Development of the Cardiac Conduction System.

The electrical impulses that coordinate the sequential, rhythmic contractions of the atria and ventricles are initiated and tightly regulated by the specialized tissues of the cardiac conduction system. In the mature heart, these impulses are generated by the pacemaker cardiomyocytes of the sinoatrial node, propagated through the atria to the atrioventricular node where they are delayed and then rapidly propagated to the atrioventricular bundle, right and left bundle branches, and finally, the peripheral ventricular conduction system. Each of these specialized components arise by complex patterning events during embryonic development. This chapter addresses the origins and transcriptional networks and signaling pathways that drive the development and maintain the function of the cardiac conduction system.

Automated quantitative evaluation of fetal atrioventricular annular plane systolic excursion.

OBJECTIVES: The primary aim of this study was to evaluate the feasibility of automated measurement of fetal atrioventricular (AV) plane displacement (AVPD) over several cardiac cycles using myocardial velocity traces obtained by color tissue Doppler imaging (cTDI). The secondary objectives were to establish reference ranges for AVPD during the second half of normal pregnancy, to assess fetal AVPD in prolonged pregnancy in relation to adverse perinatal outcome and to evaluate AVPD in fetuses with a suspicion of intrauterine growth restriction (IUGR). METHODS: The population used to develop the reference ranges consisted of women with an uncomplicated singleton pregnancy at 18-42 weeks of gestation (n = 201). The prolonged-pregnancy group comprised women with an uncomplicated singleton pregnancy at ≥ 41 + 0 weeks of gestation (n = 107). The third study cohort comprised women with a singleton pregnancy and suspicion of IUGR, defined as an estimated fetal weight < 2.5th centile or an estimated fetal weight < 10th centile and umbilical artery pulsatility index > 97.5th centile (n = 35). Cineloops of the four-chamber view of the fetal heart were recorded using cTDI. Regions of interest were placed at the AV plane in the left and right ventricular walls and the interventricular septum, and myocardial velocity traces were integrated and analyzed using an automated algorithm developed in-house to obtain mitral (MAPSE), tricuspid (TAPSE) and septal (SAPSE) annular plane systolic excursion. Gestational-age specific reference ranges were constructed and normalized for cardiac size. The correlation between AVPD measurements obtained using cTDI and those obtained by anatomic M-mode were evaluated, and agreement between these two methods was assessed using Bland-Altman analysis. The mean Z-scores of fetal AVPD in the cohort of prolonged pregnancies were compared between cases with normal and those with adverse outcome using Mann-Whitney U-test. The mean Z-scores of fetal AVPD in IUGR fetuses were compared with those in the normal reference population using Mann-Whitney U-test. Inter- and intraobserver variability for acquisition of cTDI recordings and offline analysis was assessed by calculating coefficients of variation (CV) using the root mean square method. RESULTS: Fetal MAPSE, SAPSE and TAPSE increased with gestational age but did not change significantly when normalized for cardiac size. The fitted mean was highest for TAPSE throughout the second half of gestation, followed by SAPSE and MAPSE. There was a significant correlation between MAPSE (r = 0.64; P < 0.001), SAPSE (r = 0.72; P < 0.001) and TAPSE (r = 0.84; P < 0.001) measurements obtained by M-mode and those obtained by cTDI. The geometric means of ratios between AVPD measured by cTDI and by M-mode were 1.38 (95% limits of agreement (LoA), 0.84-2.25) for MAPSE, 1.00 (95% LoA, 0.72-1.40) for SAPSE and 1.20 (95% LoA, 0.92-1.57) for TAPSE. In the prolonged-pregnancy group, the mean ± SD Z-scores for MAPSE (0.14 ± 0.97), SAPSE (0.09 ± 1.02) and TAPSE (0.15 ± 0.90) did not show any significant difference compared to the reference ranges. Twenty-one of the 107 (19.6%) prolonged pregnancies had adverse perinatal outcome. The AVPD Z-scores were not significantly different between pregnancies with normal and those with adverse outcome in the prolonged-pregnancy cohort. The mean ± SD Z-scores for SAPSE (-0.62 ± 1.07; P = 0.006) and TAPSE (-0.60 ± 0.89; P = 0.002) were significantly lower in the IUGR group compared to those in the normal reference population, but the differences were not significant when the values were corrected for cardiac size. The interobserver CVs for the automated measurement of MAPSE, SAPSE and TAPSE were 28.1%, 17.7% and 15.3%, respectively, and the respective intraobserver CVs were 33.5%, 15.0% and 17.9%. CONCLUSIONS: This study showed that fetal AVPD can be measured automatically by integrating cTDI velocities over several cardiac cycles. Automated analysis of AVPD could potentially help gather larger datasets to facilitate use of machine-learning models to study fetal cardiac function. The gestational-age associated increase in AVPD is most likely a result of increasing cardiac size, as the AVPD normalized for cardiac size did not change significantly between 18 and 42 weeks. A decrease was seen in TAPSE and SAPSE in IUGR fetuses, but not after correction for cardiac size. © 2021 The Authors. Ultrasound in Obstetrics & Gynecology published by John Wiley & Sons Ltd on behalf of International Society of Ultrasound in Obstetrics and Gynecology.

The sinus venosus myocardium contributes to the atrioventricular canal: potential role during atrioventricular node development?

The presence of distinct electrophysiological pathways within the atrioventricular node (AVN) is a prerequisite for atrioventricular nodal reentrant tachycardia to occur. In this study, the different cell contributions that may account for the anatomical and functional heterogeneity of the AVN were investigated. To study the temporal development of the AVN, the expression pattern of ISL1, expressed in cardiac progenitor cells, was studied in sequential stages performing co-staining with myocardial markers (TNNI2 and NKX2-5) and HCN4 (cardiac conduction system marker). An ISL1+/TNNI2+/HCN4+ continuity between the myocardium of the sinus venosus and atrioventricular canal was identified in the region of the putative AVN, which showed a pacemaker-like phenotype based on single cell patch-clamp experiments. Furthermore, qPCR analysis showed that even during early development, different cell populations can be identified in the region of the putative AVN. Fate mapping was performed by in ovo vital dye microinjection. Embryos were harvested and analysed 24 and 48 hrs post-injection. These experiments showed incorporation of sinus venosus myocardium in the posterior region of the atrioventricular canal. The myocardium of the sinus venosus contributes to the atrioventricular canal. It is postulated that the myocardium of the sinus venosus contributes to nodal extensions or transitional cells of the AVN since these cells are located in the posterior region of the AVN. This finding may help to understand the origin of atrioventricular nodal reentrant tachycardia.

Connexin45 provides optimal atrioventricular nodal conduction in the adult mouse heart.

RATIONALE: The gap junctional protein connexin (Cx) 45 is strongly expressed in the early embryonic myocardium. In the adult hearts of mice and humans, the expression mainly is restricted to the cardiac conduction system. Cx45 plays an essential role for development and function of the embryonic heart because general and cardiomyocyte-directed deficiencies of Cx45 in mice lead to embryonic lethality attributable to morphological and functional cardiovascular defects. The function of Cx45 in the adult mouse has not yet been cleared. OBJECTIVE: To clarify the function of Cx45 in the adult mouse heart. METHODS AND RESULTS: To circumvent the embryonic lethality resulting from Cx45 deficiency, mice were generated in which deletion of Cx45 specifically was induced in cardiomyocytes of adult mice. These Cx45-deficient mice were viable but showed a decrease in atrioventricular nodal conductivity. In addition, the Cx30.2 protein that is coexpressed with Cx45 in the cardiac conduction system was posttranscriptionally reduced by 70% in mutant hearts. Furthermore, deletion of both Cx45 and Cx30.2 resulted in viable mice that, however, showed stronger impairment of atrioventricular nodal conduction than the single Cx45-deficient mice. CONCLUSIONS: Cx45 is required for optimal impulse propagation in the atrioventricular node and stabilizes the level of the coexpressed Cx30.2 protein in the adult mouse heart. In contrast to the embryo, Cx45 is not essential for the viability of adult mice.

Electrophysiological mapping of embryonic mouse hearts: mechanisms for developmental pacemaker switch and internodal conduction pathway.

INTRODUCTION: Understanding sinoatrial node (SAN) development could help in developing therapies for SAN dysfunction. However, electrophysiological investigation of SAN development remains difficult because mutant mice with SAN dysfunctions are frequently embryonically lethal. Most research on SAN development is therefore limited to immunocytochemical observations without comparable functional studies. METHODS AND RESULTS: We applied a multielectrode array (MEA) recording system to study SAN development in mouse hearts acutely isolated at embryonic ages (E) 8.5-12.5 days. Physiological heart rates were routinely restored, enabling accurate functional assessment of SAN development. We found that dominant pacemaking activity originated from the left inflow tract (LIFT) region at E8.5, but switched to the right SAN by E12.5. Combining MEA recordings and pharmacological agents, we show that intracellular calcium (Ca(2+))-mediated automaticity develops early and is the major mechanism of pulse generation in the LIFT of E8.5 hearts. Later in development at E12.5, sarcolemmal ion channels develop in the SAN at a time when pacemaker channels are down-regulated in the LIFT, leading to a switch in the dominant pacemaker location. Additionally, low micromolar concentrations of tetrodotoxin (TTX), a sodium channel blocker, minimally affect pacemaker rhythm at E8.5-E12.5, but suppress atrial activation and reveal a TTX-resistant SAN-atrioventricular node (internodal) pathway that mediates internodal conduction in E12.5 hearts. CONCLUSIONS: Using a physiological mapping method, we demonstrate that differential mechanistic development of automaticity between the left and right inflow tract regions confers the pacemaker location switch. Moreover, a TTX-resistant pathway mediates preferential internodal conduction in E12.5 mouse hearts.

Developmental origin, growth, and three-dimensional architecture of the atrioventricular conduction axis of the mouse heart.

RATIONALE: The clinically important atrioventricular conduction axis is structurally complex and heterogeneous, and its molecular composition and developmental origin are uncertain. OBJECTIVE: To assess the molecular composition and 3D architecture of the atrioventricular conduction axis in the postnatal mouse heart and to define the developmental origin of its component parts. METHODS AND RESULTS: We generated an interactive 3D model of the atrioventricular junctions in the mouse heart using the patterns of expression of Tbx3, Hcn4, Cx40, Cx43, Cx45, and Nav1.5, which are important for conduction system function. We found extensive figure-of-eight rings of nodal and transitional cells around the mitral and tricuspid junctions and in the base of the atrial septum. The rings included the compact node and nodal extensions. We then used genetic lineage labeling tools (Tbx2(+/Cre), Mef2c-AHF-Cre, Tbx18(+/Cre)), along with morphometric analyses, to assess the developmental origin of the specific components of the axis. The majority of the atrial components, including the atrioventricular rings and compact node, are derived from the embryonic atrioventricular canal. The atrioventricular bundle, including the lower cells of the atrioventricular node, in contrast, is derived from the ventricular myocardium. No contributions to the conduction system myocardium were identified from the sinus venosus, the epicardium, or the dorsal mesenchymal protrusion. CONCLUSIONS: The atrioventricular conduction axis comprises multiple domains with distinctive molecular signatures. The atrial part proliferates from the embryonic atrioventricular canal, along with myocytes derived from the developing atrial septum. The atrioventricular bundle and lower nodal cells are derived from ventricular myocardium.

Atrioventricular conduction delay in fetuses exposed to anti-SSA/Ro and anti-SSB/La antibodies: a magnetocardiography study.

BACKGROUND: The presence of anti-SSA/Ro and anti-SSB/La antibodies during pregnancy is associated with fetal congenital heart block (CHB), which is primarily diagnosed through fetal echocardiography. Conclusive information about the complete electrophysiology of the fetal cardiac conducting system is still lacking. In addition to echocardiography, fetal magnetocardiography (fMCG) can be used. fMCG is the magnetic analogue of the fetal electrocardiogram (ECG). PATIENTS AND METHODS: Forty-eight pregnant women were enrolled in an observational study; 16 of them tested positive for anti-SSA/Ro and anti-SSB/La antibodies. In addition to routine fetal echocardiography, fMCG was used. Fetal cardiac time intervals (fCTIs) were extracted from the magnetic recordings by predefined procedures. ECGs in the neonates of the study group were performed within the first month after delivery. RESULTS: The PQ segment of the fCTI was significantly prolonged in the study group (P = 0.007), representing a delay of the electrical impulse in the atrioventricular (AV) node. Other fCTIs were within normal range. None of the anti-SSA/Ro and/or anti-SSB/La fetuses progressed to a more advanced heart block during pregnancy or after birth. CONCLUSION: The study identified a low-risk population within antibody positive mothers, where PQ segment prolongation is associated with a lack of progression of the disease.

Accessory atrioventricular myocardial pathways in mouse heart development: substrate for supraventricular tachycardias.

Atrioventricular reentry tachycardia (AVRT) requiring an accessory atrioventricular pathway (AP) is the most common type of arrhythmia in the perinatal period. The etiology of these arrhythmias is not fully understood as well as their capability to dissipate spontaneously in the first year of life. Temporary presence of APs during annulus fibrosus development might cause this specific type of arrhythmias. To study the presence of APs, electrophysiological recordings of ventricular activation patterns and immunohistochemical analyses with antibodies specifically against atrial myosin light chain 2 (MLC-2a), Periostin, Nkx2.5, and Connexin-43 were performed in embryonic mouse hearts ranging from 11.5 to 18.5 days post-conception (dpc). The electrophysiological recordings revealed the presence of functional APs in early (13.5-15.5 dpc) and late (16.5-18.5 dpc) postseptated stages of mouse heart development. These APs stained positive for MLC-2a and Nkx2.5 and negative for Periostin and Connexin-43. Longitudinal analyses showed that APs gradually decreased in number (p = 0.003) and size (p = 0.035) at subsequent developmental stages (13.5-18.5 dpc). Expression of periostin was observed in the developing annulus fibrosus, adjacent to APs and other locations where formation of fibrous tissue is essential. We conclude that functional APs are present during normal mouse heart development. These APs can serve as transient substrate for AVRTs in the perinatal period of development.

Origin and development of the atrioventricular myocardial lineage: insight into the development of accessory pathways.

Defects originating from the atrioventricular canal region are part of a wide spectrum of congenital cardiovascular malformations that frequently affect newborns. These defects include partial or complete atrioventricular septal defects, atrioventricular valve defects, and arrhythmias, such as atrioventricular re-entry tachycardia, atrioventricular nodal block, and ventricular preexcitation. Insight into the cellular origin of the atrioventricular canal myocardium and the molecular mechanisms that control its development will aid in the understanding of the etiology of the atrioventricular defects. This review discusses current knowledge concerning the origin and fate of the atrioventricular canal myocardium, the molecular mechanisms that determine its specification and differentiation, and its role in the development of certain malformations such as those that underlie ventricular preexcitation.

Transcription factor Tbx3 is required for the specification of the atrioventricular conduction system.

The cardiac conduction system consists of distinctive heart muscle cells that initiate and propagate the electric impulse required for coordinated contraction. The conduction system expresses the transcriptional repressor Tbx3, which is required for vertebrate development and controls the formation of the sinus node. In humans, mutations in Tbx3 cause ulnar-mammary syndrome. Here, we investigated the role of Tbx3 in the molecular specification of the atrioventricular conduction system. Expression analysis revealed early delineation of the atrioventricular bundle and proximal bundle branches by Tbx3 expression in human, mouse, and chicken. Tbx3-deficient mice, which die between embryonic day 12.5 and 15.5, ectopically expressed genes for connexin (Cx)43, atrial natriuretic factor (Nppa), Tbx18, and Tbx20 in the atrioventricular bundle and proximal bundle branches. Cx40 was precociously upregulated in the atrioventricular bundle of Tbx3 mutants. Moreover, the atrioventricular bundle and branches failed to exit the cell cycle in Tbx3 mutant embryos. Finally, Tbx3-deficient embryos developed outflow tract malformations and ventricular septal defects. These data reveal that Tbx3 is required for the molecular specification of the atrioventricular bundle and bundle branches and for the development of the ventricular septum and outflow tract. Our data suggest a mechanism in which Tbx3 represses differentiation into ventricular working myocardium, thereby imposing the conduction system phenotype on cells within its expression domain.

Distinct regulation of developmental and heart disease-induced atrial natriuretic factor expression by two separate distal sequences.

Nppa, encoding atrial natriuretic factor, is expressed in fetal atrial and ventricular myocardium and is downregulated in the ventricles after birth. During hypertrophy and heart failure, Nppa expression is reactivated in the ventricles and serves as a highly conserved marker of heart disease. The Nppa promoter has become a frequently used model to study mechanisms of cardiac gene regulation. Nevertheless, the regulatory sequences that provide the correct developmental pattern and ventricular reactivation during cardiac disease remain to be defined. We found that proximal Nppa fragments ranging from 250 bp to 16 kbp provide robust reporter gene activity in the atria and correct repression in the atrioventricular canal and the nodes of the conduction system in vivo. However, depending on fragment size and site of integration into the genome of mice, the fetal ventricular activity was either absent or present in an incorrect pattern. Furthermore, these fragments did not provide ventricular reactivation in heart disease models. These results indicate that the proximal promoter does not provide a physiologically relevant model for ventricular gene activity. In contrast, 2 modified bacterial artificial chromosome clones with partially overlapping genomic Nppa sequences provided appropriate reactivation of the green fluorescent protein reporter during pressure overload-induced hypertrophy and heart failure in vivo. However, only 1 of these bacterial artificial chromosomes provided correct fetal ventricular green fluorescent protein activity. These results show that distinct distal regulatory sequences and divergent regulatory pathways control fetal ventricular activity and reactivation of Nppa during cardiac disease, respectively.

Periostin regulates atrioventricular valve maturation.

Cardiac valve leaflets develop from rudimentary structures termed endocardial cushions. These pre-valve tissues arise from a complex interplay of signals between the myocardium and endocardium whereby secreted cues induce the endothelial cells to transform into migratory mesenchyme through an endothelial to mesenchymal transformation (EMT). Even though much is currently known regarding the initial EMT process, the mechanisms by which these undifferentiated cushion mesenchymal tissues are remodeled "post-EMT" into mature fibrous valve leaflets remains one of the major, unsolved questions in heart development. Expression analyses, presented in this report, demonstrate that periostin, a component of the extracellular matrix, is predominantly expressed in post-EMT valve tissues and their supporting apparatus from embryonic to adult life. Analyses of periostin gene targeted mice demonstrate that it is within these regions that significant defects are observed. Periostin null mice exhibit atrial septal defects, structural abnormalities of the AV valves and their supporting tensile apparatus, and aberrant differentiation of AV cushion mesenchyme. Rescue experiments further demonstrate that periostin functions as a hierarchical molecular switch that can promote the differentiation of mesenchymal cells into a fibroblastic lineage while repressing their transformation into other mesodermal cell lineages (e.g. myocytes). This is the first report of an extracellular matrix protein directly regulating post-EMT AV valve differentiation, a process foundational and indispensable for the morphogenesis of a cushion into a leaflet.

Left cardiac isomerism in the Sonic hedgehog null mouse.

Sonic hedgehog (Shh) is a secreted morphogen necessary for the production of sidedness in the developing embryo. In this study, we describe the morphology of the atrial chambers and atrioventricular junctions of the Shh null mouse heart. We demonstrate that the essential phenotypic feature is isomerism of the left atrial appendages, in combination with an atrioventricular septal defect and a common atrioventricular junction. These malformations are known to be frequent in humans with left isomerism. To confirm the presence of left isomerism, we show that Pitx2c, a recognized determinant of morphological leftness, is expressed in the Shh null mutants on both the right and left sides of the inflow region, and on both sides of the solitary arterial trunk exiting from the heart. It has been established that derivatives of the second heart field expressing Isl1 are asymmetrically distributed in the developing normal heart. We now show that this population is reduced in the hearts from the Shh null mutants, likely contributing to the defects. To distinguish the consequences of reduced contributions from the second heart field from those of left-right patterning disturbance, we disrupted the movement of second heart field cells into the heart by expressing dominant-negative Rho kinase in the population of cells expressing Isl1. This resulted in absence of the vestibular spine, and presence of atrioventricular septal defects closely resembling those seen in the hearts from the Shh null mutants. The primary atrial septum, however, was well formed, and there was no evidence of isomerism of the atrial appendages, suggesting that these features do not relate to disruption of the contributions made by the second heart field. We demonstrate, therefore, that the Shh null mouse is a model of isomerism of the left atrial appendages, and show that the recognized associated malformations found at the venous pole of the heart in the setting of left isomerism are likely to arise from the loss of the effects of Shh in the establishment of laterality, combined with a reduced contribution made by cells derived from the second heart field.

Pulsed Doppler fetal atrioventricular interval measurement: Assessment of a new image scoring method.

OBJECTIVES: We propose an image scoring method to improve the quality and the reproducibility of measurement of the AV interval before establishing reference tables of the measurements and studies on the prevention and treatment of first-degree AV block especially if the first child has been diagnosed AV block. METHOD: Prospective study from May 2015 to June 2016. Sonographers were asked to measure AV interval with pulsed Doppler in a five-chamber view in standard second-trimester screening before and after having received our image scoring method. Images were scored by 2 blinded reviewers. RESULTS: The intra-class correlation coefficient (ICC) between the two reviewers for the overall score was 0.91. On average, the measurement quality increased by 2.5 points/10 (95% CI 1.0-4.0). In the second set of images, after the scoring method was given, the score stared at 6.50 for the first image, with a significant improvement of 0.18 (p = 0.016) per subsequent image comparing to a non significant improvement for the first set of image. There was a significant improvement in intra-observer reliability, ICC: 0.680 [95% CI 0.606-0.854] versus 0.458 [95% CI 0.140-0.651]. CONCLUSION: The use of this scoring method is simple, reproducible and improves image quality and reproducibility of AV interval measurement in a five-chamber view.

The epicardium obscures interpretations on endothelial-to-mesenchymal transition in the mouse atrioventricular canal explant assay.

Atrioventricular septal defects often result from impaired endocardial cushion development. Endothelial-to-mesenchymal transition (EndoMT) is a critical event in endocardial cushion development that initiates in the atrioventricular canal (AVC). In ex vivo EndoMT studies, mouse AVCs are flat-mounted on a collagen gel. In the explant outgrowths, the ratio of elongated spindle-like mesenchymal cells over cobblestone-shaped cells, generally considered as endothelial cells, reflects EndoMT. Using this method, several key signalling pathways have been attributed important functions during EndoMT. Using genetic lineage tracing and cell-type-specific markers, we show that monolayers of cobblestone-shaped cells are predominantly of epicardial rather than endothelial origin. Furthermore, this epicardium is competent to undergo mesenchymal transition. Contamination by epicardium is common and inherent as this tissue progressively attaches to AVC myocardium. Inhibition of TGFß signalling, previously shown to blunt EndoMT, caused an enrichment in epicardial monolayers. The presence of epicardium thus confounds interpretations of EndoMT signalling pathways in this assay. We advocate to systematically use lineage tracers and cell-type-specific markers on stage-matched AVC explants. Furthermore, a careful reconsideration of earlier studies on EndoMT using this explant assay may identify unanticipated epicardial effects and/or the presence of epicardial-to-mesenchymal transition (EpiMT), which would alter the interpretation of results on endothelial-to-mesenchymal transition.

Fronto-nasal dysplasia and atrio-ventricular canal in a fetus with trisomy 18 identified by absent nasal bones during first trimester screening scan.

Failed ultrasonographic visualization of nasal bones is associated with an increased risk of fetal malformations. Maternal ethnicity and chromosomal abnormalities influence the incidence and visualization rate of nasal bones. A case of absent nasal bones with fronto-nasal dysplasia and septated cystic hygroma identified at 13(+5) weeks' gestation in a trisomy 18 fetus is reported. The crown-rump length was 82 mm and the absent nasal bones were associated with micrognathia and a flattened face. The risks for trisomy 21 and 18 were subsequently calculated. The couple refused chorionic villus sampling. At 19 weeks' gestation a follow-up scan revealed, apart from the resolution of septated cystic hygroma, hypertelorism, a large interventricular septum defect with an atrio-ventricular canal and an abnormal A wave Doppler pulsation at the level of the ductus venosus. Bilateral choroid plexus cysts were additional ultrasound findings. At that time, an uneventful cordocentesis was performed showing a 47,XY(+18) karyotype. Termination of pregnancy was achieved and pathologic examination confirmed the ultrasonographically detected fetal malformations. When screening the fetal face for the presence or absence of nasal bones during the first trimester pregnancy scan the following points must be taken into consideration: (i) the ethnicity of the mother; (ii) if the nasal bones are absent, measurement of nuchal translucency and risk calculations for trisomy 21 and trisomy 18 should be performed; (iii) if the calculated risks are high, karyotyping should be recommended; and (iv) determine whether the absent nasal bones are an isolated or an associated finding and, in the latter case, discriminate between minor or major fetal malformations.

The crista supraventricularis in the human heart and its role in the morphogenesis of the septomarginal trabecula.

The crista supraventricularis and septomarginal trabecula are common elements of the right ventricle, and determine many hemodynamic phenomena. The morphological analysis of both structures in regard to their mutual relations was the aim of this study. The study was carried out on the material of preserved human hearts--fetuses, children and adults. The size and development of the crista supraventricularis was carefully evaluated. The division of its lower part, and hence the possibilities of development of the septomarginal trabecula, was divided into five types (A, B, C, D and E). The most common was type B, containing two muscular trabeculae. The width of the crista varied 1/5-3/5 of the width of the interventricular septum. On the basis of this study, a conclusion of morphological unity of the septomarginal trabecula and crista supraventricularis was drawn.

A GATA-6 gene heart-region-specific enhancer provides a novel means to mark and probe a discrete component of the mouse cardiac conduction system.

The transcriptional programs that specify the distinct components of the cardiac conduction system are poorly understood, in part due to a paucity of definitive molecular markers. In the present study we show that a cGATA-6 gene enhancer can be used to selectively express transgenes in the atrioventricular (AV) conduction system as it becomes manifest in the developing multichambered mouse heart. Furthermore, our analysis of staged cGATA-6/lacZ embryos revealed that the activity of this heart-region-specific enhancer can be traced back essentially to the outset of the cardiogenic program. We provide evidence that this enhancer reads medial/lateral and anterior/posterior positional information before the heart tube forms and we show that the activity of this enhancer becomes restricted at the heart looping stage to AV myocardial cells that induce endocardial cushion formation. We infer that a deeply-rooted heart-region-specific transcriptional program serves to coordinate AV valve placement and AV conduction system formation. Lastly, we show that cGATA-6/Cre mice can be used to delete floxed genes in the respective subsets of specialized heart cells.

Cardiac morphology at late fetal stages in the mouse with trisomy 16: consequences for different formation of the atrioventricular junction when compared to humans with trisomy 21.

OBJECTIVE: The mouse with trisomy 16 (Ts16) is held to be a genetic model for humans with Down's syndrome (Ts21). Both trisomies are associated with atrioventricular septal defects, but the precise morphology in the mouse remains unclear. We have therefore characterised cardiac morphology in the mouse with Ts16. METHODS: Ts16 fetuses, from a Rb(11.16)2H/Rb(16.17)7Bnr x C57BL/6J cross, were collected on gestational days 17 or 18 (full term = 19 days) and studied using scanning electron microscopy and serial sections. RESULTS: The hearts showed a spectrum of deficient atrioventricular septation which we categorised into two types. In one, a common atrioventricular junction was separated into right and left orifices by a tongue of tissue joining two valvar leaflets that bridged the ventricular septum to varying extent. In the other, a common atrioventricular junction was connected exclusively to the left ventricle. All hearts had ostium primum atrial and ventricular septal defects, together with abnormal ventriculo-arterial connections. No heart had the typical morphology seen in the human with Down's syndrome, namely a balanced common atrioventricular junction, guarded by a common valve, with the aorta connected exclusively to the left ventricle. CONCLUSIONS: The cardiac defects seen in Ts16 mice show marked differences from the typical anatomy in human Ts21, suggesting more complex mechanisms of cardiac dysmorphogenesis in Ts16. The mouse model will prove valuable in elucidating the mechanism of normal expansion of the atrioventricular junctions, and help in charting the precise steps involved in atrial and ventricular septation.

The mouse with trisomy 16 as a model of human hearts with common atrioventricular junction.

OBJECTIVE: To establish if the mouse with trisomy 16 is a suitable animal model with which to elucidate the development of a common atrioventricular junction. METHODS: The junctional morphologies in the normal human heart and those with a common atrioventricular junction are compared and contrasted. These are then related to observations made in normal mice and those with trisomy 16. So as better to understand development, a full description is given first of the normal atrioventricular junctions. Developmental implications are discussed because failure of fusion of the endocardial cushions cannot account for all the anomalies found in RXR alpha knockout, and in iv/iv mice. RESULTS: Mice with trisomy 16 showed evidence of deficiencies of atrioventricular septation and possessed a common atrioventricular junction, but the valvar orifices were not balanced between the ventricles as is the case in humans. Whilst some mice showed affinities with human tricuspid atresia, other cardiac malformations in the mice had no counterparts in human cardiac pathology. In humans both "partial" and "complete" forms of "atrioventricular canal malformations" share a basically common muscular junctional morphology, the differences being due exclusively to the way the bridging leaflets are fused to each other and/or the septum. CONCLUSIONS: It is simplistic to use the mouse with trisomy 16 as a model for cardiac abnormalities seen in humans. A spectrum more comparable to humans is found in RXR knockout mice. Study of the iv/iv mouse may help elucidate the genetic steps involved in normal and abnormal atrioventricular septation.