Congenital malformations of the semilunar valves

Embryonic development of bicuspid aortic valves

Bicuspid aortic valve (BAV) is the most common congenital cardiac malformation, frequently associated with aortopathies and valvulopathies. The congenital origin of BAV is suspected to impact the development of the disease in the adult life. During the last decade, a number of studies dealing with the embryonic development of congenital heart disease have significantly improved our knowledge on BAV etiology. They describe the developmental defects, at the molecular, cellular and morphological levels, leading to congenital cardiac malformations, including BAV, in animal models. These models consist of a spontaneous hamster and several mouse models with different genetic manipulations in genes belonging to a variety of pathways. In this review paper, we aim to gather information on the developmental defects leading to BAV formation in these animal models, in order to tentatively explain the morphogenetic origin of the spectrum of valve morphologies that characterizes human BAV. BAV may be the only defect resulting from gene manipulation in mice, but usually it appears as the less severe defect of a spectrum of malformations, most frequently affecting the cardiac outflow tract. The genes whose alterations cause BAV belong to different genetic pathways, but many of them are direct or indirectly associated with the NOTCH pathway. These molecular alterations affect three basic cellular mechanisms during heart development, i.e., endocardial-to-mesenchymal transformation, cardiac neural crest (CNC) cell behavior and valve cushion mesenchymal cell differentiation. The defective cellular functions affect three possible morphogenetic mechanisms, i.e., outflow tract endocardial cushion formation, outflow tract septation and valve cushion excavation. While endocardial cushion abnormalities usually lead to latero-lateral BAVs and septation defects to antero-posterior BAVs, alterations in cushion excavation may give rise to both BAV types. The severity of the original defect most probably determines the specific aortic valve phenotype, which includes commissural fusions and raphes. Based on current knowledge on the developmental mechanisms of the cardiac outflow tract, we propose a unified hypothesis of BAV formation, based on the inductive role of CNC cells in the three mechanisms of BAV development. Alterations of CNC cell behavior in three possible alternative key valvulogenic processes may lead to the whole spectrum of BAV.

Stem Cell Cytoskeletal Responses to Pulsatile Flow in Heart Valve Tissue Engineering Studies

Heart valve replacement options remain exceedingly limited for pediatric patients because they cannot accommodate somatic growth. To overcome this shortcoming, heart valve tissue engineering using human bone marrow stem cells (HBMSCs) has been considered a potential solution to the treatment of critical congenital valvular defects. The mechanical environments during in vitro culture are key regulators of progenitor cell fate. Here, we report on alterations in HBMSCs, specifically in their actin cytoskeleton and their nucleus under fluid-induced shear stresses of relevance to heart valves. HBMSCs were seeded in microfluidic channels and were exposed to the following conditions: pulsatile shear stress (PSS), steady shear stress (SS), and no flow controls (n = 4/group). Changes to the actin filament structure were monitored and subsequent gene expression was evaluated. A significant increase (p < 0.05) in the number of actin filaments, filament density and angle (between 30° and 84°), and conversely a significant decrease (p < 0.05) in the length of the filaments were observed when the HBMSCs were exposed to PSS for 48 h compared to SS and no flow conditions. No significant differences in nuclear shape were observed among the groups (p > 0.05). Of particular relevance to valvulogenesis, klf2a, a critical gene in valve development, was significantly expressed only by the PSS group (p < 0.05). We conclude that HBMSCs respond to PSS by alterations to their actin filament structure that are distinct from SS and no flow conditions. These changes coupled with the subsequent gene expression findings suggest that at the cellular level, the immediate effect of PSS is to initiate a unique set of quantifiable cytoskeletal events (increased actin filament number, density and angle, but decrease in filament length) in stem cells, which could be useful in the fine-tuning of in vitro protocols in heart valve tissue engineering.

Factors other than genotype account largely for the phenotypic variation of the pulmonary valve in Syrian hamsters

Understanding of the aetiology of congenitally anomalous pulmonary valves remains incomplete. The aim of our study, therefore, was to elucidate the degree to which the phenotypic variation known to exist for the pulmonary valve relies on genotypic variation. Initially, we tested the hypothesis that genetically alike individuals would display similar valvar phenotypes if the phenotypic arrangement depended entirely, or almost entirely, on the genotype. Thus, we examined pulmonary valves from 982 Syrian hamsters belonging to two families subject to systematic inbreeding by crossing siblings. Their coefficient of inbreeding was 0.999 or higher, so they could be considered genetically alike. External environmental factors were standardized as much as possible. A further 97 Syrian hamsters from an outbred colony were used for comparative purposes. In both the inbred and outbred hamsters, we found valves with a purely trifoliate, or tricuspid, design, trifoliate valves with a more or less extensive fusion of the right and left leaflets, bifoliate, or bicuspid, valves with fused right and left leaflets, with or without a raphe located in the conjoined arterial sinus, and quadrifoliate, or quadricuspid, valves. The incidence of the different valvar morphological variants was similar in the outbred and inbred colonies, except for the bifoliate pulmonary valves, which were significantly more frequent in the hamsters from one of the two inbred families. Results of crosses between genetically alike hamsters revealed no significant association between the pulmonary valvar phenotypes as seen in the parents and their offspring. The incidence of bifoliate pulmonary valves, nonetheless, was higher than statistically expected in the offspring of crosses where at least one of the parents possessed a pulmonary valve with two leaflets. Our observations are consistent with the notion that the basic design of the pulmonary valve, in terms of whether it possesses three or two leaflets, relies on genotypic determinants. They also denote that the bifoliate condition of the valve is the consequence of complex inheritance, with reduced penetrance and variable expressivity. Moreover, in showing that the incidence of the bifoliate pulmonary valve significantly differs in two different isogenetic backgrounds, our data suggest that genetic modifiers might be implicated in directing the manifestation of such specific pulmonary valvar malformations. Finally, our findings indicate that factors other than the genotype, operating during embryonic life and creating developmental noise, or random variation, play a crucial role in the overall phenotypic variation involving the pulmonary valve.

A bicuspid pulmonary valve associated with tetralogy of fallot

Congenitally bicuspid pulmonary valves are uncommon. When they occur, it is usually in association with other congenital cardiac lesions, most often a tetralogy of Fallot. We present a rare case of a patient with a congenitally bicuspid pulmonary valve who had pulmonary valve and RVOT reconstruction. The patient did well for 17 years, but needed further reconstruction when the pulmonary valve started getting stenosed and RV pressures went up significantly. The pulmonary valve showed fibroses, thickening, and focal calcification.

Congenitally bicuspid aortic valves: a surgical pathology study of 542 cases (1991 through 1996) and a literature review of 2,715 additional cases

OBJECTIVE

To describe a clinicopathologic study of a large group of congenitally bicuspid aortic valves surgically excised at a single institution.

MATERIAL AND METHODS

The medical charts and bicuspid valves from patients undergoing aortic valve replacement at Mayo Clinic Rochester between 1991 and 1996 were retrospectively reviewed.

RESULTS

The age of the 542 patients ranged from 1 to 86 years (mean, 61), and 372 (69%) were men. Among these, 409 (75%) had pure aortic stenosis (AS), 73 (13%) had pure aortic insufficiency (regurgitation) (AI), 53 (10%) had combined AS and AI, and 7 (1%) had normal function. The mean age was higher for those with AS than AI (65 versus 46 years; P < 0.001), whereas the male-to-female ratio was higher for AI than AS (17.3:1 versus 1.7:1; P < 0.001). The two cusps were not equal in size in 95%, and a raphe was present in 76% (67% typical, 9% atypical). Raphal position was described in 315 and was between the right and left cusps in 270 (86%). Raphal absence occurred more often in valves with equal-sized cusps than unequal (33% versus 14%; P = 0.005). Moderate to severe calcification affected valves with AS more frequently than AI (99% versus 41%; P < 0.001). In contrast, annular dilatation was associated with AI more than AS (48% versus 11%; P < 0.001). Acquired commissural fusion involved valves with combined AS and AI more often than the other functional states (31% versus 14%; P = 0.002). Sixteen patients (age range, 18 to 78 years; 13 men) had infective endocarditis (6 active, 10 healed), including 10 with AI (9 men), 3 with AS plus AI, 2 with AS, and 1 with normal function but embolization.

CONCLUSION

Functionally, the most common fate of congenitally bicuspid aortic valves was calcific stenosis with or without regurgitation (85%). Because approximately 4 million US citizens have bicuspid valves and because valve replacement is currently the only treatment of symptomatic AS, this disorder will continue to affect health-care costs.

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.

Fusion of valve cushions as a key factor in the formation of congenital bicuspid aortic valves in Syrian hamsters

BACKGROUND

Bicuspid aortic valve is the most frequent congenital cardiac malformation in humans. However, the morphogenesis of the defect is still unknown. Previous work showed that, in the Syrian hamster, congenital bicuspid aortic valves with the aortic sinuses arranged in ventrodorsal orientation are expressions of a trait the variation of which takes the form of a continuous phenotypic spectrum, ranging from a tricuspid aortic valve with no fusion of the ventral commissure to a bicuspid aortic valve devoid of any raphe. The present study was designed to elucidate the mechanism involved in the formation of bicuspid aortic valves in Syrian hamsters as a possible starting point for further investigation of this process in humans.

METHODS

The sample examined consisted of 80 embryos, aged between 10 days, 16 hours and 13 days, 1 hour postcoitum. Most (n = 59) of the embryos belonged to a laboratory-inbred family of Syrian hamsters with a high incidence of bicuspid aortic valves. The study was carried out using scanning electron microscopy and histological techniques for light microscopy.

RESULTS

Twenty-three embryos showed a still undivided conotruncus. In all of these cases there were six mesenchymal semilunar valve primordia protruding into the lumen of the conotruncus. In a further 29 embryos, the conotruncus had just divided into the aortic and pulmonary channels; the embryos were at the beginning of the valvulogenesis. In 13 of these 29 embryos there were three well-defined aortic valve cushions, right, left, and dorsal, whereas in the other 16, the right and left valve cushions were more or less fused toward the lumen of the aorta; when they were completely fused, only two aortic valve cushions, a ventral and a dorsal, could be identified. In the remaining 28 embryos, the aortic valve cushions showed a marked degree of excavation. In 23 of these cases, the valve exhibited a basically tricuspid architecture, whereas it was unequivocally bicuspid in the other five.

CONCLUSIONS

All variants of the aortic valve morphologic spectrum occurring in the Syrian hamster develop from three mesenchymal valve cushions, right, left, and dorsal, after normal septation of the conotruncus. The bicuspid condition of the aortic valve is not the consequence of improper development of the conotruncal ridges, conotruncal malseptation, valve cushion agenesis, or lesions acquired after a normal valvulogenesis. Fusion of the right and left valve cushions at the beginning of the valvulogenesis appears to be a key factor in the formation of bicuspid aortic valves. Each aortic valve acquires its specific morphology prior to the end of the valvulogenetic process.

Bicuspid aortic and pulmonary valves in the Syrian hamster

We studied the conditions of the aortic and pulmonary valves of 153 Syrian hamsters belonging to a single family subjected to high endogamous pressure. Semilunar valves of 143 specimens were examined using a stereomicroscope, and in 5 of these cases a histologic study was also performed. The remaining 10 specimens were studied by means of scanning electron microscopy. In 77 specimens both semilunar valves were normal. The aortic valve was bicuspid and the pulmonary valve was normal in 46 animals, while a normal aortic valve and a bicuspid pulmonary valve occurred in 24. In the remaining 6 specimens both semilunar valves were bicuspid. All bicuspid semilunar valves detected herein showed the same morphotype, namely with the leaflets oriented ventrodorsally. The incidence of these anomalous valves did not significantly differ between sexes. Results of a chi 2-test substantiate that conditions of both outflow tract semilunar valves are independent traits. Thus, concurrence of a bicuspid aortic valve and a bicuspid pulmonary valve in an individual is a random event. The present findings support the assumption that bicuspid aortic and pulmonary valves have different morphogenetic origin, although factors producing a bicuspid aortic valve and those leading to a bicuspid pulmonary valve are not mutually exclusive.

Aplasia of semilunar valve leaflets: two case reports and developmental aspects

Two fetuses of approximately 11 weeks development with aplasia of the pulmonary as well as the aortic valve leaflets are reported. Both cases showed additional cardiac malformations. Case 1, with all leaflets missing, also had double-outlet right ventricle, hypoplastic left ventricle, large ventricular septal defect, straddling tricuspid valve, and atretic mitral valve. Case 2, with only one hypoplastic aortic valve leaflet, showed hypoplasia of the mitral valve and the left ventricle, and a subaortic ventricular septal defect. The observations made and data in the literature suggest that aplasia of semilunar valve leaflets reflects an underdevelopment of the endocardial cushion swellings at the ventriculoarterial junction, rather than resulting primarily from a malseptation of the cardiac outflow tract.

Pathogenesis of persistent left superior vena cava with a coronary sinus connection

The basis for persistence of the left superior vena cava (LSVC), usually associated with cardiac malformations, is poorly understood. We examined 351 staged, serially sectioned human embryos in the Carnegie Embryological Collection and 1208 specimens with congenital cardiovascular malformations in the Pathology Collection of the Johns Hopkins Hospital. A standardized questionnaire was answered for each embryo and autopsy case and a computer program was employed to tabulate concurrent anatomic features. In the normal embryos a symmetric venous system appeared with the heart tube at Carnegie stage 9; the sinoatrial junction translocated to the right and the relationship of the coronary sinus to the LSVC was established by stage 12. The LSVC was patent through stage 20 and subsequently underwent luminal obliteration by compression between the left atrium and the hilum of the left lung. Among the 1208 hearts with a congenital abnormality, 104 (9%) had a persistent LSVC with a coronary sinus connection. Statistically, significantly more frequent associations were found between persistent LSVC and atrioventricular canal defects, cor triatriatum, and mitral atresia and a significantly less frequent association was observed between persistent LSVC and atrial septal defect or patent foramen ovale as a primary defect. The normally late embryonic obliteration of the LSVC suggests that its persistence would be secondary to reduce cardiac compression or to blood flow redistribution at an early stage, and the malformations associated with persistent LSVC support that view. Identification of a persistent left superior vena cava with coronary sinus connection should suggest an associated malformation, especially atrioventricular canal, cor triatriatum, or mitral atresia.

Development of the coronary arteries in the embryonic human heart

It is not known why the coronary arteries almost always originate only from the right and left aortic sinuses of Valsalva, since the structure and conditions appear to be the same for all six sinuses of the embryonic great arteries. We sought a possible mechanical explanation for the phenomenon by studying the development of the coronary vasculature in 351 staged, serially sectioned human embryos of Carnegie stages 9 through 23 from the Carnegie Embryological Collection. A plexus of blind epicardial capillaries appears on the heart in Carnegie stage 14 or 15 and acquires a coronary sinus connection in stage 15, 16, or 17. The connection of the proximal coronary arteries to the aorta does not appear until stage 18. We found no histologic features of the cardiac nerves or any other component of the tissues to account for the consistent origin of coronary arteries from the right and left aortic sinuses of Valsalva. However, serial section reconstructions showed that the two sinuses where coronary arteries develop acquire a positive transverse curvature and a negative longitudinal curvature, i.e., a catenoidal or saddle-shaped configuration, before the appearance of the coronary arteries. The four noncoronary sinuses also have a positive transverse curvature, but longitudinally, in contrast, they have a positive curvature or are straight. The results suggest that the coronary arteries originate from those sinuses of Valsalva where wall tension is increased by a catenoidal configuration.

Arrested rotation of the outflow tract may explain tetralogy of Fallot and transposition of the great arteries

In a previous study we investigated the rapidly changing spatial relationship of the junction of the outflow tract and great arteries in normal human embryos of Carnegie stages 15 through 19. The results suggested that the malformation complexes tetralogy of Fallot and transposition of the great arteries could be accounted for as arrests in the progression of this process of rotation. To further study this question we reviewed hearts obtained at autopsy: 28 were normal, 16 had tetralogy of Fallot, and 27 had transposition of the great arteries. The angle of the aortic to pulmonary valve axis relative to the inferior surface of the heart, as viewed from apex to base, was measured from postmortem radiographs. For normal hearts the angle was 72 degrees +/- 3 SE, 48 degrees +/- 5 for tetralogy of Fallot, and 333 degrees +/- 3 for transposition of the great arteries. Although direct comparison of hearts and embryos is difficult because of the different methods of determining angles, the valve positions in normal hearts were most similar to stage 19 embryos. Likewise, hearts with tetralogy of Fallot and transposition of the great arteries resembled stages 18 and 15, respectively. The results of the two studies are consistent with the hypothesis that tetralogy of Fallot and transposition of the great arteries arise as a result of arrests in the normal rotation of the region of the junction of the outflow tract and the great arteries.

Surgical pathology of combined aortic stenosis and insufficiency: a study of 213 cases

The gross surgical pathologic features of the aortic valve were reviewed in 213 patients who had had clinically combined aortic stenosis and insufficiency and aortic valve replacement at our institution during the years 1965, 1970, 1975, and 1980. The three most common causes were postinflammatory disease (69%) and calcification of congenitally bicuspid (19%) and unicommissural (6%) aortic valves. Other causes included infective endocarditis (2%) and congenitally quadricuspid or malformed tricuspid aortic valves (1% each); the cause was indeterminate in 1%. In the postinflammatory and bicuspid states, calcification tended to be more extensive in men than in women. The relative incidence of postinflammatory disease in our study did not change appreciably from 1965 to 1980, despite the steadily decreasing incidence of acute rheumatic fever reported in western countries.

Surgical pathology of pure aortic insufficiency: a study of 225 cases

The gross surgical pathologic features of the aortic valve were reviewed in 225 patients who had had clinically pure aortic insufficiency and aortic valve replacement at our institution during the years 1965, 1970, 1975, and 1980. The four most common causes of aortic regurgitation were postinflammatory disease (46%), aortic root dilatation (21%), incomplete closure of a congenitally bicuspid aortic valve (20%), and infective endocarditis (9%). Other causes of aortic incompetence in our study included ventricular septal defects (2%) and quadricuspid aortic valves (1%); the cause was indeterminate in 1%. The mean age of patients at valve replacement was approximately 50 years for all etiologic factors except a ventricular septal defect. All forms of aortic insufficiency were much more common in male than in female patients, except the postinflammatory and indeterminate types, which occurred approximately equally in both sexes. Moreover, the incidences of postinflammatory disease and aortic root dilatation changed appreciably with time. Before 1980, their incidences were 51% and 17%, respectively, but during 1980, they were 29% and 37%, respectively. Accordingly, aortic root dilatation is now the most common cause of pure aortic regurgitation in our surgical population. The decrease in the incidence of postinflammatory disease may be a result of the decreasing incidence of acute rheumatic fever reported in western countries.

Surgical pathology of pure aortic stenosis: a study of 374 cases

The gross surgical pathologic features of the aortic valve were reviewed in 374 patients who had had clinically pure aortic stenosis and aortic valve replacement at our institution during the years 1965, 1970, 1975, and 1980. The most common cause of aortic stenosis, accounting for 46% of our cases, was calcification of a congenitally bicuspid valve. In the remainder, stenosis was produced by postinflammatory fibrocalcific disease (including rheumatic disease) in 35%, by degenerative calcification of an aging valve in 10%, and by calcification of a congenitally unicommissural valve in 6%. The cause of aortic stenosis was indeterminate in 4%. Valvular lesions included various degrees of dystrophic calcification, commissural fusion, and cuspid fibrosis. Calcification tended to occur more extensively and at a younger age in men than in women. Furthermore, it tended to produce stenosis and to necessitate valve replacement earliest in patients with unicommissural valves (mean age, 48 years), later in those with bicuspid or postinflammatory valves (mean age, 59 and 60 years, respectively), and latest in those with degenerative stenosis (mean age, 72 years). In our study, the relative incidence of postinflammatory aortic stenosis remained unchanged from 1965 to 1980, despite the steadily decreasing incidence of acute rheumatic fever reported in western countries. Our data suggest that (1) the incidence of chronic rheumatic heart disease has not yet begun to decrease appreciably, (2) many episodes of acute rheumatic fever may be subclinical, or (3) some forms of nonrheumatic aortic valve disease may produce gross alterations indistinguishable from those of classic chronic rheumatic valvulitis.

Risk factors for aortic dissection: a necropsy study of 161 cases

Among 161 necropsy cases of aortic dissection, 87 (54%) were type I, 34 (21%) type II, and 40 (25%) type III, and an intimal tear was identified in each. Systemic hypertension had been present in 63 of 121 cases (52%) with type I or II dissection and in 30 of 40 (75%) with type III dissection. Aortic dissection involved 7 of 16 cases (44%) with the Marfan syndrome. In the 154 cases without the Marfan syndrome, grade 3 or 4 medial degeneration (cystic medial necrosis) was observed in the ascending aorta in only 27 (18%). The risk of aortic dissection in persons with congenitally bicuspid and unicommissural aortic valves, respectively, was 9 and 18 times that in subjects with tricuspid aortic valves. The mean age of those with aortic dissection and tricuspid, bicuspid and unicommissural aortic valves was 63, 55 and 40 years, respectively, and aortic dissection was more common in men than in women. Grade 3 or 4 atherosclerosis involved the intimal tear in only 11 of 121 type I or II dissections (9%) but 32 of 40 type III dissections (80%). Accordingly, the major risk factors for aortic dissection were systemic hypertension, the Marfan syndrome, and, for type I and II dissections, congenitally bicuspid or unicommissural aortic valves. Aortic medial degeneration was a less important risk factor. Rupture of ulcerocalcific aortic atheromas may have initiated the intimal tear in some type III dissections.

Malformations of the semilunar valves produced in chick embryos by mechanical interference with cardiogenesis. An experimental approach to the role of hemodynamics in valvular development

The purpose of the present work was to analyze the role of hemodynamics in the morphogenesis and histogenesis of the semilunar valves. To achieve this goal we have studied the development of the chick semilunar valves in conditions of abnormal local flow. To obtain an abnormal pattern of local flow we have induced alterations of the cardiac septation process by mechanical interference of the development of the conus cordis. The malformations obtained by this procedure consisted of a spectrum of alterations in the process of incorporation of the aortic conus into the left ventricle. These malformations ranged from a simple widening of the outflow tract of the left ventricle to severe forms of double-outlet right ventricle and ventricular septal defects. Malformations of the semilunar valves consisting of extensive thickening of the leaflets and lack of maturation of the valve tissues were very often present in the malformed hearts. The malformation of the valve leaflets was more frequent and severe in the aortic valve at more advanced stages of development and in the hearts showing more severe alteration of the septation process. The absence of alterations in the semilunar valves of the control embryos and in the experimental embryos without alteration of the cardiac septation suggest a close relationship between the semilunar valves anomalies and the hemodynamic alterations present in the malformed hearts.

Changes in the endothelial morphology of the developing semilunar heart valves. A TEM and SEM study in the chick

In view of recent evidence showing that shape and orientation of endothelial cells is determined by blood flow, the endothelium of the semilunar valves was studied in the developing chick heart using transmission and scanning electron microscopy. The results reveal significant developmental modifications of endothelial morphology and structure. These modifications can be linked to modifications of local blood flow and can also explain several aspects of valvular morphogenesis. The results substantially support the hypothesis of an involvement of hemodynamics in the development of the semilunar valves.

The cardiac malformations. Double inlet left ventricle and corrected transposition explained as deviations in the normal development of the interventricular septum

To examine the hypothesis that malpositions of cardiac ventricles could be explained by altered development of the interventricular septum, we studied hearts from the Johns Hopkins Hospital autopsy files with double inlet left ventricle (16 cases) or corrected transposition (nine cases). In double inlet left ventricle (16 cases) or corrected transposition (nine cases). In double inlet ventricle both atrioventricular valves connect normally developed and positioned right and left atria to a posterior morphologic left ventricle. In hearts with corrected transposition the atria are normally positioned and the morphologic right ventricle is on the left and is continuous with the anteriorly positioned aorta. The morphologic left ventricle is on the right, connected to the posteriorly positioned pulmonary trunk. Normal ventricular septation may be understood as arising from the mechanics of a spiral fold in the primary heart tube produced by the left interventricular sulcus. The ventral limb of the spiral induces the muscular interventricular septum while the dorsal limb becomes a component of the crista supraventricularis. We propose that double inlet left ventricle and corrected transposition are the result of minor deviations in the position of the interventricular sulcus on the primary heart tube. Double inlet left ventricle may develop from the formation of a closed unspiraled ring around the interventricular canal. Corrected transposition may result from a left interventricular sulcus whose ventral limb gives rise to a left sided crista supraventricularis, which determines in part the right ventricular morphology of the left sided ventricle. The dorsal limb spirals toward th atrioventricular canal, gives rise to a malpositioned interventricular septum, and displaces the embryonic trabeculated right ventricle to the left. The concept presented accounts for the morphologic findings characteristic of double inlet left ventricle and corrected transposition.

Symbolic logic analysis of congenital heart disease

In identifying the primary, genetic or environmental event in human cardiac malformations, it is desirable to separate out the apparently secondary lesions. We have employed the general principle that multiple cardiac lesions within a single patient may often represent a pathogenetic process in which one primary event gives rise to a sequence of secondary lesions. The mechanical paradigm of cardiogenesis suggests that abnormal primary lesions may give rise to secondary lesions through altered blood flow or pressure relations, if the secondary structure normally appears later and represents a reasonable tissue response to altered mechanics. Symbolic logic is a mathematical method in which descriptive statements are placed in formal relation to one another and their logical consequences are deduced. Using the mechanical paradigm as a technique of hypothesis generation and symbolic logic as a technique of hypothesis testing, we have analyzed the following malformation complexes in large series of autopsied patients: coarctation of the aorta, tetralogy of Fallot, arterious malformation, interrupted aortic arch, and semilunar valve malformations.

Development of the outflow tract and closure of the interventricular septum in the normal human heart

The majority of congenital heart malformations in humans involve defects in the atrioventricular valves, the crest of the interventricular septum, and/or the outflow tract, but the position and timing of these structures during cardiac development is controversial. We examined all 622 staged, serially sectioned normal human embryos and fetuses in the Carnegie Embryological Collection, and obtained a statistical tabulation of the appearance of the endocardial cushion components and surrounding structures for 382 embryos in good condition between stages 9 and 23 inclusive, when the heart normally develops. Accurately scaled drawings of ventral and lateral views of the hearts of seven embryos from stage 13 through 22 were prepared from graphic reconstructions in order to visualize the relationships of the structures under consideration. We found that development of the outflow tract septum follows the apparent functional separation of both the left and right ventricles and the blood streams leaving them. Elevations of the endocardial cushion material are continuous throughout the outflow tract and develop as a consequence of the elliptical configuration imposed on the circular cross section of the outflow tract. The membranous interventricular septum is formed of cushion material in the space bounded by the outflow tract septum, interventricular septum, and the fused AV cushion and right outflow tract cushion. The results of this study are consistent with the assertion that functional separation of the aortic and pulmonary outflow tracts precedes anatomic septation, and that anatomic septation is brought about by mechanical modeling of developing myocardium and endocardial cushion material.