Mechanisms of heart valve development and disease

The valves of the heart are crucial for ensuring that blood flows in one direction from the heart, through the lungs and back to the rest of the body. Heart valve development is regulated by complex interactions between different cardiac cell types and is subject to blood flow-driven forces. Recent work has begun to elucidate the important roles of developmental pathways, valve cell heterogeneity and hemodynamics in determining the structure and function of developing valves. Furthermore, this work has revealed that many key genetic pathways involved in cardiac valve development are also implicated in diseased valves. Here, we review recent discoveries that have furthered our understanding of the molecular, cellular and mechanosensitive mechanisms of valve development, and highlight new insights into congenital and acquired valve disease.

New imaging techniques project the cellular and molecular alterations underlying bicuspid aortic valve development

Bicuspid aortic valve (BAV) disease is the most common congenital cardiac malformation associated with an increased lifetime risk and a high rate of surgically-relevant valve deterioration and aortic dilatation. Genomic data revealed that different genes are associated with BAV. A dominant genetic factor for the recent past was the basis to the recommendation for a more extensive aortic intervention. However very recent evidence that hemodynamic stressors and alterations of wall shear stress play an important role independent from the genetic trait led to more conservative treatment recommendations. Therefore, there is a current need to improve the ability to risk stratify BAV patients in order to obtain an early detection of valvulopathy and aortopathy while also to predict valve dysfunction and/or aortic disease development. Imaging studies based on new cutting-edge technologies, such us 4-dimensional (4D) flow magnetic resonance imaging (MRI), two-dimensional (2D) or three-dimensional (3D) speckle-tracking imaging (STI) and computation fluid dynamics, combined with studies demonstrating new gene mutations, specific signal pathways alterations, hemodynamic influences, circulating biomarkers modifications, endothelial progenitor cell impairment and immune/inflammatory response, all detected BAV valvulopathy progression and aortic wall abnormality. Overall, the main purpose of this review article is to merge the evidences of imaging and basic science studies in a coherent hypothesis that underlies and thus projects the development of both BAV during embryogenesis and BAV-associated aortopathy and its complications in the adult life, with the final goal to identifying aneurysm formation/rupture susceptibility to improve diagnosis and management of patients with BAV-related aortopathy.

Bicuspid Aortic Valve: A Review of its Genetics and Clinical Significance

The aim of this review was to describe recent advancements in the understanding of bicuspid aortic valve (BAV). BAV is the most common congenital cardiac anomaly, and affects between 0.46% and 1.37% of the population. There is a male predominance of approximately 3:1.While isolated BAV is found in certain patients, it is often associated with other congenital cardiac lesions, including dilatation of the thoracic aorta, coarctation of the aorta and abnormalities of the coronary anatomy. In most cases, it remains undetected until the patient contracts infective endocarditis, or calcification occurs. Alternatively, the BAV may remain functional for the entirety of the subjects' life, or it may develop progressive calcification, stenosis and regurgitation, with or without infection. Additionally, BAV is associated with aortic aneurysm formation and aortic dissection. Because BAV is a disease of both the valve and the aorta, surgical decision-making is complicated and remains an important challenge to the surgeon. Although recent reports have improved the current knowledge of the disease, many questions remain unresolved. The present review summarizes the current knowledge regarding the genetic basis of BAV and highlights some of the recent findings that have shed a light on the complications of this disease.

Quadricuspid aortic valves in Syrian hamsters and their formation according to current knowledge on valvulogenesis

Occurrence of quadricuspid aortic valves has been reported in humans, in nine dogs and in a greater white-toothed shrew. Moreover, two cases of developing aortic valves with four anticipated leaflets have been described in Syrian hamster embryos. Currently, however, no case of quadricuspid aortic valve in adult hamsters has been recorded. The aim here is to present four adults of this rodent species, two of them with unequivocally quadricuspid aortic valves and the other two with quadricuspid-like aortic valves. The four anomalous aortic valves were detected among 4,190 Syrian hamsters examined in our laboratory, representing an incidence of 0.09%. None of the affected hamsters showed apparent signs of disease. The present findings are considered on the light of current empirical knowledge about the morphogenesis of quadricuspid and bicuspid aortic and pulmonary valves. Quadricuspid aortic valves result from the partition of one of the normal mesenchymal cushions which normally give rise to normal (tricuspid) valves, while quadricuspid-like valves might be the product of a combined mechanism of fusion and partition of the cushions at the onset of the valvulogenesis. The presence of aortic valves with four leaflets in ancient mammalian lineages such as insectivors and rodents suggest that quadricuspid aortic valves, although showing almost certainly a low incidence, may be widespread among the different groups of mammals, including domestic animals.

Maternal aspects of fetal cardiac intervention

OBJECTIVES

Fetal cardiac interventions have the potential to alter natural disease progression and reduce morbidity and mortality in children. Although there are already encouraging data on fetal outcome, information on maternal morbidity and mortality after intervention is scarce. The aim of the present study was to assess maternal aspects, pregnancy-associated risks and adverse events in 53 intrauterine cardiac interventions.

METHODS

Between October 2000 and December 2012, 53 fetal cardiac interventions were performed in 47 patients (43 aortic valve dilations in 39 patients, seven pulmonary valve dilations in six patients and three balloon atrioseptostomies in two patients). Median gestational age was 26 + 4 (range, 20 + 3 to 33 + 1) weeks. Interventions were performed by an ultrasound-guided percutaneous approach under general anesthesia. All medical records and patient charts were analyzed retrospectively.

RESULTS

All women were considered to be healthy in the preoperative assessment; 39 (83%) patients continued pregnancy until term and eight of 47 patients had an intrauterine fetal death (IUFD) and were induced. Postoperative nausea was reported in 29.8% of patients and abdominal pain in 36.2% of patients on the day of surgery. Preterm contractions were observed in two patients; no preterm prelabor rupture of membranes occurred. One severe postpartum hemorrhage was observed in a patient with IUFD and subsequent induction; however, this was unrelated to the balloon valvuloplasty. No intensive care unit admission and no major anesthesia-associated complications (aspiration, anaphylactic reaction, cardiovascular collapse, damage to teeth, laryngeal damage, awareness or hypoxic brain damage) were observed. Maternal mortality was zero. A significant learning curve was observed in terms of duration of intervention.

CONCLUSION

In our experience, percutaneous needle-guided fetal cardiac intervention seems to be a safe procedure for the mother. In 53 procedures no major maternal complication directly related to the intervention was observed.

RNA-seq analysis to identify novel roles of scleraxis during embryonic mouse heart valve remodeling

Heart valve disease affects up to 30% of the population and has been shown to have origins during embryonic development. Valvulogenesis begins with formation of endocardial cushions in the atrioventricular canal and outflow tract regions. Subsequently, endocardial cushions remodel, elongate and progressively form mature valve structures composed of a highly organized connective tissue that provides the necessary biomechanical function throughout life. While endocardial cushion formation has been well studied, the processes required for valve remodeling are less well understood. The transcription factor Scleraxis (Scx) is detected in mouse valves from E15.5 during initial stages of remodeling, and expression remains high until birth when formation of the highly organized mature structure is complete. Heart valves from Scx-/- mice are abnormally thick and develop fibrotic phenotypes similar to human disease by juvenile stages. These phenotypes begin around E15.5 and are associated with defects in connective tissue organization and valve interstitial cell differentiation. In order to understand the etiology of this phenotype, we analyzed the transcriptome of remodeling valves isolated from E15.5 Scx-/- embryos using RNA-seq. From this, we have identified a profile of protein and non-protein mRNAs that are dependent on Scx function and using bioinformatics we can predict the molecular functions and biological processes affected by these genes. These include processes and functions associated with gene regulation (methyltransferase activity, DNA binding, Notch signaling), vitamin A metabolism (retinoic acid biosynthesis) and cellular development (cell morphology, cell assembly and organization). In addition, several mRNAs are affected by alternative splicing events in the absence of Scx, suggesting additional roles in post-transcriptional modification. In summary, our findings have identified transcriptome profiles from abnormal heart valves isolated from E15.5 Scx-/- embryos that could be used in the future to understand mechanisms of heart valve disease in the human population.

Assessment of fetal cardiomyopathy in early-stage twin-twin transfusion syndrome: comparison between commonly reported cardiovascular assessment scores

OBJECTIVES

To assess the relationship between commonly reported fetal cardiomyopathy scoring systems in early-stage twin-twin transfusion syndrome (TTTS).

METHODS

We reviewed retrospectively 100 cases of Quintero Stages I and II TTTS referred to our center for evaluation from 2008 to 2010. The cases were divided into groups of 25, representing each of four grades of TTTS cardiomyopathy as assessed by Cincinnati stage: no cardiomyopathy, Stage IIIa, Stage IIIb and Stage IIIc. Spearman correlation (rs ) was calculated between the Children's Hospital of Philadelphia (CHOP) score, cardiovascular profile score (CVPS), Cincinnati stage and myocardial performance index (MPI).

RESULTS

There was a weak correlation between the Cincinnati stage and the CHOP score (rs  = 0.36) and CVPS (rs  = -0.39), while correlation was strong between the CHOP score and CVPS (rs  = -0.72). MPI elevation was concordant with Cincinnati stage more frequently (82% of cases) than were ventricular hypertrophy (43%) or atrioventricular valve regurgitation (28%). 51% of fetuses with minimally elevated CHOP score (0-1) and 48% of fetuses with minimally depressed CVPS (9-10) had significant elevation (Z-score ≥ +3) in right ventricular or left ventricular MPI.

CONCLUSIONS

MPI has a strong influence on grading the severity of fetal cardiomyopathy using the Cincinnati stage among fetuses with mild TTTS. Furthermore, significant elevation of the MPI is common among fetuses with mild disease as assessed by the CHOP score and CVPS. These differences should be understood when assessing and grading cardiomyopathy in TTTS, particularly in early (Quintero Stages I and II) disease.

Atrioventricular valve development: new perspectives on an old theme

Atrioventricular valve development commences with an EMT event whereby endocardial cells transform into mesenchyme. The molecular events that induce this phenotypic change are well understood and include many growth factors, signaling components, and transcription factors. Besides their clear importance in valve development, the role of these transformed mesenchyme and the function they serve in the developing prevalve leaflets is less understood. Indeed, we know that these cells migrate, but how and why do they migrate? We also know that they undergo a transition to a mature, committed cell, largely defined as an interstitial fibroblast due to their ability to secrete various matrix components including collagen type I. However, we have yet to uncover mechanisms by which the matrix is synthesized, how it is secreted, and how it is organized. As valve disease is largely characterized by altered cell number, cell activation, and matrix disorganization, answering questions of how the valves are built will likely provide us with information of real clinical relevance. Although expression profiling and descriptive or correlative analyses are insightful, to advance the field, we must now move past the simplicity of these assays and ask fundamental, mechanistic based questions aimed at understanding how valves are "built". Herein we review current understandings of atrioventricular valve development and present what is known and what isn't known. In most cases, basic, biological questions and hypotheses that were presented decades ago on valve development still are yet to be answered but likely hold keys to uncovering new discoveries with relevance to both embryonic development and the developmental basis of adult heart valve diseases. Thus, the goal of this review is to remind us of these questions and provide new perspectives on an old theme of valve development.

Implications of bidirectional flow in the great arteries at the 11-14-week scan

OBJECTIVE

To determine the implications of a bidirectional Doppler flow pattern detected in one or both of the fetal great arteries in the first trimester.

METHODS

Database records were reviewed for cases of bidirectional flow found on fetal echocardiography performed at less than 15 weeks of gestation between January 1999 and October 2006 inclusive. Bidirectional flow consisted of antegrade flow in systole and retrograde flow in diastole. Data including indication for fetal echocardiography, gestational age at diagnosis, the anatomical features, karyotype, nuchal translucency measurement and outcome were collected.

RESULTS

From almost 2500 scans, bidirectional flow was detected in a total of 15 fetuses. The abnormal flow pattern was confined to the aorta in four, to the pulmonary artery in four, and was present in both great arteries in a further seven fetuses. Eight of the 15 fetuses had regurgitation at one or both atrioventricular valves. Additional cardiac abnormalities were detected by ultrasound in eight cases. Of the 13 cases for which the karyotype was known, 12 were abnormal and five of these had trisomy 18. None of the fetuses survived. There were two intrauterine deaths and 13 terminations of pregnancy.

CONCLUSION

Bidirectional flow in one or both great arteries is an unusual finding at the first-trimester scan and must be distinguished from the retrograde flow occurring only in systole in duct-dependent heart defects. It carries a poor prognosis, which is a consequence of both the high chance of underlying chromosomal abnormality and the hemodynamic compromise associated with severe arterial valve regurgitation.

Cardiogenic effects of trichloroethylene and trichloroacetic acid following exposure during heart specification of avian development

Trichloroethylene (TCE) and its metabolite trichloroacetic acid (TCA) are common drinking water contaminants in the United States. Both chemicals have been implicated in causing congenital heart defects (CHD) in human epidemiological and animal model studies. However, the latter studies have primarily focused on assessment of cardiac morphology at late embryonic stages. Here, we tested whether treating avian embryos with TCE or TCA during an exposure window encompassing cardiac specification (Hamburger-Hamilton [HH] 3+) until the onset of chambering (HH 17) informs the etiology of CHD at later stages of development. Embryos were exposed to TCE or TCA via direct injection into the yolk, over a range of doses that included each compound's maximum contaminant level as established by the U.S. Environmental Protection Agency. A modified TUNEL (Terminal deoxynucleotide transferase mediated dUTP-biotin Nick-End Labeling) assay indicated that neither compound induced apoptotic cell death in ventricular myocytes or endocardiocytes at HH 18. However, mid-range dosages of TCE increased myocyte and endocardiocyte proliferation by this time, as determined by monitoring BrdU incorporation; in contrast, an intermediate dose of TCA inhibited proliferation in endocardiocytes. These cellular changes had no apparent functional consequences because all measured hemodynamic parameters were normal for TCE- and TCA-exposed embryos at HH 18, HH 21, and HH 23. In summary, TCE or TCA exposure during the cardiac specification window has only minimal effects on the developing avian heart. These results sharply contrast with our previously reported observations following administration of equivalent doses during a window of valvuloseptal morphogenesis. Taken together, these findings indicate that, as for other teratogens, sensitivity is dictated by the embryo's stage of development.

Left ventricular myocardial performance in the fetus with severe tricuspid valve disease and tricuspid insufficiency

We sought to determine whether left ventricular (LV) function is abnormal in a fetus with severe tricuspid valve disease (TVD), and whether it may contribute to the outcome. We measured the LV Tei index (including isovolumic relaxation [IRT], contraction, and ejection times) in 31 fetuses with TVD, compared the data with measurements from 32 normal fetuses, and correlated these measurements with pulmonary morphology and outcome. LV Tei index was significantly greater, ejection times shorter, and IRT longer in TVD compared with controls. The Tei index was highest in fetuses with either pulmonary insufficiency or atresia compared with those with forward flow. Finally, LV Tei index was significantly greater in fetuses with fetal (n = 5) or neonatal (n = 9) demise than in neonatal survivors (n = 7). Global LV performance as assessed by the Tei index is abnormal in TVD and likely contribute to the high mortality associated with TVD.

Acute Cardiovascular Effects of Fetal Surgery in the Human

Background— Prenatal surgery for congenital anomalies can prevent fetal demise or alter the course of organ development, resulting in a more favorable condition at birth. The indications for fetal surgery continue to expand, yet little is known about the acute sequelae of fetal surgery on the human cardiovascular system.

Methods and Results— Echocardiography was used to evaluate the heart before, during, and early after fetal surgery for congenital anomalies, including repair of myelomeningocele (MMC, n=51), resection of intrathoracic masses (ITM, n=15), tracheal occlusion for congenital diaphragmatic hernia (CDH, n=13), and resection of sacrococcygeal teratoma (SCT, n=4). Fetuses with MMC all had normal cardiovascular systems entering into fetal surgery, whereas those with ITM, CDH, and SCT all exhibited secondary cardiovascular sequelae of the anomaly present. At fetal surgery, heart rate increased acutely, and combined cardiac output diminished at the time of fetal incision for all groups including those with MMC, which suggests diminished stroke volume. Ventricular dysfunction and valvular dysfunction were identified in all groups, as was acute constriction of the ductus arteriosus. Fetuses with ITM and SCT had the most significant changes at surgery.

Conclusions— Acute cardiovascular changes take place during fetal surgery that are likely a consequence of the physiology of the anomaly and the general effects of surgical stress, tocolytic agents, and anesthesia. Echocardiographic monitoring during fetal surgery is an important adjunct in the management of these patients.

[3D-sonography as "2D-M-mode sonography in fetal echocardiography]

AIM

Establishing a technique employing 3D-ultrasound for the echocardiographic examination of foetuses.

METHOD

M-mode sonography has reached considerable importance in the detection of arrhythmia as well as pathological changes of the heart valves and contraction disorders of the foetal heart. The variability of the position of the foetus leads to problems typically arising with this method. The ventro-anterior position along the long axis of the foetal heart, being the optimal position for prenatal echocardiography, allows no examination in M-mode because the plane of the valve lies vertically in relation to the direction of the ultrasound. A special appliance of 3D-sonography solves the problem by providing a 2-dimensional examination technique of the heart in M-mode independent of the position of the foetal heart. The method allows simultaneous measurement of the movement of different parts of the myocardium in several different planes. By fixing the position of the applicator (in B-mode) and choosing the 3D-sequence in the "free-hand-mode" one gets a 2-dimensional time-tissue-block which contains all movements in the chosen axis. These "time planes" can be deliberately selected for the examination of different planes in the fixed B-mode image which shows their variations along the time axis.

RESULTS

Our study describes the application and evaluation of this examination during pregnancy between the 28th and 40th week. Reliable images of the AV-valves or the aortic valve could be produced in 73% of examinations.

Abnormal cardiac conduction and morphogenesis in connexin40 and connexin43 double-deficient mice

Connexin40-deficient (Cx40(-/-)/Cx43(+/+)) and connexin43-heterozygous knockout mice (Cx40(+/+)/Cx43(+/-)) are viable but show cardiac conduction abnormalities. The ECGs of adult double heterozygous animals (Cx40(+/-)/Cx43(+/-)) suggest additive effects of Cx40 and Cx43 haploinsufficiency on ventricular, but not on atrial, conduction. We also observed additive effects of both connexins on cardiac morphogenesis. Approximately half of the Cx40(-/-)/Cx43(+/+) embryos died during the septation period, and an additional 16% died after birth. The majority of the latter mice had cardiac hypertrophy in conjunction with common atrioventricular junction or a ventricular septal defect. All Cx40(-/-)/Cx43(+/-) progeny exhibited cardiac malformations and died neonatally. The most frequent defect was common atrioventricular junction with abnormal atrioventricular connection, which was more severe than that seen in Cx40(-/-)/Cx43(+/+) mice. Furthermore, muscular ventricular septal defects, premature closure of the ductus arteriosus, and subcutaneous edema were noticed in these embryos. Cx40(+/-)/Cx43(-/-) embryos showed the same phenotype (ie, obstructed right ventricular outflow tract) as reported for Cx40(+/+)/Cx43(-/-) mice. These findings demonstrate that Cx43 haploinsufficiency aggravates the abnormalities observed in the Cx40(-/-) phenotype, whereas Cx40 haploinsufficiency does not worsen the Cx43(-/-) phenotype. We conclude that the gap-junctional proteins Cx40 and Cx43 contribute to morphogenesis of the heart in an isotype-specific manner.

Anatomy and formation of congenital bicuspid and quadricuspid pulmonary valves in Syrian hamsters

BACKGROUND

Congenital bicuspid and quadricuspid pulmonary valves have received little attention because of their limited clinical relevance. However, knowledge of the mechanisms by which these anomalous valves develop is essential to obtain a more accurate survey of the etiological factors implicated in the malformations of the cardiac outflow tract in mammals. The present study was designed to assess the anatomical features of bicuspid and quadricuspid pulmonary valves in Syrian hamsters as well as to elucidate the mechanisms involved in the formation of these defective valves.

METHODS

The sample examined consisted of 206 adults and 28 embryos belonging to a laboratory-inbred family of Syrian hamsters with a high incidence of congenital anomalies of the pulmonary and aortic valves. The study was carried out using histological techniques for light microscopy, semithin sections, and scanning electron microscopy.

RESULTS

The pulmonary valve was tricuspid in 140 of the 206 adult hamsters, and in 124 of these tricuspid valves the dorsal commissure was more or less extensively fused. Another 45 hamsters possessed a bicuspid pulmonary valve with the sinuses oriented ventrodorsally. In 43 of these bicuspid valves, a raphe was located in the dorsal pulmonary sinus. The pulmonary valve was quadricuspid in a further nine specimens. The remaining 12 hamsters had a tricuspid pulmonary valve with a raphe-like ridge located in the right pulmonary sinus. In seven of these valves, the dorsal commissure showed a more or less extensive fusion. The embryos examined, aged between 11 days, 3 hours and 12 days, 6 hours postcoitum, were at the beginning of the valvulogenesis. In five of the 28 embryos, the pulmonary valve consisted of three mesenchymal valve cushions, right, left, and dorsal. In a further 17 embryos, the right and left valve cushions were more or less fused toward the lumen of the pulmonary artery. In the remaining six embryos, the left and dorsal valve cushions were normal, whereas the right cushion was divided into two lobes.

CONCLUSIONS

The present findings suggest that in the Syrian hamster: (1) bicuspid pulmonary valves result from the extensive fusion of the right and left pulmonary valve cushions at the beginning of the valvulogenesis, (2) the partial fusion of the right and left pulmonary valve cushions leads to the formation of tricuspid pulmonary valves with a more or less extensive fusion of the dorsal commissure, (3) quadricuspid pulmonary valves result from the partition of one of the three valve cushions at a very early stage of the valvulogenesis, and (4) the partial division of the right pulmonary valve cushion may lead to the development of tricupsid pulmonary valves with a raphe-like ridge located in the right pulmonary sinus. In addition, the present findings, together with previous observations in Syrian hamsters, indicate that in this species the mechanisms by which bicuspid and quadricuspid pulmonary valves develop are similar to those by which bicuspid and quadricuspid aortic valves form, respectively. However, the primary factor or factors that induce the malformations of the pulmonary valve operate independently from those inducing the malformations of the aortic valve.

Valvar vegetations in the neonate due to fetal endocarditis

Mass lesions were observed on the tricuspid valve by cross-sectional echocardiography in 5 neonates, 4 of whom presented with fetal distress. Four went on to develop severe respiratory distress, 3 from meconium aspiration and 1 from hyaline membrane disease, with 2 deaths despite intensive treatment. Autopsy confirmed meconium aspiration pneumonia. Sterile, unorganised vegetations were seen on the tricuspid valve (and in 1 infant the mitral valve too). Perinatal factors such as hypoxia, haemodynamic and coagulation disturbances may have contributed to the formation of the vegetations and may reflect the severity and refractoriness of persistent pulmonary hypertension.

Fetal cardiovascular morphology of truncus arteriosus with or without truncal valve insufficiency in the rat

BACKGROUND

Recent advances in fetal echocardiography have necessitated further study on fetal in situ cardiovascular morphology of truncus arteriosus and the effects of truncal valve insufficiency.

METHODS AND RESULTS

We studied 55 fetal rats with truncus arteriosus among 300 fetuses from 40 virgin females treated with 200 mg fertilysin on the 10th day of pregnancy. After rapid whole-body freezing on the 21st day, the fetuses were studied by means of serial cross-sectional photographs of the frozen thorax. Thirty-five fetuses with a normal heart treated with fertilysin served as controls. Truncus arteriosus was characterized by a large ventricular septal defect, a solitary artery (truncus arteriosus) overriding the ventricular septum, the right and left pulmonary arteries originating from the truncus arteriosus with or without a common trunk (main pulmonary artery), and absent ductus arteriosus. Fetuses with truncal valve insufficiency had thick truncal valves, a large truncus arteriosus, and large ventricles. The subgroup of 12 fetuses with a large truncus (truncal diameter greater than 160% of the ascending aorta diameter in the controls) showed significantly greater values for right ventricular volume (200% of control) and mass (120% of control), left ventricular volume (170% of control) and mass (110% of control), right (120% of control) and left (110% of control) atrial volume, and pericardial fluid (140% of control) than the controls. These changes were less prominent and ventricular volumes were not increased in the remaining subgroup with a truncal diameter of 160% or less of aorta diameter in the controls.

CONCLUSIONS

In fetal truncus arteriosus, truncal valve insufficiency was associated with increased ventricular volume load and incomplete cardiac compensation in rats.

Atrioventricular canal malformation interpreted as secondary to reduced compression upon the developing heart

This study was undertaken to evaluate the nature and pathogenesis of malformations of the atrioventricular canal in relation to normal cardiogenesis. Serial histologic sections of normal human embryos and fetuses were made, from which three-dimensional images were reconstructed to show the relationship between the developing heart and its surrounding structures, and the course of development of the atrial septum and atrioventricular valves. Based on these reconstructions and on examination of the hearts of 59 patients with atrioventricular canal malformations, it is suggested that the spectrum of atrioventricular malformations may arise as a result of reduced compression of the developing atria by surrounding structures during embryonic Stages 13 through 18. Comparison of hearts with atrioventricular canal defects with normal embryos indicated that the malformations may be classified as primitive canals, complete canals, or partial canals, corresponding to failure of completion of normal development in Stages 14 through 18. In primitive canal the atrial septum was absent or had only a portion of septum primum. In complete canal both atrial septums were present, but the atrioventricular valve material was not subdivided and the four chambers were in communication. In partial canal, the atrioventricular valve was divided, but atrial and ventricular septal defects and valve clefts were present in varying degrees of severity. It is proposed that the spectrum of cardiac abnormalities which constitutes atrioventricular canal malformations may be understood as arising from varying degrees of lack of normal compression of the developing heart by surrounding structures. (Am J Pathol 95.579-598, 1979)