinus horn rapidly loses its importance (Fig. 11.10B). When the left common cardinal vein is obliterated at 10 weeks, all that remains of the left sinus horn is the oblique vein of the left atrium and the coronary sinus (Fig. 11.11).

As a result of left-to-right shunts of blood, the right sinus horn and veins enlarge greatly. The right horn, which now forms the only communication between the original sinus venosus and the atrium, is incorporated into the right atrium to form the smooth-walled part of the right atrium (Fig. 11.12). Its entrance, the sinuatrial orifice, is flanked on each side by a valvular fold, the right and left venous valves (Fig. 11.12 A). Dorsocranially the valves fuse, forming a ridge known as the septum spurium (Fig. 11.12 A). Initially the valves are large, but when the right sinus horn is incorporated into the wall of the atrium, the left venous valve and the septum spurium fuse with the developing atrial septum (Fig. 11.12C). The superior portion of the right venous valve disappears entirely. The inferior portion develops into two parts: (a) the valve of

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Responses

  • baldovino
    What is sulcus of the heart?
    8 years ago
  • Rhoda
    What is the location of the cardiogenic area in the presomite embryo?
    7 years ago

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