2 Cardiac Development

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2 Cardiac Development

BRAD J. MARTINSEN, PhD AND JAMIE L. LOHR, MD

CONTENTS

INTRODUCTION TO HUMAN HEART EMBRYOLOGY AND DEVELOPMENT PRIMARY HEART FIELD AND LINEAR HEART TUBE FORMATION

SECONDARY HEART FIELD, OUTFLOW TRACT FORMATION, AND CARDIAC LOOPING CARDIAC NEURAL CREST AND OUTFLOW TRACT AND ATRIAL

AND VENTRICULAR SEPTATION

PROEPICARDIUM AND CORONARY ARTERY DEVELOPMENT CARDIAC MATURATION

SUMMARY OF EMBRYONIC CONTRIBUTION TO HEART DEVELOPMENT REFERENCES

1. I N T R O D U C T I O N TO H U M A N HEART EMBRYOLOGY A N D DEVELOPMENT

The primary heart field, secondary heart field, cardiac neural crest, and proepicardium are the four major embryonic regions involved in the process of vertebrate heart development (Fig. 1). They each make an important contribution to overall cardiac development, which occurs with complex developmental timing and regulation. This chapter describes how these regions interact to form the final structure of the heart in relationship to the generalized developmental timeline of human embryology (Table 1).

The heart is the first organ to fully form and function during vertebrate development, and many of the underlying mechanisms are considered molecularly and developmentally con- served (1). The description presented here is based on heart development research from the chick, mouse, frog, and human model systems. Importantly, numerous research findings have redefined the understanding of the primary heart field, which gives rise to the main structure of the heart (atria and ventricles) and have led to exciting discoveries of the secondary heart field, which gives rise to the outflow tracts of the mature heart (2-4).

These discoveries were a critical step in advancing our understanding of how the outflow tracts of the heart form, an area in which many congenital heart defects arise, and thus have had important implications for the understanding and prevention of From: Handbook of Cardiac Anatomy, Physiology, and Devices Edited by: P. A. Iaizzo © Humana Press Inc., Totowa, NJ

human congenital heart disease (5). In addition, great strides have also been made in our knowledge of the contribution of the cardiac neural crest and the epicardium to overall heart development.

2. PRIMARY HEART FIELD

A N D LINEAR HEART TUBE F O R M A T I O N

The cells that will become the heart are among the first cell lineages formed in the vertebrate embryo (6,7). By day 15 of human development, the primitive streak has formed (8), and the first mesodermal cells to migrate (gastrulate) through the primitive streak are cells fated to become the heart (9,10) (Fig. 2). These mesodermal cells migrate to an anterior and lateral position where they form bilateral primary heart fields (Fig. 1A) (11). Studies of chick development have not sup- ported the previously held notion of a medial cardiac crescent that bridges the two bilateral primary heart fields (12). Com- plete comparative molecular and developmental studies between the chick and mouse are required to confirm these results. The posterior border of the bilateral primary heart field reaches to the first somite in the lateral mesoderm on both sides of the midline (Fig. 1A) (3,12).

At day 18 of human development, the lateral plate mesoderm is split into two layers: somatopleuric and splanchnopleuric (8). It is the splanchnopleuric mesoderm layer that contains the myocardial and endocardial cardiogenic precursors in the region of the primary heart fields as defined above.

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Fig. 1. The four major contributors to heart development illustrated in the chick model system: primary heart field, secondary heart field, cardiac neural crest, and proepicardium. (A) Day 1 chick embryo (equivalent to day 20 of human development). Red denotes primary heart field cells.

(B) Day 2.5 chick embryo (equivalent to -5 wk of human development). Green denotes cardiac neural crest cells; yellow denotes secondary heart field cells; blue denotes proepicardial cells. (C) Day 8 chick heart (equivalent to - 9 wk of human development). Green dots represent derivatives of the cardiac neural crest; yellow dots represent derivatives of the secondary heart field; red dots represent derivatives of the primary heart fields; blue dots represent the derivatives of the proepicardium. Ao, aorta; APP, anterior parasympathetic plexis; Co, coronary vessels; E, eye; H, heart; IFT, inflow tract; LA, left atrium; LV, left ventricle; Mb, midbrain; NF, neural folds; OFT, outflow tract; Otc, otic placode; P, pulmonary artery; RA, right atrium; RV, right ventricle; T, trunk.

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CHAPTER 2 / C'ARDIAC DEVELOPMENT 17

Days of human development

Developmental process

Table 1

Developmental Timeline of Human Heart Embryology

0 1-4 5-12 13-14 15-17 17-18 18-26 20 21-22 22 22-28 22-28 22-28 32-37 57+

Birth

(3-4 wk) (3-4 wk) (3-4 wk) (5-6 wk) (9 wk)

Fertilization.

Cleavage and movement down the oviduct to the uterus.

Implantation of the embryo into the uterus.

Primitive streak formation (midstreak level contains precardiac cells).

Formation of the three primary germ layers (gastrulation): ectoderm, mesoderm, and endoderm. Midlevel primitive streak cells that migrate to an anterior and lateral position form the bilateral primary heart field.

Lateral plate mesoderm splits into the somatopleuric mesoderm and splanchnopleuric mesoderm. Splanchnopleuric mesoderm contains the myocardial and endocardial cardiogenic precursors in the region of the primary heart field.

Neurulation (formation of the neural tube)

Cephalocaudal and lateral folding brings the bilateral endocardial tubes into the ventral midline of the embryo.

Heart tube fusion.

Heart tube begins to beat.

Heart looping and the accretion of cells from the primary and secondary heart fields.

Proepicardial cells invest the outer layer of the heart tube and eventually form the epicardium and coronary vasculature.

Neural crest migration starts.

Cardiac neural crest migrates through the aortic arches and enters the outflow tract of the heart.

Outflow tract and ventricular septation complete.

Functional septation of the atrial chambers as well as the pulmonary and systemic circulatory systems.

Most of the human developmental timing information is from ref. 8, except for the human staging of the secondary heart field and proepicardium, which was correlated from other model systems (2~l,14).

Presumptive endocardial cells delaminate from the splanchnopleuric mesoderm and coalesce via vasculogenesis to form two lateral endocardial tubes (Fig. 2A) (13).

During the third week of human development, two bilateral layers of myocardium surrounding the endocardial tubes are brought into the ventral midline during closure of the ventral foregut via cephalic and lateral folding of the embryo (Fig. 2A) (8). The lateral borders of the myocardial mesoderm layers are the first heart structures to fuse, followed by the fusion of the two endocardial tubes to form one endocardial tube surrounded by splanchnopleuric-derived myocardium (Fig. 2B,C). The medial borders of the myocardial mesoderm layers are the last to fuse (14). Thus, the early heart is continuous with noncardiac splanchnopleuric mesoderm across the dorsal mesocardium (Fig. 2C). This will eventually partially break down to form the ventral aspect of the linear heart tube with a posterior inflow (venous pole) and anterior outflow (arterial pole), as well as the dorsal wall of the pericardial cavity (5,14). During the fusion of the endocardial tubes, the myocardium secretes an acellular matrix, forming the cardiac jelly layer that separates the myocardium and endocardium.

By day 22 of human development, the linear heart tube begins to beat. As the human heart begins to fold and loop from days 22-28 (described in the next section), epicardial cells will invest the outer layer of the heart tube (Fig. 1B and Fig. 3A), resulting in a heart tube with four primary layers: endocardium, cardiac jelly, myocardium, and epicardium (Fig. 3B) (8).

3. S E C O N D A R Y HEART FIELD, O U T F L O W TRACT F O R M A T I O N , A N D C A R D I A C L O O P I N G

A cascade of signals identifying the left and right sides of the embryo are thought to initiate the process of primary linear

Neural fold Dorsal aorta

Somatopleuric planchnopleuric

igrating hnopleuric cells

A Endocardial tube

g lateral ardium

~rest cells y heart field 'sal

;ocardium endocardial

nyocardium

Fig. 2. Cross-sectional view of human heart tube fusion: (A) Day 20, cephalocaudal and lateral folding brings bilateral endocardial tubes into the ventral midline of the embryo. (B) Day 21, start of heart tube fusion. (C) Day 22, complete fusion, resulting in the beating primitive heart tube. Ectoderm, dark gray; mesoderm, gray; endoderm, white.

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Mvocardium.

Er pican rating Lrdial

\

'enosl Jm

~rsum

A

Fig. 3. Origin and migration of proepicardial cells. (A) Whole mount view of the looping human heart within the pericardial cavity at day 28.

Proepicardial cells (dark gray dots, mesoderm origin) emigrate from the sinus venosus and possibly the septum transversum and then migrate out over the outer surface of the ventricles, eventually surrounding the entire heart. (B) Cross-sectional view of the looping heart showing the four layers of the heart: epicardium, myocardium, cardiac jelly, and endocardium. LV, left ventricle; RV, right ventricle.

S

Linear heart Looping & Septation

tube Accretion

Day 22 Day 28 9 weeks

Fig. 4. Looping and septation of the human primary linear heart tube. Dark gray and white regions represent tissue added during the looping process from the primary and secondary heart field, respectively. Ao, aorta; AV, atrioventricular region; LA, left atrium; LV, left ventricle;

P, pulmonary artery; RA, right atrium; RV, right ventricle.

heart tube looping (15). The primary heart tube loops to the right of the embryo and bends, to allow convergence of the inflow (venous) and outflow (arterial) ends, from days 22-28 of human development (Fig. 4). This process occurs prior to the division of the heart tube into four chambers and is required for proper alignment and septation of the mature cardiac chambers.

During the looping process, the primary heart tube increases dramatically in length (four- to fivefold), and this process dis- places atrial myocardium posteriorly and superiorly dorsal to the forming ventricular chambers (5,8,15). During the looping process, the inflow (venous) pole, atria, and atrioventricular region are added to, or accreted from, the posterior region of the paired primary heart fields; the myocardium of proximal out-

flow tract (conus) and distal outflow tract (truncus) are added to the arterial pole from the secondary heart field (2-4).

The secondary heart field (Fig. 1B and Fig. 2C) is located along the splanchnopleuric mesoderm (beneath the floor of the foregut) at the attachment site of the dorsal mesocardium (2- 4,14). During looping, the secondary heart field cells undergo epithelial-to-myocardial transformation at the outflow (arterial) pole and add additional myocardial cells onto the developing outflow tract. This lengthening of the primary heart tube appears to be an important process for the proper alignment of the inflow and outflow tracts prior to septation. If this process does not occur normally, ventricular septal defects and malpositioning of the aorta may occur (14).

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CHAPTER 2 / CARDIAC DEVELOPMENT 19 Thus, by day 28 of human development, the chambers of

the heart are in position and are demarcated by visible con- strictions and expansions that denote the sinus venosus, common atrial chamber, atrioventricular sulcus, ventricular chamber, and conotruncus (proximal and distal outflow tract) (Fig. 4) (8,14).

4. C A R D I A C NEURAL CREST

A N D O U T F L O W TRACT A N D ATRIAL A N D V E N T R I C U L A R SEPTATION

Once the chambers are in their correct positions after looping, extensive remodeling of the primitive vasculature and septation of the heart can occur. The cardiac neural crest is an extracardiac (from outside the primary or secondary heart fields) population of cells that arises from the neural tube in the region of the first three somites up to the midotic placode level (rhombomeres 6-8) (Fig. 5). Cardiac neural crest cells leave the neural tube during weeks 3-4 of human development and migrate through aortic arches 3, 4, and 6 (Fig. 1B) and then eventually move into the developing outflow tract of the heart during weeks 5 and 6. These cells are necessary for complete septation of the outflow tract and ventricles (which is completed by week 9 of human development), as well as for the formation of the anterior parasympathetic plexis, which contributes to cardiac innervation and the regulation of heart rate (8,16-18).

The primitive vasculature of the heart is bilaterally symmetrical, but during weeks 4 to 8 of human development, there is remodeling of the inflow end of the heart so that all systemic blood will flow into the future right atrium (8). In addition, there is extensive remodeling of the initially bilaterally symmetrical aortic arch arteries into the great arteries (septation of the aortic and pulmonary vessels) that is dependent on the presence of the cardiac neural crest (14,19). The distal outflow tract (truncus) septates into the aorta and pulmonary trunk via the fusion of two streams or prongs of cardiac neural crest that migrate into the distal outflow tract. In contrast, the proximal outflow tract septates by fusion of the endocardial cushions and eventually joins proximally with the atrioventricular endocardial cushion tissue and the ventricular septum (20,2 l). The endocardial cushions are formed by both atrioventricular canal and outflow tract endocardial cells that migrate into the cardiac jelly, forming bulges or cushions.

Despite its clinical importance, to date almost nothing is known about the molecular pathways that determine cell lineages in the cardiac neural crest or that regulate outflow tract septation (14). However, it is known that if the cardiac neural crest is removed before it begins to migrate, the conotruncal septa completely fails to develop, and blood leaves both the ventricles through what is termed a persistent truncus arterio- sus, a rare congenital heart anomaly in humans. Failure of outflow tract septation may also be responsible for other forms of congenital heart disease, including transposition of the great vessels, high ventricular septal defects, and tetralogy of Fallot (8,16,18).

The septation of the outflow tract (conotruncus) is tightly coordinated with the septation of both the ventricles and atria to produce a functional heart. All of these septa eventually

Fig. 5. Origin of the cardiac neural crest within a 34-h chick embryo.

Green dots represent cardiac neural crest cells in the neural folds of hindbrain rhombomeres 6-8 (the region of the first three somites up to the midotic placode level). Fb, forebrain; Mb, midbrain.

fuse with the atrioventricular cushions, which also divide the left and right atrioventricular canals and serve as the source of cells for the atrioventricular valves. Prior to septation, the right atrioventricular canal and right ventricle expand to the right, causing a realignment of the atria and ventricles so that they are directly over each other. This allows venous blood entering from the sinus venosus to flow directly from the right atrium to the presumptive right ventricle without flowing through the presumptive left atrium and ventricle (8,14). The new alignment also simultaneously provides the left ventricle with a direct outflow path to the truncus arteriosus and subse- quently to the aorta.

Between weeks 4 and 7 of human development, the left and right atria undergo extensive remodeling and are eventually septated. However, during the septation process, a right-to-left shunting of oxygenated blood (oxygenated by the placenta) is created via shunts, ducts, and foramens (Fig. 6). Prior to birth,

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Fora OV

A

Fig. 6. Transition from fetal dependence on the placenta for oxygenated blood to self-oxygenation via the lungs. (A) Circulation in the fetal heart before birth. Pink arrows show right-to-left shunting of placentally oxygenated blood through the foramen ovale and ostium secundum.

(B) Circulation in the infant heart after birth. The first breath of the infant and cessation of blood flow from the placenta cause final septation of the heart chambers (closure of the foramen ovale and ostium secundum) and thus separation of the pulmonary and systemic circulatory systems. Blue arrows show the pulmonary circulation, and the red arrows show the systemic circulation within the heart. AV, atrioventricular;

LA, left atrium; LV, left ventricle; RA, right atrium; RV, right ventricle.

the use of the pulmonary system is not necessary, but eventually a complete separation of the systemic and pulmonary circulatory systems will be necessary for normal cardiac and systemic function (8). Initially, the right sinus horn is incorporated into the right posterior wall of the primitive atrium, and the trunk of the pulmonary venous system is incorporated into the posterior wall of the left atrium via a process called intussusception.

At day 26 of human development, a crescent-shaped wedge of tissue, called the septum primum, begins to extend into the atrium from the mesenchyme of the dorsal mesocardium. As it grows, it diminishes the ostium primum, a foramen that allows shunting of blood from the right to the left atrium. However, programmed cell death near the superior edge of the septum primum creates a new foramen, the ostium secundum, which continues the right-to-left shunting of oxygenated blood. An incomplete, ridged septum secundum with a foramen ovale near the floor of the right atrium forms next to the septum primum;

both fuse with the septum intermedium of the atrioventricular cushions (8).

At the same time as atrial septation is beginning, at about the end of the fourth week of human development, the muscular ventricular septum begins to grow toward the septum intermedium (created by the fusion of the atrioventricular cushions), creating a partial ventricular septum. By the end of the ninth week of human development, the outflow tract septum has grown down onto the upper ridge of this muscular ventricular septum and onto the inferior endocardial cushion, which completely separates the right and left ventricular chambers.

Not until after birth, however, does the heart become func- tionally septated in the atrial region. At birth, dramatic changes

in the circulatory system occur because of the transition from fetal dependence on the placenta for oxygenated blood to self- oxygenation via the lungs. During fetal life, only small amounts of blood are flowing through the pulmonary system because the fluid-filled lungs create high resistance, resulting in low-pressure and low-volume flow into the left atrium from the pulmonary veins. This allows the high-volume blood flow coming from the placenta to pass through the inferior vena cava into the right atrium, where it is then directed across the foramen ovale into the left atrium. The oxygenated blood then flows into the left ventricle and directly out to the body via the aorta. At birth, the umbilical blood flow is interrupted, stop- ping the high-volume flow from the placenta. In addition, the alveoli and pulmonary vessels open when the infant takes its first breath, dropping the resistance in the lungs and allowing more flow into the left atrium from the lungs. This reverse in pressure difference between the atria pushes the flexible septum primum against the ridged septum secundum and closes the foramen ovale and ostium secundum, resulting in the complete septation of the heart chambers (Fig. 6) (8).

5. P R O E P I C A R D I U M A N D C O R O N A R Y A R T E R Y D E V E L O P M E N T

The last major contributor to vertebrate heart development discussed in this chapter is the proepicardium. Prior to heart looping, the primary heart tube consists of endocardium, cardiac jelly, and myocardium. It is not until the start of heart looping that the epicardium surrounds the myocardium, forming the fourth layer of the primary heart tube (Fig. 3) (22). This population of cells will eventually give rise to the coronary

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CHAPTER 2 / CARDIAC DEVELOPMENT 21

vasculature. A neural crest origin of the coronary vessels was originally hypothesized, but lineage tracing studies have shown that the neural crest gives rise to cells of the tunica media of the aortic and pulmonary trunks, but not to the coronary arteries (13,23). These investigators also concluded that the coronary vasculature is derived from the proepicardial organ, a nest of cells in the dorsal mesocardium of the sinus venosus or septum transversum. These cells, which are derived from an indepen- dent population of splanchnopleuric mesoderm cells, migrate onto the primary heart tube between day 22 and day 28 of human development (Fig. 3),just as the heart begins its looping (8,14). Prior to migration, these cells are collectively called the proepicardium or the proepicardial organ.

Interestingly, three lineages of the coronary vessel cells (smooth muscle, endothelial, and connective tissue cells) are segregated in the proepicardium prior to migration into the heart tube (13,24). These cells will coalesce to form coronary vessels de novo via the process of vasculogenesis (25). It has also been shown that the epicardium provides an intrinsic factor needed for normal myocardial development and is a source of cells needed for forming the interstitial myocardium and cushion mesenchyme (14,26). It should be noted that an understanding of the embryological origin of the vascular system and its molecular regulation is thought to be important in helping to explain the varying susceptibility of different components of the vascular system to atherosclerosis (13,27).

6. CARDIAC MATURATION

Although the embryonic heart is fully formed and functional by the 1 lth week of pregnancy, the fetal and neonatal heart continue to grow and mature rapidly, with many clinically relevant changes taking place after birth. During fetal development, or from the time after the embryo is completely formed in the first trimester of pregnancy until birth, the heart grows primarily by the process of cell division (28-31). Within a few weeks after birth, the predominant mechanism of cardiac growth is cell hypertrophy; in other words, existing cardiac cells become larger rather than increasing significantly in number (28-30).

The exact timing of this process and the mechanisms regulating these changes are not yet completely elucidated. In the classic explanation, mature cardiac cells lose the ability to divide; however, more recent work suggests that a limited amount of cell division can occur in adult human hearts dam- aged by ischemia (32). This finding has led to a renewed inter- est in understanding the regulation of cell division during cardiac maturation. Additional maturational changes in both the fetal and neonatal heart include alterations in the compo- sition of cardiac muscle, differences in energy production, and maturation of the contractile function. These changes, along with associated physiological changes in the transitional circulation, as discussed in Section 4, affect the treatments of newborns with congenital heart disease, particularly those requiring interventional procedures or cardiac surgery.

The hemodynamic changes associated with birth include a significant increase in left ventricular cardiac output to meet the increased metabolic needs of the newborn infant. This improve- ment in cardiac output occurs despite the fact that the neonatal

myocardium has less muscle mass and less cellular organization than the mature myocardium. The newborn myocardium consists of 30% contractile proteins and 70% noncontractile mass (membranes, connective tissues, and organelles), in contrast to the adult myocardium, which is 60% contractile mass (30). The myocardial cells of the fetus are rounded, and both the myocardial cells and myofibrils within them are oriented ran- domly. As the fetal heart matures, these myofibrils increase in size and number and orient to the long axis of the cell, which further contributes to improved myocardial function (28). The fetal myocardial cell contains higher amounts of glycogen than the mature myocardium, suggesting a higher dependence on glucose for energy production; in experiments in nonprimate model systems, the fetal myocardium is able to meet its metabolic needs with lactate and glucose as the only fuels (33). In contrast, the preferred substrate for energy metabolism in the adult heart is long-chain fatty acids, although the adult heart is able to utilize carbohydrates as well (33,34). This change is considered to be triggered in the first few days or weeks of life by an increase in serum long-chain fatty acids with feeding, but the timing and clinical impact of this transition relative to an ill or nonfeeding neonate with cardiovascular disease is currently unknown.

In addition, the maturing myocardial cells undergo changes in the expression of their contractile proteins, which may be responsible for some of the maturational differences in cardiovascular function. Changes in expression of contractile proteins that may be important in humans include a gradual increase in the expression of myosin light chain 2 (MLC 2) in the ventricle from the neonatal period through adolescence. In the fetal ventricle, two forms of myosin light chain, MLC 1 and MLC 2, are expressed in equal amounts (30,35). Increased MLC 1 expression is associated with increased contractility; for example, it has been documented in isolated muscle from patients with tetralogy of Fallot that both MLC 1 expression and contractility are increased (36). After birth, there is a gradual increase in the amount of MLC 2, or the "regulatory" myosin light chain, which has a slower rate of force development but can be phosphorylated to increase calcium-dependent force development in mature cardiac muscle (30,37).

There is also variability in actin isoform expression during cardiac development. The human fetal heart predominantly expresses cardiac ct-actin; the more mature human heart expresses skeletal c~-actin (28,38). Actin is responsible for interacting with myosin cross bridges and regulating adenos- ine triphosphatase (ATPase) activity, and work done in the mouse model system suggests that the change to skeletal actin may be an additional mechanism of enhanced contractility in the mature heart (28,39,40).

There are also developmental changes of potential functional significance within the regulatory proteins of the sarcomere.

More specifically, the fetal heart expresses both ~- and [3-tropomyosin, a regulatory filament, in nearly equal amounts; after birth, the proportion of [3-tropomyosin decreases, and c~-tropomyosin increases, potentially to optimize diastolic relaxation (28,41,42). Interestingly, an expression of high levels of 13-tropomyosin in the neonatal heart has been linked to early death caused by myocardial dysfunction (43).

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Last, the isoform of the inhibitory troponin, troponin I, also changes after birth. The fetal myocardium contains mostly the skeletal isoform of troponin I (28,44); after birth, the myocar- dium begins to express cardiac troponin I, and by approx 9 mo of age, only cardiac troponin I is present (28,45,46). It should also be noted that cardiac troponin I can be phosphorylated to improve calcium responsivity and contractility, which may improve function in the more mature heart; it is thought that the skeletal form of troponin I may serve to protect the fetal and neonatal myocardium from acidosis (30,39,47).

In summary, the full impact of these developmental changes in contractile proteins and their effect on cardiac function or perioperative treatment of the newborn with heart disease remains unclear at the present time. However, such insights may provide future modes for optimizing therapy in children.

Two of the most clinically relevant features of the immature myocardium are its requirement for high levels of extracellular calcium and a decreased sensitivity to [3-adrenergic inotropic agents. The neonatal heart has a decrease in both volume and functional maturity of the sarcoplasmic reticulum, which stores intracellular calcium (28). The paucity of intracellular calcium storage and subsequent release via the sarcoplasmic reticulum in the fetal and neonatal myocardium increases the require- ments of these myocardia for extracellular calcium, so that exogenous administration of calcium can be used to augment cardiac contractility in the appropriate clinical setting. In addition, neonates and infants are significantly more sensitive to calcium channel blocking drugs than older children and adults and thus are at a higher risk for severe depression of myocardial contractility with the administration of these agents (28,30,48).

Last, although data in humans are limited, there appears to be significantly decreased sensitivity to [3-agonist agents in the immature myocardium and in older children with congeni- tal heart disease ( 3 0 , 4 9 - 5 1 ) . This may be attributable to a paucity of receptors, sensitization to endogenous catechola- mines at birth or with heart failure, or a combination of these and/or additional factors. Because of this decreased responsiveness to [3-agonists, there is a common requirement clinically for administrating higher doses of [3-agonist inotropic agents in newborns and infants. Importantly, alternative medi- cations, including phosphodiesterase inhibitors, are often use- ful adjuncts to improve contractility in newborns with myocardial dysfunction (30).

Although the structure of the heart is complete in the first trimester of pregnancy, cardiac growth and maturation occur in the fetus, newborn, and child. Many of these developmental changes, particularly decreased intracellular calcium stores in the immature sarcoplasmic reticulum and a decreased responsiveness to [3-agonist inotropic agents, have a significant impact on the care of newborns, infants, and children with congenital heart disease, particularly those requiring surgical inter- vention early in life.

7. S U M M A R Y O F E M B R Y O N I C

C O N T R I B U T I O N T O H E A R T D E V E L O P M E N T The contribution of the four major embryonic regions (primary heart field, secondary heart field, cardiac neural crest, and proepicardium; Fig. 1) to heart development illustrates

the complexity of human heart development. Each of these regions makes a unique contribution to the heart, but they ultimately depend on each other for the creation of a fully functional organ.

An understanding of the mechanisms of human heart development provides clues to the etiology of congenital heart disease. Nevertheless, to date, the genetic regulatory mechanisms of these developmental processes are just starting to be charac- terized. A molecular review of heart development is outside the scope of this chapter, but several interesting molecular heart reviews have been published (14,52,53). A better understand- ing of the embryological origins of the heart combined with the characterization of the genes that control heart development (54) may lead to many new clinical applications to treat congenital and adult heart disease.

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