414.1 Early Cardiac Morphogenesis

Daniel Bernstein

In the early presomite embryo, the 1st identifiable cardiac progenitor cell clusters are arranged in the anterior lateral plate mesoderm on both sides of the embryo’s central axis; these clusters form paired cardiac tubes by 18 days of gestation. The paired tubes fuse in the midline on the ventral surface of the embryo to form the primitive heart tube by 22 days. This straight heart tube is composed of an outer myocardial layer, an inner endocardium, and a middle layer of extracellular matrix known as the cardiac jelly. There are 2 distinct cell lineages: the primary heart field provides precursor cells for the left ventricle, whereas the secondary heart field provides precursors for the atria and right ventricle. Premyocardial cells, including epicardial cells and cells derived from the neural crest, continue their migration into the region of the heart tube. Regulation of this early phase of cardiac morphogenesis is controlled in part by the interaction of specific signaling molecules or ligands, usually expressed by 1 cell type, with specific receptors, usually expressed by another cell type. Positional information is conveyed to the developing cardiac mesoderm by factors such as retinoids (isoforms of vitamin A), which bind to specific nuclear receptors and regulate gene transcription. Migration of epithelial cells into the developing heart tube is directed by extracellular matrix proteins (such as fibronectin) interacting with cell surface receptors (the integrins). Other important regulatory molecules include bone morphogenetic protein 2 (BMP2); fibroblast growth factor 4 (FGF4), the transcription factors Nkx2.5, GATA4, Mesp1, and Mesp2; and members of the Wnt/β-catenin signaling pathway. The clinical importance of these ligands is revealed by the spectrum of cardiac teratogenic effects caused by the retinoid-like drug isotretinoin.

As early as 20-22 days, before cardiac looping, the embryonic heart begins to contract and exhibit phases of the cardiac cycle that are surprisingly similar to those in the mature heart. Morphologists initially identified segments of the heart tube that were believed to correspond to structures in the mature heart (Web Fig. 414-1): the sinus venosus and atrium (right and left atria), the primitive ventricle (left ventricle), the bulbus cordis (right ventricle), and the truncus arteriosus (aorta and pulmonary artery). However, this model is oversimplified. Only the trabecular (most heavily muscularized) portions of the left ventricular myocardium are present in the early cardiac tube; the cells that will become the inlet portion of the left ventricle migrate into the cardiac tube at a later stage (after looping is initiated). Even later to appear are the primordial cells that give rise to the great arteries (truncus arteriosus), including cells derived from the neural crest, which are not present until after cardiac looping is complete. Chamber-specific transcription factors participate in the differentiation of the right and left ventricles. The basic helix-loop-helix (bHLH) transcription factor dHAND is expressed in the developing right ventricle; disruption of this gene or of other transcriptional factors such as myocyte enhancer factors 2C (MEF2C) in mice leads to hypoplasia of the right ventricle. The transcription factor eHAND is expressed in the developing left ventricle and conotruncus and is also critical to their development.

Web Figure 414-1 Timeline of cardiac morphogenesis.

(From Larsen WJ: Essentials of human embryology, New York, 1998, Churchill Livingstone.)

414.2 Cardiac Looping

Daniel Bernstein

At ≈22-24 days, the heart tube begins to bend ventrally and toward the right (see Web Fig. 414-1). The heart is the 1st organ to escape from the bilateral symmetry of the early embryo. Looping brings the future left ventricle leftward and in continuity with the sinus venosus (future left and right atria), whereas the future right ventricle is shifted rightward and in continuity with the truncus arteriosus (future aorta and pulmonary artery). This pattern of development explains the relatively common occurrence of the cardiac anomalies double-outlet right ventricle and double-inlet left ventricle and the extreme rarity of double-outlet left ventricle and double-inlet right ventricle (Chapter 424.5). When cardiac looping is abnormal (situs inversus, heterotaxia), the incidence of serious cardiac malformations is high and there are usually associated abnormalities in the L-R patterning of the lungs and abdominal viscera.

Potential mechanisms of cardiac looping include differential growth rates for myocytes on the convex vs the concave surface of the curve, differential rates of programmed cell death (apoptosis), and mechanical forces generated within myocardial cells via their actin cytoskeleton. The signal for this directionality is contained in a concentration gradient between the right and left sides of the embryo in the expression of critical signaling molecules. A number of signaling pathways have been identified as regulators of this L-R asymmetry, including sonic hedgehog (Shh), transforming growth factor-β (TGF-β), nodal, and LR dynein. Interestingly, mice in which the LR dynein gene has been inactivated display random L-R orientation of the heart and abdominal viscera, with 50% of their hearts looping to the right and 50% looping to the left.

414.3 Cardiac Septation

Daniel Bernstein

When looping is complete, the external appearance of the heart is similar to that of a mature heart; internally, the structure resembles a single tube, although it now has several bulges resulting in the appearance of primitive chambers. The common atrium (comprising both the right and left atria) is connected to the primitive ventricle (future left ventricle) via the atrioventricular canal. The primitive ventricle is connected to the bulbus cordis (future right ventricle) via the bulboventricular foramen. The distal portion of the bulbus cordis is connected to the truncus arteriosus via an outlet segment (the conus).

The heart tube now consists of several layers of myocardium and a single layer of endocardium separated by cardiac jelly, an acellular extracellular matrix secreted by the myocardium. Septation of the heart begins at approximately day 26 with the ingrowth of large tissue masses, the endocardial cushions, at both the atrioventricular and conotruncal junctions (see Web Fig. 414-1). These cushions consist of protrusions of cardiac jelly, which, in addition to their role in development, also serve a physiologic function as primitive heart valves. Endocardial cells dedifferentiate and migrate into the cardiac jelly in the region of the endocardial cushions, eventually becoming mesenchymal cells that will form part of the atrioventricular valves.

Complete septation of the atrioventricular canal occurs with fusion of the endocardial cushions. Most of the atrioventricular valve tissue is derived from the ventricular myocardium in a process involving undermining of the ventricular walls. Because this process occurs asymmetrically, the tricuspid valve annulus sits closer to the apex of the heart than the mitral valve annulus does. Physical separation of these 2 valves produces the atrioventricular septum, the absence of which is the primary common defect in patients with atrioventricular canal defects (Chapter 420.5). If the process of undermining is incomplete, one of the atrioventricular valves may not separate normally from the ventricular myocardium, a possible cause of Ebstein anomaly (Chapter 424.7).

Septation of the atria begins at ≈30 days with growth of the septum primum downward toward the endocardial cushions (see Web Fig. 414-1). The orifice that remains is the ostium primum. The endocardial cushions then fuse and, together with the completed septum primum, divide the atrioventricular canal into right and left segments. A 2nd opening appears in the posterior portion of the septum primum, the ostium secundum, and it allows a portion of the fetal venous return to the right atrium to pass across to the left atrium. Finally, the septum secundum grows downward, just to the right of the septum primum. Together with a flap of the septum primum, the ostium secundum forms the foramen ovale, through which fetal blood passes from the inferior vena cava to the left atrium (Chapter 415).

Septation of the ventricles begins at about embryonic day 25 with protrusions of endocardium in both the inlet (primitive ventricle) and outlet (bulbus cordis) segments of the heart. The inlet protrusions fuse into the bulboventricular septum and extend posteriorly toward the inferior endocardial cushion, where they give rise to the inlet and trabecular portions of the interventricular septum. Ventricular septal defects can occur in any portion of the developing interventricular septum (Chapter 420.6). The outlet or conotruncal septum develops from ridges of cardiac jelly, similar to the atrioventricular cushions. These ridges fuse to form a spiral septum that brings the future pulmonary artery into communication with the anterior and rightward right ventricle and the future aorta into communication with the posterior and leftward left ventricle. Differences in cell growth of the outlet septum lead to lengthening of the segment of smooth muscle beneath the pulmonary valve (conus), a process that separates the tricuspid and pulmonary valves. In contrast, disappearance of the segment beneath the aortic valve leads to fibrous continuity of the mitral and aortic valves. Defects in these processes are responsible for conotruncal and aortic arch defects (truncus arteriosus, tetralogy of Fallot, pulmonary atresia, double-outlet right ventricle, interrupted aortic arch), a group of cardiac anomalies often associated with deletions of the DiGeorge critical region of chromosome 22q11 (Chapters 417 and 418). The transcription factor Tbx1 has been implicated as a candidate gene, which may be responsible for DiGeorge syndrome. Several genes have been implicated in valve formation, including Ptpn11, which encodes the tyrosine phosphatase Shp-2, and when present in a mutated from leads to Noonan syndrome, associated with pulmonary valve stenosis; and NOTCH1, a regulator of cell differentiation that has been associated with aortic valve disease.

414.4 Aortic Arch Development

Daniel Bernstein

The aortic arch, head and neck vessels, proximal pulmonary arteries, and ductus arteriosus develop from the aortic sac, arterial arches, and dorsal aortae. When the straight heart tube develops, the distal outflow portion bifurcates into the right and left 1st aortic arches, which join the paired dorsal aortae (Fig. 414-1). The dorsal aortae will fuse to form the descending aorta. The proximal aorta from the aortic valve to the left carotid artery arises from the aortic sac. The 1st and 2nd arches largely regress by about 22 days, with the 1st aortic arch giving rise to the maxillary artery and the 2nd to the stapedial and hyoid arteries. The 3rd arches participate in the formation of the innominate artery and the common and internal carotid arteries. The right 4th arch gives rise to the innominate and right subclavian arteries, and the left 4th arch participates in formation of the segment of the aortic arch between the left carotid artery and the ductus arteriosus. The 5th arch does not persist as a major structure in the mature circulation. The 6th arches join the more distal pulmonary arteries, with the right 6th arch giving rise to a portion of the proximal right pulmonary artery and the left 6th arch giving rise to the ductus arteriosus. The aortic arch between the ductus arteriosus and the left subclavian artery is derived from the left-sided dorsal aorta, whereas the aortic arch distal to the left subclavian artery is derived from the fused right and left dorsal aortae. Abnormalities in development of the paired aortic arches are responsible for right aortic arch, double aortic arch, and vascular rings (Chapter 426.1).

Figure 414-1 Schematic drawings illustrating the changes that result during transformation of the truncus arteriosus, aortic sac, aortic arches, and dorsal aortae into the adult arterial pattern. The vessels that are not shaded or colored are not derived from these structures. A, Aortic arches at 6 wk; by this stage the 1st 2 pairs of aortic arches have largely disappeared. B, Aortic arches at 7 wk; the parts of the dorsal aortae and aortic arches that normally disappear are indicated by broken lines. C, Arterial vessels of a 6 mo old infant.

(From Moore KL, Persaud TVN, Torchia M: The developing human, Philadelphia, 2007, Elsevier.)

414.5 Cardiac Differentiation

Daniel Bernstein

The process by which the totipotential cells of the early embryo become committed to specific cell lineages is differentiation. Precardiac mesodermal cells differentiate into mature cardiac muscle cells with an appropriate complement of cardiac-specific contractile elements, regulatory proteins, receptors, and ion channels. Expression of the contractile protein myosin occurs at an early stage of cardiac development, even before fusion of the bilateral heart primordia. Differentiation in these early mesodermal cells is regulated by signals from the anterior endoderm, a process known as induction. Several putative early signaling molecules include fibroblast growth factor, activin, and insulin. Signaling molecules interact with receptors on the cell surface; these receptors activate 2nd messengers, which, in turn, activate specific nuclear transcription factors (GATA-4, MEF2, Nkx, bHLH, and the retinoic acid receptor family) that induce the expression of specific gene products to regulate cardiac differentiation. Some of the primary disorders of cardiac muscle, the cardiomyopathies, may be related to defects in some of these signaling molecules (Chapter 433).

Developmental processes are chamber specific. Early in development, ventricular myocytes express both ventricular and atrial isoforms of several proteins, such as atrial natriuretic peptide (ANP) and myosin light chain (MLC). Mature ventricular myocytes do not express ANP and express only a ventricular-specific MLC 2v isoform, whereas mature atrial myocytes express ANP and an atrial-specific MLC 2a isoform. Heart failure (Chapter 436), volume overload (Chapters 420 and 422), and pressure overload hypertrophy (Chapter 421) are associated with a recapitulation of fetal cell phenotypes in which mature myocytes re-express fetal proteins. Because different isoforms have different contractile behavior (fast vs slow activation, high vs low adenosine triphosphatase activity), expression of different isoforms may have important functional consequences.

The extent to which stem cells can be made to differentiate into cardiac muscle cells is the focus of investigation in the field of regenerative cardiology. Some investigators believe that cardiac precursor cells known as cardiomyoblasts can replace damaged myocytes and, if stimulated with the proper regulatory factors, could be induced to regenerate damaged cardiac muscle. Others believe that circulating stem cells or bone marrow–derived cells may support cardiac regeneration. Progress is being made in growing functional cardiomyocytes from both embryonic and adult stem cell populations from various sources. These cells may one day be utilized to repopulate damaged cardiac tissue or, using biomechanical scaffolds, to build a replacement ventricle for patients with hypoplastic left or right heart.

414.6 Developmental Changes in Cardiac Function

Daniel Bernstein

During development, the composition of the myocardium undergoes profound changes that result in an increase in the number and size of myocytes. During prenatal life, this process involves myocyte division (hyperplasia), whereas after the 1st few postnatal weeks, subsequent cardiac growth occurs by an increase in myocyte size (hypertrophy). The myocytes themselves change shape from round to cylindrical, the proportion of myofibrils (which contain the contractile apparatus) increases, and the myofibrils become more regular in their orientation.

The plasma membrane (known as the sarcolemma in myocytes) is the location of the ion channels and transmembrane receptors that regulate the exchange of chemical information from the cell surface to the cell interior. Ion fluxes through these channels control the processes of depolarization and repolarization. Developmental changes have been described for the sodium-potassium pump, the sodium-hydrogen exchanger, and voltage-dependent calcium channels. As the myocyte matures, extensions of the sarcolemma develop toward the interior of the cell (the t-tubule system), which dramatically increases its surface area and enhances rapid activation of the myocyte. Regulation of the membrane’s α- and β-adrenergic receptors with development enhances the ability of the sympathetic nervous system to control cardiac function as the heart matures.

The sarcoplasmic reticulum (SR), a series of tubules surrounding the myofibrils, controls the intracellular calcium concentration. A series of pumps regulate calcium release to the myofibrils for initiation of contraction (ryanodine-sensitive calcium channel) and calcium uptake for initiation of relaxation (adenosine triphosphate–dependent SR calcium pump). In immature hearts, this SR calcium transport system is less well developed, and such hearts consequently have an increased dependence on transport of calcium from outside the cell for contraction. In a mature heart, the majority of the calcium to activate contraction comes from the SR. This developmental phenomenon may explain the sensitivity of the infant heart to sarcolemmal calcium channel blockers such as verapamil, which often results in a marked depression in contractility and cardiac arrest (Chapter 429).

The major contractile proteins (myosin, actin, tropomyosin, and troponin) are organized into the functional unit of cardiac contraction, the sarcomere. Each has several isoforms that are expressed differentially by location (atrium vs ventricle) and by developmental stage (embryo, fetus, newborn, adult).

Changes in myocardial structure and myocyte biochemistry result in easily quantifiable differences in cardiac function with development. Fetal cardiac function is less responsive to changes in both preload (filling volume) and afterload (systemic resistance). The most effective means of increasing ventricular function in a fetus is through increasing the heart rate. After birth and with further maturation, preload and afterload play an increasing role in regulating cardiac function. The rate of cardiac relaxation is also developmentally regulated. The decreased ability of the immature SR calcium pump to remove calcium from the contractile apparatus is manifested as a decreased ability of the fetal heart to enhance relaxation in response to sympathetic stimulation. This decreased ability of the immature myocardium to use preload effectively may partly explain the difficulty that most premature infants have in compensating for the left-to-right shunt through a patent ductus arteriosus Chapters 95.3 and 420.8).