Function and form in the developing cardiovascular system

SPOTLIGHT REVIEW

Cardiovascular Research doi:10.1093/cvr/cvr062

5

10

60

Function and form in the developing cardiovascular system David Sedmera 1,2*

65

70

1 Department of Cardiovascular Morphogenesis, Institute of Physiology, Academy of Sciences of the Czech Republic, Videnska 1083, 14220 Prague, Czech Republic; and 2Institute of Anatomy, First Faculty of Medicine, Charles University in Prague, U nemocnice 3, 12800 Prague, Czech Republic

Received 26 November 2010; revised 23 February 2011; accepted 23 February 2011 15 75

Abstract

20

25

Function of the developing heart is dictated by changes in its morphology. For simplicity, we distinguish four stages with different contraction mechanics and conduction parameters. Straight or looped tubular hearts, similar to those of invertebrates such as Drosophila or Ciona, operate as suction pumps and are characterized by a caudally localized pacemaker and slow, peristaltoid conduction and contraction. There is a complete occlusion of the lumen during systole. When the atrial and ventricular chambers appear, the preseptation heart is in many functional aspects similar to the adult heart, but the same function is achieved by different means. There are parallels in design among the hearts of lower vertebrates, such as a spongy ventricle without coronary vasculature and a myocardial atrioventricular canal. Even after septation, considerable maturation of cardiac morphology and function occurs during the foetal and early postnatal period.

----------------------------------------------------------------------------------------------------------------------------------------------------------Keywords

80

85

Myocardium † Conduction system † Embryonic heart † Ontogenesis † Phylogenesis

----------------------------------------------------------------------------------------------------------------------------------------------------------This article is part of the Spotlight Issue on: Cardiac Development 30 90

35

40

45

50

55

1. Introduction

2. Tubular heart

Unlike most other organ systems, function of the cardiovascular system is critical for embryonic survival and, as a result, the heart has to beat in order to support circulation well before its morphogenesis is complete. The function of the heart influences its morphology,1 and vice versa. The resulting constraints force the hearts into very similar functional designs that are conserved between widely divergent organisms, and make the developmental pathway between the simple cardiac tube and the mature four-chambered organ of homeotherm vertebrates rather tortuous (Figure 1). In this review, I will go through different stages of morphogenesis of the mammalian heart and correlate the structure of the main pumping chamber(s), the ventricle(s), with ventricular function, and give examples of similar functional arrangements from ‘lower’ classes of vertebrates or invertebrates. In parallel, the sequence of activation of different cardiac compartments will be correlated with emerging function and structure of the cardiac conduction system. The main idea is that cardiac design is tightly dictated by demands of life style of the particular organism. Due to evolutionary pressure, this is likely to be a very good fit. As such, we should refrain from calling the designs of ‘lower’ vertebrates2,3 ‘primitive’, especially those of sharks4 should we happen to meet them in their natural habitat.

The first cardiac contractions start at the straight tube stage (Figure 1), approximately ED8.5 in the mouse5 and 22 days in humans.6 The first heart beats are irregular and do not propel blood through the circulation,7 but activity quickly becomes coordinated and primitive blood starts to circulate through the embryo, thus enabling its further growth by decreasing diffusion distance for nutrients and oxygen. Many invertebrates, such as the model organism Drosophila melanogaster, are able to function with this tubular design. In vertebrates, the heart is consistently activated by a pacemaker situated at the inflow part, while in invertebrates with an open circulatory system, the direction of flow can be altered—clearly an advantage allowing sequential perfusion of different parts of the body.8 However, the similarities end with the structure and function—at the cellular level, these pulsatile dorsal vessels share properties of skeletal muscle, as is the case of the ‘lymphatic hearts’.9 In the chordate Amphioxus,10 there are multiple pulsatile vessels. Of note, although there is an epicardial layer,8 the hearts of invertebrates supporting open circulatory systems lack endocardium11,12 (Figure 2).

95

100

105

110

3. Cardiac looping The next stage of cardiac morphogenesis is the transition from a straight tube to a looped heart (Figure 1). This process is termed

* Corresponding author. Tel: +420 2 2496 5941; fax: +420 2 24596 5770, Email: [email protected] Published on behalf of the European Society of Cardiology. All rights reserved. & The Author 2011. For permissions please email: [email protected].

115

Page 2 of 8

D. Sedmera

120 180

125 185

130 190

135 195

140

Figure 1 Four key stages in cardiac morphogenesis—tube, loop, chamber formation, completion of septation with deployment of coronary circulation. Art work by Ivan Helekal based on chick heart development studies by Manner14 and Sedmera et al.90,91 200

145 205

150 210

155 215

160 220

165 225

170

175

Figure 2 Similarities in design of hearts without coronary circulation. (A) Blue Crab, transverse paraffin section stained with hematoxylin– eosin. (B) Oyster, transverse section through ventricle stained with rhodamine – phalloidine. Scale bars 100 mm. Note that these two invertebrate hearts do not contain endocardium (four times magnified view of trabeculae, direct interface of myocytes with haemolymph indicated by arrowheads in A). (C) Frog, sagittal section. (D) Chick embryo, Stage 29 (embryonic day 6), transverse section through the right ventricle, scanning electron micrographs. Most of the ventricular myocardial mass is contained in the trabeculae (Tr). Outer compact myocardium (Co) is very thin. En indicates endocardium covering the myocytes. Partly based on studies by Sedmera et al.47,90

230

235

Page 3 of 8

Development of function and form

240

245

250

255

260

cardiac looping,13,14 and for some it does not end with formation of the loop, but continues by further ‘twisting’ well through the septation process.15,16 Description of mechanics of the tubular heart,12 initially described as peristaltic,17,18 is now being gradually replaced by a more precise, albeit more complex, suction pump model.19 At this stage, there is complete obstruction of the lumen by apposition of endocardium reinforced by cardiac jelly, as demonstrated using high-resolution ultrasound20 or optical coherence tomography (OCT21,22). Such tubular hearts have 100% ejection fractions, which is never achieved at later stages; this geometry is at the extreme end between cylindrical, optimal for generating pressure, and spherical, optimal for volume displacement.23 In homeotherm embryos, pumping function of the tubular heart is required from this stage on for embryonic survival. Therefore, long-term consequences of perturbations resulting in significant cardiac dysmorphogenesis cannot be analysed in these model species. In contrast, during zebrafish development, the onset of circulation precedes functional requirement by several days, allowing a rare window of opportunity to analyse various mutants with severely dysfunctional or non-functional hearts and dissect the effects of function or blood flow on cardiac morphogenesis.24 The conduction system at that stage has a well-defined pacemaker at the inflow, but the general speed of conduction is slow, correlating with the ‘peristaltoid’ mode of contraction. Gradually, conduction

remains slow in the regions of the atrioventricular canal and ventricular outflow, while there is some acceleration in the prospective ventricular region (17,25; reviewed by Gourdie et al.26). For illustration, see the Supplementary material online, Movie S1. 300

4. Chamber formation Soon after the beginning of looping, morphological differentiation of myocardium along the cardiac tube becomes apparent.27 This includes disappearance of cardiac jelly in the atria and ventricles, coinciding with the process of development of trabeculae, and, soon afterwards, pectinate muscles in the atria (Figure 3). The morphological differences are accompanied by changes in gene expression patterns.28,29 This process is well conserved in vertebrates, no matter whether there is one or two atria or ventricles. Ventricular trabeculae provide a means of increasing myocardial mass in the absence of coronary circulation30 and are also seen in the hearts of more complex invertebrates (Figure 2). Some of these (mollusks) even have separate atrial and ventricular chambers separated by a valve.12 Recently, a cardiac phenotype very similar to that of ErbB2 null mice lacking ventricular trabeculae31 was described in zebrafish,32 showing conserved mechanism of ventricular trabeculation initiation via endocardium– myocardium neuregulin-Erb signalling in vertebrates.33 In this respect, it is interesting to speculate about the

305

310

315

320

265 325

270 330

275 335

280 340

285 345

290

295

Figure 3 Comparative morphology of adult atrial and ventricular chambers in the zebrafish and mouse. Despite considerable evolutionary distance and dramatically different life styles, the main features of chamber myocardial design (ventricles with trabeculae carneae, Tr; atria with pectinate muscles, pm) are conserved. Not visible is the similar ventricular activation pattern from the apex.47,92 In the zebrafish, this is achieved by trabeculae connecting the atrioventricular canal myocardium with ventricular apex (curved arrow), while in the mouse, by His bundle and its branches contained in the interventricular septum. The differences are in higher proportion of trabeculated ventricular myocardium and presence of a single atrial and ventricular chamber in the fish (expertly dissected and imaged by Norman Hu, Utah). Previously published scanning electron micrograph from Sedmera et al.91 and related to Wessels and Sedmera.70

350

Page 4 of 8

D. Sedmera

355 415

360 420

365 425

370

Figure 4 Complete activation map of ED12.5 mouse embryonic heart. Both atrial and ventricular activation maps are shown in 1 ms isochronal intervals; delay between the chambers (due to conduction through the atrioventricular canal, invisible from the outside) was 115 ms. Those areas first activated (intercaval region in the atria, right ventricular apex) are indicated by orange asterisks, and spread of activation is highlighted by orange arrows. Note also slow conduction (evidenced by isochronal crowding) in the outflow tract (OT) myocardium. Scale bar 100 mm. Previously unpublished figure related to a study by Sedmera et al.37

430

375 435

380

385

390

395

400

405

410

molecular mechanism of trabecular formation in invertebrates (Figure 2) that lack the endocardium. The conduction system starts to show mature characteristics once the chambers are formed, i.e. activation from a well-defined pacemaker,34 spreading through the atrial tissues using preferential pathways,35 and atrioventricular delay based upon slow conduction through the myocardium of the atrioventricular canal.5,17,36 The outflow valve function is exercised, rather efficiently,20 by a combination of close apposition of the outflow tract cushions and contraction of the slowly conducting myocardium of the outflow tract17,25,37 (Figure 4, Supplementary material online, Movie S2). At this stage, the embryonic heart is big enough for imaging and acquisition of functional parameters. It was shown that many functional aspects of the trabeculated heart are comparable with the adult heart, and the same parameters can be meaningfully determined after some modification of the equipment. Video recordings (Supplementary material online, Movie S3) of chick hearts,38 together with direct servo-null pressure measurements and Doppler flow waveforms,39 have taught us that the trabeculated embryonic heart observes the Frank –Starling relationship, dorsal aortic blood flow scales with embryo weight, and that the increase in heart rate results in decreased cardiac output.40 Video recordings in mice followed shortly,41 and ultrasound studies even allowed longitudinal follow-up of individual embryos42 and provided functional insights into the demise of some prenatally lethal mouse mutants, such as NFATc1 mutants that lack outflow valves.43 Likewise, insufficient valve function of the cushions in the outflow tract in ED11.5 mouse embryos leads to diastolic flow reversal and death at ED12.5.44 Recently, results from early human embryos have been reported using advances in gynecological ultrasound.45

5. Maturation of ventricular impulse conduction The heart with formed cardiac chambers is from the conduction system point of view directly comparable with the matured one.

The pacemaking activity is localized to the sino-atrial node, which starts to acquire its autonomic innervation pattern for neural modulation.46 The function of delay generation between the activation of atria and ventricles begins to shift from the slowly conducting myocardium of the atrioventricular canal5,17,36 that is similar to arrangement in the adult hearts of lower vertebrates47 to the atrioventricular node.48 The transition from primitive base-to-apex activation of the ventricles to mature apex-to-base sequence correlates with ventricular septation.49 This is rather logical, as the proximal part of the fast component of the ventricular conduction system, His bundle and bundle branches, is located in the interventricular septum.50,51 However, the actual transition occurs before the completion of septation, and is structurally supported by trabecular sheets connected directly to the atrioventricular canal myocardium.52 This is similar to situation in the lower vertebrates where apex-to-base activation is achieved by trabeculae connecting the ventricular apex and atrioventricular canal myocardium (Figure 3), which is insulated from the ventricular compact zone by a fibrous wedge of subepicardial tissue.47 The first precursor of the ventricular conduction system is the primary ring, described first by immunohistochemistry53,54 and later shown to mediate early ventricular activation in the mouse,55 chick,52,56 and rat.57 Interestingly, the apex to base activation of the ventricle is first heralded by an epicardial breakthrough localized to the right of the interventricular septum, near the apex of the forming right ventricle. This was documented in the mouse,58 chick,25 rat,57 and rabbit59 and is indicative of earlier right bundle branch functionality. This precocious functionality suggests that geometry of this bundle—narrow with fewer side branches compared with left bundle branch—plays an important role in conduction speed. Earlier functionality of the right bundle is conserved among species despite molecularly more advanced differentiation of the left ventricle evidenced by earlier and higher level expression of connexin40, shown both in the chick60,61 and mouse.51 Examples of our interpretation of ventricular activation patterns, from the least mature to most, are shown in Figure 5. Conservation and potential utility of this arrangement could be explained by the close proximity of the ventricular inflow and outflow at the base

440

445

450

455

460

465

470

Page 5 of 8

Development of function and form

475 535

480 540

485 545

490 550

495 555

500

Figure 5 Maturation of ventricular activation sequence in birds. (A) Activation through primary ring (along the forming interventricular septum). Stage 24 (embryonic day 4) chick embryonic heart (from Sankova et al.56). (B) Single epicardial breakthrough near the right ventricular apex indicates functionality of the right bundle branch. (C) The presence of two epicardial breakthrough sites near left and right ventricular apices signifies that both right and left bundle branch are functional. Both are unpublished maps of Stage 34 (embryonic day 8) quail embryonic hearts related to study by Nanka et al.71 Scale bar 1 mm.

560

505 565

of the heart, which favours beginning of contraction from the opposite end—the apex. 510

6. Remodelling of atrioventricular myocardium 515

520

525

530

The myocardium of the atrioventricular canal initially mediates, together with the atrioventricular cushions, valve function in the preseptation heart. In higher vertebrates, atrioventricular canal myocardium gradually regresses, at least in part through apoptosis,62 and is replaced by fibrous insulating tissue derived from the epicardium.63 The function of delay generator between the atrium and ventricle is then localized to the atrioventricular node.48,64 This structure is prominent and well recognized in mammals, and has recently been characterized at the molecular level as a rather complex structure with several functional subdomains with differential gene expression.29 It is well recognized that the adult node is derived from a slowly conducting embryonic myocardial segment expressing, among other genes, Tbx265 that is down-regulated in the developing chambers. A molecular model of conduction system differentiation was recently presented by the Amsterdam group.66 Detailed understanding of the normal development of atrioventricular canal myocardium is essential for our ability to explain the developmental origins of cardiac arrhythmias such as ventricular pre-excitation, characterized by the presence of rapidly conducting anomalous

myocardial connections between the atria and ventricles. Abnormal atrioventricular canal remodelling in the quail model of epicardial ablation can lead to persistence of accessory pathways that can lead in turn to ventricular pre-excitation63 or form a substrate of atrioventricular reentry circuit resulting in reciprocating tachycardia. This situation is observed in the early embryonic mouse heart,67 where these connections are still normally present and functional. Problems can arise also from the opposite situation, where conduction from the atrioventricular node is slowed or even completely blocked. The atrioventricular node is one of the targets of the autoantibodies present in mothers with lupus. These patients stand up to 5% chance68 of having a child with congenital heart block. Third-degree atrioventricular block results often in profound bradycardia, leading to circulatory failure with accumulation of fluid in foetal tissues and cavities (hydrops) and rapid demise. Fortunately, diagnosis is nowadays possible in utero, so serial monitoring of at-risk pregnancies is possible69 and treatment can be initiated as soon as the condition develops; however, no firm therapeutic guidelines are currently established.

570

575

580

585

7. Foetal circulation While the interest of developmental biologists wanes after completion of septation, in the clinical settings, this is when it just starts, since the earliest stages under 12 weeks are seldom studied apart

590

Page 6 of 8

595

600

605

610

615

620

625

630

from confirming the presence of a beating heart. Many interesting things happen between septation and birth, especially in humans where this period lasts 6 months rather than 6 days for the mouse. One important step in myocardial morphogenesis and maturation is deployment of the coronary supply to the ventricular myocardium, occurring in all higher vertebrates and some (but not all) lower vertebrates. While the evolutionary necessity of ventricular compaction is debated,12 its importance for homeotherms is clearly shown by lethality of mouse mutants with impaired compaction (reviewed by Wessels and Sedmera70) or hypoxic quail embryos with absent coronary arteries and thin compact myocardium.71 The foetal heart not only grows in size, but further develops spiralling myocardial architecture of the compact layer.72 This development was shown to depend on haemodynamic loading, being accelerated by increased pressure load and delayed by decreased preload.73 Abnormal haemodynamics can lead to development of some forms of severe congenital heart disease, such as aortic atresia in hearts that were perfectly normal at the routine scan at 19 weeks of human gestation.74 From clinical perspective, the period between 20 and 28 weeks of gestation is the time when foetal intervention can be attempted for some severe lesions with predictably devastating consequences such as severe aortic stenosis.75 Recently, a rare valvar anomaly (mitral fibrous arcade) was linked to volume overload in the recipient twin in the settings of twin–twin transfusion syndrome,76 confirming the role of haemodynamic loading not only in myocardial changes, but also valve maturation. Impaired performance of malformed hearts also affects the other foetal systems, most critically the central nervous system.77 It is thus likely that foetal interventions improving cardiac output, such as balloon dilation of the stenotic aortic78 or pulmonary valve,79 could have potential benefits exceeding the simple restoration of normal cardiac anatomy and circulation. Several foetal models were developed to test the effects of prenatal interventions80 or to study the growth response of the foetal heart to haemodynamic challenge.81,82 It is important to know the limits of adaptation of the foetal circulatory system, since some procedures that might work well in theory may not be tolerated by the whole organism (e.g. requirement of three-stage palliation of the hypoplastic left heart syndrome ending up in Fontan circulation rather than ‘all at once’ procedure; reviewed by Sedmera et al.83). Interventions in prenatal life have the potential to take advantage of the growth capacity of foetal myocardium based upon myocyte proliferation rather than hypertrophy. Such restoration of myocardial mass with appropriate vascularization might also result in superior performance in the long term.

635

8. Adaptation to postnatal circulation 640

645

Changes in circulatory patterns after birth are well known from embryology and obstetrics textbooks. The early postnatal period differs from subsequent growth in childhood, as there is a timewindow during which cardiomyocytes are still capable of proliferation. In rodents, this privileged period lasts for about 3 days after birth,84 while in humans, it is believed to last about 6 months, and surgical interventions85 such as biventricular repair for hypoplastic left heart syndrome are most successful during this window.85,86 The ability of the adult mammalian myocardium to regenerate is limited. Nevertheless, because of the potential significance of

D. Sedmera

regeneration to counter the current human plague of infarction and resulting chronic heart failure in survivors, exploration of regenerative potential is a subject of intensive research. The identification of resident cardiac stem cells87 generated much hope as well as much controversy, and the current state of the field is nicely summarized by Rosenthal and Harvey.88 Proliferative activity of adult cardiac myocytes has recently been documented in the normal human heart.89 However, it appears to be more related to homeostatic renewal than further growth of myocardium or regeneration after ischaemic attack. Myocardial infarction is healed by scar tissue, thus sacrificing long-term output for immediate strength and decreased risk of rupture. A further insight into normal developmental processes is necessary to define optimal therapeutic strategies for regeneration and repair of the diseased adult heart.

650

9. Concluding remarks

665

During heart morphogenesis, cardiac performance increases dramatically and a sequence of different functional designs (tubular heart, spongy ventricles), tested successfully during the course of evolution, meld smoothly into each other. Each morphological stage has a developmentally appropriate pacemaking and conduction system. A better understanding of the links between cardiac structure and function is important for devising treatment strategies for the developing heart and new in vivo imaging technologies, such as high-resolution ultrasound and OCT, are changing the way we look at the structure – function relationship in cardiac morphogenesis and dysmorphogenesis.

655

660

670

675

Supplementary material Supplementary material is available at Cardiovascular Research online.

680

Acknowledgements I would like to thank to all my past and present collaborators who were instrumental in generating results discussed in this paper and helped to shape my thinking about the developing heart. Ivan Helekal is acknowledged for generating the Figure 2. Norman Hu (Salt Lake City, UT) prepared the zebrafish heart specimen shown in Figure 3. I would also like to thank Robert Kelly for final critical reading of the manuscript and linguistic corrections. Conflict of interest: none declared.

685

690

Funding Supported by Ministry of Education VZ 0021620806, Academy of Sciences Purkinje Fellowship to D.S., and AV0Z50110509, Grant Agency 695 of the Czech Republic 304/08/0615 and P302/11/1308.

References 1. Sedmera D. Form follows function: developmental and physiological view on ventricular myocardial architecture. Eur J Cardiothorac Surg 2005;28:526 –528. 2. Grimes AC, Kirby ML. The outflow tract of the heart in fishes: anatomy, genes and evolution. J Fish Biol 2009;74:983 – 1036. 3. Icardo JM, Imbrogno S, Gattuso A, Colvee E, Tota B. The heart of Sparus auratus: a reappraisal of cardiac functional morphology in teleosts. J Exp Zool A Comp Exp Biol 2005;303:665 –675. 4. Sanchez-Quintana D, Hurle JM. Ventricular myocardial architecture in marine fishes. Anat Rec 1987;217:263–273. 5. Chen F, De Diego C, Chang MG, McHarg JL, John S, Klitzner TS et al. Atrioventricular conduction and arrhythmias at the initiation of beating in embryonic mouse hearts. Dev Dyn 2010;239:1941 –1949. 6. Sadler TW. Langman’s Medical Embryology. 11th edn. Baltimore: Lippincot Williams & Wilkings; 2011.

700

705

Development of function and form

710

715

720

725

730

735

740

745

750

755

760

765

7. Hirota A, Kamino K, Komuro H, Sakai T, Yada T. Early events in development of electrical activity and contraction in embryonic rat heart assessed by optical recording. J Physiol 1985;369:209 –227. 8. Buechling T, Akasaka T, Vogler G, Ruiz-Lozano P, Ocorr K, Bodmer R. Nonautonomous modulation of heart rhythm, contractility and morphology in adult fruit flies. Dev Biol 2009;328:483 –492. 9. Valasek P, Macharia R, Neuhuber WL, Wilting J, Becker DL, Patel K. Lymph heart in chick—somitic origin, development and embryonic oedema. Development 2007;134: 4427 –4436. 10. Holland ND, Venkatesh TV, Holland LZ, Jacobs DK, Bodmer R. AmphiNk2-tin, an amphioxus homeobox gene expressed in myocardial progenitors: insights into evolution of the vertebrate heart. Dev Biol 2003;255:128 –137. 11. Kucera T, Strilic B, Regener K, Schubert M, Laudet V, Lammert E. Ancestral vascular lumen formation via basal cell surfaces. PLoS ONE 2009;4:e4132. 12. Xavier-Neto J, Davidson B, Simoes-Costa MS, Castro RA, Castillo HA, Sampaio AC et al. Evolutionary Origins of Hearts. In: Rosenthal N, Harvey RP, eds. Heart Development and Regeneration. London: Elsevier; 2010:3–46. 13. Patten BM. The formation of the cardiac loop in the chick. Am J Anat 1922;30: 373 –397. 14. Manner J. Cardiac looping in the chick embryo: a morphological review with special reference to terminological and biomechanical aspects of the looping process. Anat Rec 2000;259:248 –262. 15. Bartram U, Molin DG, Wisse LJ, Mohamad A, Sanford LP, Doetschman T et al. Double-outlet right ventricle and overriding tricuspid valve reflect disturbances of looping, myocardialization, endocardial cushion differentiation, and apoptosis in TGF-beta(2)-knockout mice. Circulation 2001;103:2745 –2752. 16. Bouman HG, Broekhuizen ML, Baasten AM, Gittenberger-De Groot AC, Wenink AC. Spectrum of looping disturbances in stage 34 chicken hearts after retinoic acid treatment. Anat Rec 1995;243:101 –108. 17. de Jong F, Opthof T, Wilde AA, Janse MJ, Charles R, Lamers WH et al. Persisting zones of slow impulse conduction in developing chicken hearts. Circ Res 1992;71: 240 –250. 18. Sakai T, Hirota A, Kamino K. Video-imaging assessment of initial beating patterns of the early embryonic chick heart. Jpn J Physiol 1996;46:465 – 472. 19. Forouhar AS, Liebling M, Hickerson A, Nasiraei-Moghaddam A, Tsai HJ, Hove JR et al. The embryonic vertebrate heart tube is a dynamic suction pump. Science 2006;312: 751 –753. 20. McQuinn TC, Bratoeva M, Dealmeida A, Remond M, Thompson RP, Sedmera D. High-frequency ultrasonographic imaging of avian cardiovascular development. Dev Dyn 2007;236:3503 –3513. 21. Yelbuz TM, Choma MA, Thrane L, Kirby ML, Izatt JA. Optical coherence tomography: a new high-resolution imaging technology to study cardiac development in chick embryos. Circulation 2002;106:2771 –2774. 22. Manner J, Thrane L, Norozi K, Yelbuz TM. In vivo imaging of the cyclic changes in cross-sectional shape of the ventricular segment of pulsating embryonic chick hearts at stages 14 to 17: A contribution to the understanding of the ontogenesis of cardiac pumping function. Dev Dyn 2009;238:3273 –3284. 23. Hutchins GM, Bulkley BH, Moore GW, Piasio MA, Lohr FT. Shape of the human cardiac ventricles. Am J Cardiol 1978;41:646–654. 24. Warren KS, Fishman MC. "Physiological genomics": mutant screens in zebrafish. Am J Physiol 1998;275:H1 –H7. 25. Reckova M, Rosengarten C, deAlmeida A, Stanley CP, Wessels A, Gourdie RG et al. Hemodynamics is a key epigenetic factor in development of the cardiac conduction system. Circ Res 2003;93:77 –85. 26. Gourdie RG, Harris BS, Bond J, Justus C, Hewett KW, O’Brien TX et al. Development of the cardiac pacemaking and conduction system. Birth Defects Res 2003;69C:46–57. 27. Knaapen MW, Vrolijk BC, Wenink AC. Nuclear and cellular size of myocytes in different segments of the developing rat heart. Anat Rec 1996;244:118 –125. 28. Moorman AF, Christoffels VM. Cardiac chamber formation: development, genes, and evolution. Physiol Rev 2003;83:1223 –1267. 29. Aanhaanen WT, Mommersteeg MT, Norden J, Wakker V, de Gier-de Vries C, Anderson RH et al. Developmental origin, growth, and three-dimensional architecture of the atrioventricular conduction axis of the mouse heart. Circ Res 2010;107: 728 –736. 30. Minot CS. On a hitherto unrecognised circulation without capillaries in the organs of Vertebrata. Proc Boston Soc Nat Hist 1901;29:185 –215. 31. Lee KF, Simon H, Chen H, Bates B, Hung MC, Hauser C. Requirement for neuregulin receptor erbB2 in neural and cardiac development. Nature 1995;378:394 –398. 32. Liu J, Bressan M, Hassel D, Huisken J, Staudt D, Kikuchi K et al. A dual role for ErbB2 signaling in cardiac trabeculation. Development 2010;137:3867 –3875. 33. Lai D, Liu X, Forrai A, Wolstein O, Michalicek J, Ahmed I et al. Neuregulin 1 sustains the gene regulatory network in both trabecular and nontrabecular myocardium. Circ Res 2010;107:715 –727. 34. Leaf DE, Feig JE, Vasquez C, Riva PL, Yu C, Lader JM et al. Connexin40 imparts conduction heterogeneity to atrial tissue. Circ Res 2008;103:1001 –1008. 35. Sedmera D, Wessels A, Trusk TC, Thompson RP, Hewett KW, Gourdie RG. Changes in activation sequence of embryonic chick atria correlate with developing myocardial architecture. Am J Physiol Heart Circ Physiol 2006;291:H1646 –H1652.

Page 7 of 8 36. Arguello C, Alanis J, Pantoja O, Valenzuela B. Electrophysiological and ultrastructural study of the atrioventricular canal during the development of the chick embryo. J Mol Cell Cardiol 1986;18:499 –510. 37. Sedmera D, Reckova M, DeAlmeida A, Coppen SR, Kubalak SW, Gourdie RG et al. Spatiotemporal pattern of commitment to slowed proliferation in the embryonic mouse heart indicates progressive differentiation of the cardiac conduction system. Anat Rec 2003;274A:773 –777. 38. Keller BB, Hu N, Clark EB. Correlation of ventricular area, perimeter, and conotruncal diameter with ventricular mass and function in the chick embryo from stages 12 to 24. Circ Res 1990;66:109 –114. 39. Clark EB, Hu N, Dummett JL, Vandekieft GK, Olson C, Tomanek R. Ventricular function and morphology in chick embryo from stages 18 to 29. Am J Physiol 1986;250: H407 –H413. 40. Dunnigan A, Hu N, Benson DW Jr, Clark EB. Effect of heart rate increase on dorsal aortic flow in the stage 24 chick embryo. Pediatr Res 1987;22:442 –444. 41. Keller BB, MacLennan MJ, Tinney JP, Yoshigi M. In vivo assessment of embryonic cardiovascular dimensions and function in day-10.5 to -14.5 mouse embryos. Circ Res 1996;79:247 –255. 42. Phoon CK. Imaging tools for the developmental biologist: ultrasound biomicroscopy of mouse embryonic development. Pediatr Res 2006;60:14–21. 43. Phoon CK, Ji RP, Aristizabal O, Worrad DM, Zhou B, Baldwin HS et al. Embryonic heart failure in NFATc1-/- mice: novel mechanistic insights from in utero ultrasound biomicroscopy. Circ Res 2004;95:92 –99. 44. Nomura-Kitabayashi A, Phoon CKL, Kishigami S, Rosenthal J, Yamauchi Y¨, Abe K et al. Outflow tract cushions perform a critical valve-like function in the early embryonic heart requiring BMPRIA-mediated signaling in cardiac neural crest. Am J Physiol Heart Circ Physiol 2009;297:H1617 – H1628. 45. Rein AJ, Mevorach D, Perles Z, Gavri S, Nadjari M, Nir A et al. Early diagnosis and treatment of atrioventricular block in the fetus exposed to maternal anti-SSA/ Ro-SSB/La antibodies: a prospective, observational, fetal kinetocardiogram-based study. Circulation 2009;119:1867 –1872. 46. Hildreth V, Anderson RH, Henderson DJ. Autonomic innervation of the developing heart: origins and function. Clin Anat 2009;22:36–46. 47. Sedmera D, Reckova M, DeAlmeida A, Sedmerova M, Biermann M, Volejnik J et al. Functional and morphological evidence for a ventricular conduction system in the zebrafish and Xenopus heart. Am J Physiol Heart Circ Physiol 2003;284:H1152 –H1160. 48. Viragh S, Challice CE. The development of the conduction system in the mouse embryo heart. II. Histogenesis of the atrioventricular node and bundle. Dev Biol 1977;56:397 –411. 49. Chuck ET, Freeman DM, Watanabe M, Rosenbaum DS. Changing activation sequence in the embryonic chick heart. Implications for the development of the His-Purkinje system. Circ Res 1997;81:470 – 476. 50. Vassall-Adams PR. The development of the atrioventricular bundle and its branches in the avian heart. J Anat 1982;134:169 – 183. 51. Miquerol L, Meysen S, Mangoni M, Bois P, van Rijen HV, Abran P et al. Architectural and functional asymmetry of the His-Purkinje system of the murine heart. Cardiovasc Res 2004;63:77 –86. 52. Sedmera D, Reckova M, Bigelow MR, DeAlmeida A, Stanley CP, Mikawa T et al. Developmental transitions in electrical activation patterns in chick embryonic heart. Anat Rec 2004;280A:1001 –1009. 53. Wessels A, Vermeulen JL, Verbeek FJ, Viragh S, Kalman F, Lamers WH et al. Spatial distribution of "tissue-specific" antigens in the developing human heart and skeletal muscle. III. An immunohistochemical analysis of the distribution of the neural tissue antigen G1N2 in the embryonic heart; implications for the development of the atrioventricular conduction system. Anat Rec 1992;232:97– 111. 54. Lamers WH, Wessels A, Verbeek FJ, Moorman AF, Viragh S, Wenink AC et al. New findings concerning ventricular septation in the human heart. Implications for maldevelopment. Circulation 1992;86:1194 –1205. 55. Rentschler S, Zander J, Meyers K, France D, Levine R, Porter G et al. Neuregulin-1 promotes formation of the murine cardiac conduction system. Proc Natl Acad Sci USA 2002;99:10464 –10469. 56. Sankova B, Machalek J, Sedmera D. Effects of mechanical loading on early conduction system differentiation in the chick. Am J Physiol Heart Circ Physiol 2010;298: H1571 –H1576. 57. Sedmera D, Reckova M, Rosengarten C, Torres MI, Gourdie RG, Thompson RP. Optical mapping of electrical activation in developing heart. Microscop Microanal 2005;11:209 –215. 58. Rentschler S, Vaidya DM, Tamaddon H, Degenhardt K, Sassoon D, Morley GE et al. Visualization and functional characterization of the developing murine cardiac conduction system. Development 2001;128:1785 –1792. 59. Rothenberg F, Nikolski V, Watanabe M, Efimov I. Electrophysiology and anatomy of embryonic rabbit hearts before and after septation. Am J Physiol Heart Circ Physiol 2005;288:H344 –H351. 60. Gourdie RG, Green CR, Severs NJ, Anderson RH, Thompson RP. Evidence for a distinct gap-junctional phenotype in ventricular conduction tissues of the developing and mature avian heart. Circ Res 1993;72:278 –289. 61. Hall CE, Hurtado R, Hewett KW, Shulimovich M, Poma CP, Reckova M et al. Hemodynamic-dependent patterning of endothelin converting enzyme 1 expression

770

775

780

785

790

795

800

805

810

815

820

825

Page 8 of 8

62. 830

63.

64.

65. 835 66.

840

67. 68. 69.

845

70. 71.

72. 850 73.

74. 855

75.

76.

and differentiation of impulse-conducting Purkinje fibers in the embryonic heart. Development 2004;131:581 –592. Cheng G, Wessels A, Gourdie RG, Thompson RP. Spatiotemporal distribution of apoptosis in embryonic chicken heart. Dev Dyn 2002;223:119 –133. Kolditz DP, Wijffels MC, Blom NA, van der Laarse A, Hahurij ND, Lie-Venema H et al. Epicardium-derived cells in development of annulus fibrosis and persistence of accessory pathways. Circulation 2008;117:1508 –1517. Viragh S, Challice CE. The development of the conduction system in the mouse embryo heart. I. The first embryonic A-V conduction pathway. Dev Biol 1977;56: 382 –396. Aanhaanen WT, Brons JF, Dominguez JN, Rana MS, Norden J, Airik R et al. The Tbx2+ primary myocardium of the atrioventricular canal forms the atrioventricular node and the base of the left ventricle. Circ Res 2009;104:1267 –1274. Christoffels VM, Hoogaars WM, Moorman AF. Patterning and development of the condustion system of the heart: origins of the conduction system in development. In: Rosenthal N, Harvey RP, eds. Heart Development and Regeneration. London: Elsevier, 2010:171 –194. Valderrabano M, Chen F, Dave AS, Lamp ST, Klitzner TS, Weiss JN. Atrioventricular ring reentry in embryonic mouse hearts. Circulation 2006;114:543 –549. Friedman DM, Rupel A, Glickstein J, Buyon JP. Congenital heart block in neonatal lupus: the pediatric cardiologist’s perspective. Indian J Pediatr 2002;69:517 –522. Friedman DM, Kim MY, Copel JA, Davis C, Phoon CK, Glickstein JS et al. Utility of cardiac monitoring in fetuses at risk for congenital heart block: the PR Interval and Dexamethasone Evaluation (PRIDE) prospective study. Circulation 2008;117:485 –493. Wessels A, Sedmera D. Developmental anatomy of the heart: a tale of mice and man. Physiol Genomics 2003;15:165–176. Nanka O, Krizova P, Fikrle M, Tuma M, Blaha M, Grim M et al. Abnormal myocardial and coronary vasculature development in experimental hypoxia. Anat Rec (Hoboken) 2008;291:1187 –1199. Jouk PS, Usson Y, Michalowicz G, Grossi L. Three-dimensional cartography of the pattern of the myofibres in the second trimester fetal human heart. Anat Embryol (Berl) 2000;202:103 –118. Tobita K, Garrison JB, Li JJ, Tinney JP, Keller BB. Three-dimensional myofiber architecture of the embryonic left ventricle during normal development and altered mechanical loads. Anat Rec A Discov Mol Cell Evol Biol 2005;283:193 – 201. Hornberger LK, Sanders SP, Rein AJ, Spevak PJ, Parness IA, Colan SD. Left heart obstructive lesions and left ventricular growth in the midtrimester fetus. A longitudinal study. Circulation 1995;92:1531 –1538. Tworetzky W, Wilkins-Haug L, Jennings RW, van der Velde ME, Marshall AC, Marx GR et al. Balloon dilation of severe aortic stenosis in the fetus: potential for prevention of hypoplastic left heart syndrome: candidate selection, technique, and results of successful intervention. Circulation 2004;110:2125 –2131. Losada E, Moon-Grady AJ, Strohsnitter WC, Wu D, Ursell PC. Anomalous mitral arcade in twin-twin transfusion syndrome. Circulation 2010;122:1456 –1463.

D. Sedmera

77. Limperopoulos C, Tworetzky W, McElhinney DB, Newburger JW, Brown DW, Robertson RL Jr et al. Brain volume and metabolism in fetuses with congenital heart disease: evaluation with quantitative magnetic resonance imaging and spectroscopy. Circulation 2010;121:26–33. 78. Marshall AC, Tworetzky W, Bergersen L, McElhinney DB, Benson CB, Jennings RW et al. Aortic valvuloplasty in the fetus: technical characteristics of successful balloon dilation. J Pediatr 2005;147:535 – 539. 79. Tworetzky W, McElhinney DB, Marx GR, Benson CB, Brusseau R, Morash D et al. In utero valvuloplasty for pulmonary atresia with hypoplastic right ventricle: techniques and outcomes. Pediatrics 2009;124:e510– e518. 80. deAlmeida A, McQuinn T, Sedmera D. Increased ventricular preload is compensated by myocyte proliferation in normal and hypoplastic fetal chick left ventricle. Circ Res 2007;100:1363 –1370. 81. Saiki Y, Konig A, Waddell J, Rebeyka IM. Hemodynamic alteration by fetal surgery accelerates myocyte proliferation in fetal guinea pig hearts. Surgery 1997;122: 412 –419. 82. Rudolph AM. Myocardial growth before and after birth: clinical implications. Acta Paediatr 2000;89:129 –133. 83. Sedmera D, Cook AC, Shirali G, McQuinn TC. Current issues and perspectives in hypoplasia of the left heart. Cardiol Young 2005;15:56– 72. 84. Li F, Wang X, Capasso JM, Gerdes AM. Rapid transition of cardiac myocytes from hyperplasia to hypertrophy during postnatal development. J Mol Cell Cardiol 1996; 28:1737 –1746. 85. Foker JE, Berry J, Steinberger J. Ventricular growth stimulation to achieve twoventricular repair in unbalanced common atrioventricular canal. Prog Pediatric Cardiol 1999;10:173 – 186. 86. Tchervenkov CI, Jacobs ML, Tahta SA. Congenital Heart Surgery Nomenclature and Database Project: hypoplastic left heart syndrome. Ann Thorac Surg 2000;69: S170 –S179. 87. Beltrami AP, Barlucchi L, Torella D, Baker M, Limana F, Chimenti S et al. Adult cardiac stem cells are multipotent and support myocardial regeneration. Cell 2003;114: 763 –776. 88. Rosenthal N, Harvey RP. Heart Development and Regeneration. Vol. 2. London: Elsevier, 2010. 89. Bergmann O, Bhardwaj RD, Bernard S, Zdunek S, Barnabe-Heider F, Walsh S et al. Evidence for cardiomyocyte renewal in humans. Science 2009;324:98–102. 90. Sedmera D, Pexieder T, Hu N, Clark EB. Developmental changes in the myocardial architecture of the chick. Anat Rec 1997;248:421 –432. 91. Sedmera D, Pexieder T, Vuillemin M, Thompson RP, Anderson RH. Developmental patterning of the myocardium. Anat Rec 2000;258:319 – 337. 92. Hewett KW, Norman LW, Sedmera D, Barker RJ, Justus C, Zhang J et al. Knockout of the neural and heart expressed gene HF-1b results in apical deficits of ventricular structure and activation. Cardiovasc Res 2005;67:548 – 560.

890

895

900

905

910

915

860 920

865 925

870 930

875 935

880 940

885