Effects of vitamin A on endocardial cushion development in the mouse heart

Role of carotenoids and retinoids during heart development

The nutritional requirements of the developing embryo are complex. In the case of dietary vitamin A (retinol, retinyl esters and provitamin A carotenoids), maternal derived nutrients serve as precursors to signaling molecules such as retinoic acid, which is required for embryonic patterning and organogenesis. Despite variations in the composition and levels of maternal vitamin A, embryonic tissues need to generate a precise amount of retinoic acid to avoid congenital malformations. Here, we summarize recent findings regarding the role and metabolism of vitamin A during heart development and we survey the association of genes known to affect retinoid metabolism or signaling with various inherited disorders. A better understanding of the roles of vitamin A in the heart and of the factors that affect retinoid metabolism and signaling can help design strategies to meet nutritional needs and to prevent birth defects and disorders associated with altered retinoid metabolism. This article is part of a Special Issue entitled Carotenoids recent advances in cell and molecular biology edited by Johannes von Lintig and Loredana Quadro.

The retinaldehyde reductase DHRS3 is essential for preventing the formation of excess retinoic acid during embryonic development

Oxidation of retinol via retinaldehyde results in the formation of the essential morphogen all-trans-retinoic acid (ATRA). Previous studies have identified critical roles in the regulation of embryonic ATRA levels for retinol, retinaldehyde, and ATRA-oxidizing enzymes; however, the contribution of retinaldehyde reductases to ATRA metabolism is not completely understood. Herein, we investigate the role of the retinaldehyde reductase Dhrs3 in embryonic retinoid metabolism using a Dhrs3-deficient mouse. Lack of DHRS3 leads to a 40% increase in the levels of ATRA and a 60% and 55% decrease in the levels of retinol and retinyl esters, respectively, in Dhrs3(-/-) embryos compared to wild-type littermates. Furthermore, accumulation of excess ATRA is accompanied by a compensatory 30-50% reduction in the expression of ATRA synthetic genes and a 120% increase in the expression of the ATRA catabolic enzyme Cyp26a1 in Dhrs3(-/-) embryos vs. controls. Excess ATRA also leads to alterations (40-80%) in the expression of several developmentally important ATRA target genes. Consequently, Dhrs3(-/-) embryos die late in gestation and display defects in cardiac outflow tract formation, atrial and ventricular septation, skeletal development, and palatogenesis. These data demonstrate that the reduction of retinaldehyde by DHRS3 is critical for preventing formation of excess ATRA during embryonic development.

Embryological origin of the endocardium and derived valve progenitor cells: from developmental biology to stem cell-based valve repair

The cardiac valves are targets of both congenital and acquired diseases. The formation of valves during embryogenesis (i.e., valvulogenesis) originates from endocardial cells lining the myocardium. These cells undergo an endothelial-mesenchymal transition, proliferate and migrate within an extracellular matrix. This leads to the formation of bilateral cardiac cushions in both the atrioventricular canal and the outflow tract. The embryonic origin of both the endocardium and prospective valve cells is still elusive. Endocardial and myocardial lineages are segregated early during embryogenesis and such a cell fate decision can be recapitulated in vitro by embryonic stem cells (ESC). Besides genetically modified mice and ex vivo heart explants, ESCs provide a cellular model to study the early steps of valve development and might constitute a human therapeutic cell source for decellularized tissue-engineered valves. This article is part of a Special Issue entitled: Cardiomyocyte Biology: Cardiac Pathways of Differentiation, Metabolism and Contraction.

The expanding role for retinoid signaling in heart development

The importance of retinoid signaling during cardiac development has long been appreciated, but recently has become a rapidly expanding field of research. Experiments performed over 50 years ago showed that too much or too little maternal intake of vitamin A proved detrimental for embryos, resulting in a cadre of predictable cardiac developmental defects. Germline and conditional knockout mice have revealed which molecular players in the vitamin A signaling cascade are potentially responsible for regulating specific developmental events, and many of these molecules have been temporally and spatially characterized. It is evident that intact and controlled retinoid signaling is necessary for each stage of cardiac development to proceed normally, including cardiac lineage determination, heart tube formation, looping, epicardium formation, ventricular maturation, chamber and outflow tract septation, and coronary arteriogenesis. This review summarizes many of the significant milestones in this field and particular attention is given to recently uncovered cross-talk between retinoid signaling and other developmentally significant pathways. It is our hope that this review of the role of retinoid signaling during formation, remodeling, and maturation of the developing heart will serve as a tool for future discoveries.

Retinoic acid is required for endodermal pouch morphogenesis and not for pharyngeal endoderm specification

Because tissues from all three germ layers contribute to the pharyngeal arches, it is not surprising that all major signaling pathways are involved in their development. We focus on the role of retinoic acid (RA) signaling because it has been recognized for quite some time that alterations in this pathway lead to craniofacial malformations. Several studies exist that describe phenotypes observed upon RA perturbations in pharyngeal arch development; however, these studies did not address whether RA plays multiple roles at distinct time points during development. Here, we report the resulting phenotypes in the hindbrain, the neural crest-derived tissues, and the pharyngeal endoderm when RA synthesis is disrupted during zebrafish gastrulation and pharyngeal arch morphogenesis. Our results demonstrate that RA is required for the post-gastrulation morphogenesis and segmentation of endodermal pouches, and that loss of RA does not affect the length of the pharyngeal ectoderm or medial endoderm along the anterior-posterior axis. We also provide evidence that RA is not required for the specification of pharyngeal pouch endoderm and that the pharyngeal endoderm consists of at least two different cell populations, of which the pouch endoderm is sensitive to RA and the more medial pharyngeal endoderm is not. These results demonstrate that the developmental processes underlying pharyngeal arch defects differ depending on when RA signaling is disturbed during development.

Cardiovascular malformations induced by prenatal exposure to phenobarbital in rats

The effects of prenatal exposure to phenobarbital (PB) on the cardiovascular system were examined in rat fetuses and pups. PB was administered at a dose of 80 or 120 mg/kg/day by gavage to Sprague Dawley (SD) rats on two consecutive gestational days (GD): 7-8, 8-9, 9-10, or 10-11. Fetuses were examined for cardiovascular malformations on GD 20. In addition, pups were examined for PB-induced cardiovascular malformations. Incidences of ventricular septal defect (VSD), overriding aorta, double outlet right ventricle and transposition of great arteries were significantly increased in the fetuses whose dams were administered PB at 120 mg/kg on GD 8-9, 9-10 or 10-11. GD 8-11 was the critical period for the cardiovascular malformations associated with administration of PB in rats. Various types of cardiovascular malformations were detected in pups from the PB-administered dam. Severe cardiovascular malformations induced by PB caused deaths on early postnatal days. However, slight malformations such as isolated VSD persisted until weaning, and did not affect postnatal viability.

Cellular anomalies underlying retinoid-induced phocomelia

The question of how alterations in cell behavior produced by retinoic acid (RA) influenced the development of skeletogenic mesenchyme of the limb bud was examined in this study. Our established model was employed, which involves treatment of pregnant mice with a teratogenic dose of RA (100 mg/kg) on 11 days postcoitum (dpc) resulting in a severe truncation of all long bones of the forelimbs in virtually every exposed fetus. It is shown that RA, administered at a stage to induce phocomelia in virtually all exposed embryos, resulted in immediate appearance of enhanced cell death within the mesenchyme in the central core of the limb bud, an area destined for chondrogenesis. The central core mesenchyme, which in the untreated limb buds experiences a sharp decline in cell proliferation heralding the onset of chondrogenesis, demonstrated a reversal of the process; this mesenchyme maintained a higher rate of cell proliferation upon RA exposure. These events resulted in a truncation and disorganization of the chondrogenic anlage, more pronounced in zeugopodal mesenchyme than in the autopod. We conclude that an inhibition of chondrogenesis was secondary to a disruption in cellular behavior caused by RA, a likely consequence of misregulation in the growth factor signaling cascade.

Early events in valvulogenesis: a signaling perspective

The proper formation and function of the vertebrate heart requires a multitude of specific cell and tissue interactions. These interactions drive the early specification and assembly of components of the cardiovascular system that lead to a functioning system before the attainment of the definitive cardiac and vascular structures seen in the adult. Many of these adult structures are hypothesized to require both proper molecular and physical cues to form correctly. Unlike any other organ system in the embryo, the cardiovascular system requires concurrent function and formation for the embryo to survive. An example of this complex interaction between molecular and physical cues is the formation of the valves of the heart. Both molecular cues that regulate cell transformation, migration, and extracellular matrix deposition, and physical cues emanating from the beating heart, as well as hemodynamic forces, are required for valvulogenesis. This review will focus on molecules and emerging pathways that guide early events in valvulogenesis.

Retinoic acid administration is associated with changes in the extracellular matrix and cardiac mesenchyme within the endocardial cushion

Retinoic acid has been associated with a number of cardiac defects, some of which seem to be related to changes in the endocardial cushions. Studies in mice and older chick embryos have suggested that these defects may be associated with a decrease in mesenchymal cell formation within the cushion. In a previous report we showed that retinoic acid lowered the number of mesenchymal cells in a culture bioassay of mesenchyme formation and that this response was due to retinoic acid modifying the production of particulate matrix from the myocardium. In this study, we have extended these observations to the embryo by implanting a retinoic acid coated bead into the embryo and examined the effect on cardiac mesenchyme formation and in the production of the particulate matrix. In all cases the addition of retinoic acid resulted in a decrease in the number of mesenchymal cells invading the endocardial cushions. In addition retinoic acid increased the production of hLAMP-1 and fibronectin but not transferrin, confirming our earlier report. Finally, we measured the volume of the cushion and calculated the cell density of both the inferior and superior cushions. The results suggest that the superior cushion is more sensitive to retinoic acid treatment than the inferior cushion. Collectively, these results support our earlier work that suggests that the mechanism of retinoic acid cardiac abnormalities involves a disruption in the production of particulate matrix from the myocardium and a subsequent decrease in cardiac mesenchyme cells that results in a malformed cardiac cushions.

Retinoic acid inhibition of cardiac mesenchyme formation in vitro correlates with changes in the secretion of particulate matrix from the myocardium

Retinoic acid has been associated with a variety of cardiac defects. A percentage of these defects are related to changes in the endocardial cushions. Studies in mice and older chick embryos have shown a decrease in mesenchymal cell formation attributable to retinoic acid and have suggested that retinoic acid was affecting the extracellular matrix. In this study we have tested the effect of retinoic acid on cardiac mesenchyme formation in vitro and then tested retinoic acid treated myocyte cultures for changes in the expression of hLAMP-1, fibronectin and transferrin members of the particulate matrix that is required for mesenchyme formation. Initial experiments tested the effect of retinoic acid on mesenchymal cell formation first in atrioventricular canal and outflow tract explant cultures and then in AV endothelial monolayer cultures using myocyte conditioned media or the particulate matrix fraction from retinoic acid treated myocyte cultures. In all cases, mesenchymal cell formation was suppressed while no suppression was observed when MyoCM was included with retinoic acid. Protein analysis showed that retinoic acid had a stimulatory effect on protein synthesis. ELISA assays revealed that retinoic acid treated myocyte cultures contained significantly more hLAMP-1 and fibronectin than either normal or DMSO controls. However, transferrin was not affected by retinoic acid treatment in these experiments. Our results suggest that retinoic acid affects the expression of the particulate matrix and that these changes may be responsible for the observed decrease in mesenchymal cell formation.

Forebrain induction, retinoic acid, and vulnerability to schizophrenia: insights from molecular and genetic analysis in developing mice

Schizophrenia is thought to be a disease of early development that ultimately affects forebrain neurons and circuits. There may be a relationship between disrupted forebrain development; malformations of the limb, face, and heart; and signaling via the steroid-like hormone retinoic acid (RA) in some schizophrenic patients. The limbs, face, heart, and forebrain all develop from sites where neural crest-derived, RA-producing mesenchyme contributes to induction and differentiation of adjacent epithelia. Induction between neural crest-derived, RA-producing mesenchyme, the anterior neural tube, and the anterior surface epithelium of the embryo guides regional differentiation and pathway formation during forebrain development. Furthermore, there are at least two mouse mutations--in the Pax-6 and Gli-3 genes--that cause peripheral malformations and specifically disrupt neural crest mediated, RA-dependent induction and differentiation in the forebrain. These observations suggest that induction might provide a common target for genes that alter morphogenesis of peripheral structures, disrupt RA-signaling, and compromise forebrain development. In the forebrain, some of these disruptions might influence the numbers or cellular properties of neurons and circuits. Such changes might be reflected in the aberrant forebrain function that characterizes schizophrenia.

Apoptosis in embryos of diabetic rats

The aim of the present study was to determine whether maternal diabetes affects rat embryo and yolk sac apoptosis during the postimplantation period. Severely malformed and growth-retarded embryos of gestational day 12 from diabetic rats exhibited pronounced DNA laddering on agarose gels. On the other hand, no DNA laddering could be observed in any of the non-malformed embryos from control and diabetic rats, or in their corresponding yolk sacs. Analysis of embryos of gestational day 10 revealed only a few scattered TUNEL positive cells mainly located in the allantois, the foregut epithelium, the cranial neuroepithelium and in the cranial mesenchyme. Embryonic tissue of gestational day 12 showed numerous aggregates of TUNEL-positive cells, indicating developmental remodelling of multiple organs. Analysis of non-malformed embryos of day 10 and 12 revealed a distribution and frequency of TUNEL positive cells unaffected by the diabetic state of the mother on both days. In vitro incubation (2-8 hr) of normal day-12 yolk sacs resulted in strong DNA laddering, but not in the corresponding embryos. Dispersed yolk sac cells generated higher levels of reactive oxygen species than dispersed embryonic cells. Reactive oxygen species levels in both embryonic and yolk sac cells were unaffected by the diabetic state of the mother. Moreover, immunoblot analysis showed high Bcl-2 and undetectable caspase-1 levels in embryos from both normal and diabetic rats and low Bcl-2 and high caspase-1 levels in the corresponding yolk sacs. Immunohistochemical analysis of embryos demonstrated caspase-1-reactivity in a small subpopulation of cells located in proximity to TUNEL-positive cells. We conclude that the inherent capacity of embryonic cells to enter apoptosis in vitro is low as compared to yolk sac cells, and that wide-spread apoptosis is not likely to play a major role in diabetes-induced dysmorphogenesis but rather in early phases of resorption of severely malformed and developmentally retarded embryos.

Diminished growth of atrioventricular cushion tissue in stage 24 retinoic acid-treated chicken embryos

Stage 34 chicken hearts have shown a spectrum of looping disturbances, changed hemodynamics, and changed growth of both right ventricular myocardium and atrioventricular cushion tissue after retinoic acid treatment. To obtain more information about the onset of the malformations we studied stage 24, the stage between the previously studied stage 34 and the moment of treatment. Sixteen stage 24 chicken embryos were examined after treatment with 1 microg all-trans retinoic acid at stage 15 and compared with 6 sham operated embryos. Morphological examination was supported by graphic reconstructions. Absolute volumes of atrial, atrioventricular, and ventricular myocardia were measured by a point counting method. The absolute volumes of the endocardial cushions were measured as well. Fifteen (15/16) retinoic acid-treated hearts did not show marked malformations as far as could be detected with our current macroscopic and microscopic techniques. One (1/16) retinoic acid-treated heart showed an abnormal tubular C-shape with a less bended inner curvature and with an abnormal horizontally oriented atrioventricular canal. The dorsal cushion tissue of this atrioventricular canal was discontinuous with the dorsal mesocardium and covered the malpositioned myocardial border between the atrium and the atrioventricular canal. The volume measurements did show a difference between retinoic acid treatment and sham operations. The retinoic acid-treated hearts showed a significant volume decrease of the atrioventricular cushions. No significant differences were found in the volumes of the ventricular myocardium compared to the sham operated embryos. We hypothesize that, between stages 15 and 24, retinoic acid directly affects the myocardial wall and the cushion tissue formation. In the present material this has resulted in decreased atrioventricular cushion growth, in changed hemodynamics, and in a severe looping disturbance of one embryo. We further hypothesize that, between stages 24 and 34, the malformations with minor looping disturbances will become apparent. Thus, development beyond stage 24 would result in the spectrum of looping disturbances as has been found at stage 34. These latter morphological malformations would lead to increasing hemodynamic changes, resulting in changes in growth as a secondary effect.

Anomalous looping, atrioventricular cushion dysplasia, and unilateral ventricular hypoplasia in the mouse embryos with right isomerism induced by retinoic acid

BACKGROUND

Visceroatrial heterotaxy syndrome is characterized by abnormality of visceral laterality and complex cardiovascular anomalies usually involving both the outflow and inflow tract. Morishima et al. (1995) showed that mouse embryos treated with all-trans retinoic acid at embryonic day 6.5 (primitive streak stage) induces this syndrome.

METHODS

To investigate the morphogenetic process of visceroatrial heterotaxy syndrome, we examined retinoic acid-treated mouse embryos at embryonic days 9-15 using scanning electron microscopy.

RESULTS

The sinoatrial connection was first distinguished for the determination of situs as early as at embryonic day 10.5. Normal visceroatrial situs was found in 57% of all treated embryos, and the rest had abnormal situs, in which right isomerism was found in 81%. In the right-isomeric mouse, the cardiac morphology was characterized by abnormal looping together with dysplasia of the inflow and outflow tract cushion; that is, the primitive right ventricle was usually deviated cranially to various degrees, the atrioventricular cushion appeared trilobed in a half of them, and unilateral ventricular hypoplasia was noted in about one-third of them after embryonic day 14.5.

CONCLUSIONS

An anomalous relation between the atrioventricular cushions and the interventricular septum appeared to have caused a restrictive inflow to the unilateral ventricle, leading to ventricular chamber hypoplasia on the ipsilateral side. Thus, we clarified that retinoic-acid treatment at the primitive streak stage disturbed cardiac looping and formation of atrioventricular cushion development, which secondarily influenced ventricular chamber development.

Distribution of fibronectin, type I collagen, type IV collagen, and laminin in the cardiac jelly of the mouse embryonic heart with retinoic acid-induced complete transposition of the great arteries

BACKGROUND

In the mouse model of complete transposition of the great arteries (TGA) produced by all-trans retinoic acid (RA), parietal and septal ridges in the outflow tract (OT) are hypoplastic. At first, these ridges are generated by an expanded cardiac jelly (mainly myocardial basement membrane). Thereafter, endothelial cells delaminate and invade into the adjacent cardiac jelly to form endocardial cushion tissue (formation of cushion ridge). During cushion tissue formation, basement membrane antigens play an important role in the regulation of this endothelial-mesenchymal transformation.

METHODS

To examine whether the myocardial basement membrane components are altered in the RA-treated heart OT, immunohistochemistry for fibronectin, type I collagen, type IV collagen, and laminin was carried out in mouse embryonic hearts at 9.5 and 10.5 ED (embryonic day; vaginal plug = day 0) with or without prior exposure to RA.

RESULTS

Particulate/fibrillar fibronectin and fibrillar type I collagen were observed in the thick cardiac jelly of the control heart at the onset of mesenchymal formation. In the RA-treated heart, an intermittent patchy staining for fibronectin and a sparse distribution of type I collagen were observed in the thin cardiac jelly. Laminin and type IV collagen were distributed continuously on the basal surface (layer adjacent to the basal plasma membrane) of endocardium and myocardium in both control and RA-treated hearts.

CONCLUSIONS

The alterations in the antigens of the myocardial basement membrane (cardiac jelly) may be responsible for the hypoplasticity of parietal and septal ridges that characterizes RA-induced TGA morphology. This may be one of the reasons why mesenchymal cell formation is inhibited in the RA-induced TGA.

Features of DiGeorge syndrome and CHARGE association in five patients

We report on five patients presenting with features of two congenital disorders, DiGeorge syndrome (DGS) and CHARGE association. CHARGE association is usually sporadic and its origin is as yet unknown. Conversely, more than 90% of DGS patients are monosomic for the 22q11.2 chromosomal region. In each of the five patients, both cytogenetic and molecular analysis for the 22q11.2 region were normal. In view of the broad clinical spectrum and the likely genetic heterogeneity of both disorders, these cases are consistent with the extended phenotype of either DGS without 22q11.2 deletion or CHARGE association, especially as several features of CHARGE association have been reported in rare patients with 22q11.2 deletion association phenotypes. On the other hand, these could be novel cases of an independent association involving a complex defect of neural crest cells originating from the pharyngeal pouches.

Stereological study of stage 34 chicken hearts with looping disturbances after retinoic acid treatment: disturbed growth of myocardium and atrioventricular cushion tissue

BACKGROUND

In a previous study retinoic acid treatment of chicken hearts has resulted in a spectrum of looping disturbances. Because of a decrease in contraction force of these hearts, the myocardial volume was hypothesized to be altered. Because retinoic acid has been suggested to influence endocardial cushion volumes, these were estimated as well.

METHODS

The previously studied hearts were used for estimating the absolute volumes of the atrial and ventricular myocardium and of the endocardial cushions by means of Cavalieri's principle. To measure the surface density of the trabeculations according to the isector method, we used retinoic acid treated hearts, which were perfusion fixed and in which the sections were isotropic uniform random. The volumes and surface densities found in the three morphologically distinguished groups, i.e., intact septum, isolated ventricular septal defect and double outlet right ventricle, were compared with those in shams.

RESULTS

A significant volume decrease was found in the right ventricular free wall myocardium of the double outlet right ventricle. No significant differences were found in the surface densities of the trabeculae. The volume of the atrioventricular cushion tissue in the double outlet right ventricle hearts was significantly increased. The morphological spectrum observed previously was also expressed in the right ventricular myocardial volume, which appeared to decrease from the least to the most malformed hearts, and in the volume of the atrioventricular cushion tissue, which appeared to increase.

CONCLUSIONS

Several studies have shown pathology in myocardial and cushion tissue after retinoic acid treatment. In this study we have found a decreased growth of the right ventricular myocardium and an increased growth of the atrioventricular cushion tissue. We suggest that the previously found looping disturbance causes changed hemodynamics, as reported elsewhere, and that these result in changes in growth. We cannot exclude a direct effect of retinoic acid on the myocardium, which has to explain the looping disturbance.

Altered distribution of collagen type I and hyaluronic acid in the cardiac outflow tract of mouse embryos destined to develop transposition of the great arteries

Complete transposition of the great arteries (TGA) is inducible by treatment with all-trans retinoic acid in the ICR mouse. In this model, hypoplasia and dysplasia of the proximal outflow tract cushion tissue lead to non-spiral septation. In order to evaluate the effect of retinoic acid on the extracellular matrix of the cardiac outflow tract, we examined the distribution of collagen type I and hyaluronic acid, immunohistochemically, on days 8-9 of gestation. In controls, collagen type I fibrils ran mainly in a radial direction, extending towards the endocardium in the cardiac jelly of the proximal outflow tract. Also, a pair of longitudinal fiber bundles were formed stretching to the distal outflow tract. As for hyaluronic acid, intense staining was observed in the submyocardial and intermyocardial space of the outer curvature of the heart. On the other hand, in retinoic acid-treated embryos, the submyocardial radial fibrils or longitudinal fiber bundles of collagen type I were diminished, and irregular and dense deposits of collagen type I were observed along the endocardium. Furthermore, hyaluronic acid showed a loss of differential localization between the outer and inner curvature. Instead, irregular and intense staining was observed uniformly along the outflow myocardium. Thus, retinoic acid appeared to have perturbed the differentiation in the proximal outflow tract causing an altered organization of multiple extracellular matrix molecules, including collagen type I and hyaluronic acid, which led to an abnormal molecular network of the cardiac jelly in the cardiac outflow tract, abnormal septation and, further, to TGA or TGA-type anomalies.

Expression of the beta 4 integrin subunit in the mouse heart during embryonic development: retinoic acid advances beta 4 expression

Using immunohistochemical techniques as well as in situ hybridization we were able to elicit the expression pattern of the beta 4 integrin subunit in the murine heart during development. We show that beta 4 is not expressed in the heart before E13 and is afterwards restricted to the endocardium of the atrioventricular canal, the outflow tract, and the venous valves in the right atrium. As these are all sites of high shear stress in the heart, we propose a role for alpha 6 beta 4 in the tight adhesion of the endocardial cells to their basement membranes in these segments. Moreover, mouse embryos were treated with all-trans retinoic acid, which was previously shown to induce congenital malformations, among which malformations of the heart. We show an advanced expression without ectopic localization of cardiac beta 4 after the administration of retinoic acid. This advanced appearance of beta 4 was also shown in extracardiac tissue like migrating neural crest cells. Several hypotheses on the mechanism of beta 4 up-regulation and a possible role for alpha 6 beta 4 in the development of heart malformations after the administration of retinoic acid are discussed.

Hypoplasia of cushion ridges in the proximal outflow tract elicits formation of a right ventricle-to-aortic route in retinoic acid-induced complete transposition of the great arteries in the mouse: scanning electron microscopic observations of corrosion cast models

BACKGROUND

The major morphologic change associated with retinoic acid (RA)-induced complete transposition of the great arteries (TGA), a congenital malformation of the heart, was investigated in a mouse model in which TGA was found in 80% of surviving fetuses.

METHODS

Corrosion casts of embryonic hearts with or without prior exposure to retinoic acid were observed under a scanning electron microscope.

RESULTS

In control hearts, indentations caused by expanded parietal and septal ridges in the outflow tract established right ventricle-to-left ventral pulmonic and left ventricle-to-right dorsal aortic routes before the aorticopulmonary septum completion. In RA-treated hearts, indentations of proximal regions of the parietal and septal ridges were small in the proximal outflow tract, whereas those in the distal regions developed well. These morphological features in the RA-treated hearts elicited right ventricle-to-right ventral aortic and left ventricle-to-left dorsal pulmonic routes in the TGA morphology.

CONCLUSIONS

Hypoplasticity of the proximal regions of parietal and septal ridges in the outflow tract is one of the primary morphological abnormalities of the RA-induced TGA.

Fibulin-1, vitronectin, and fibronectin expression during avian cardiac valve and septa development

BACKGROUND

Extracellular matrix (ECM) proteins have been implicated as mediators of events important to valvuloseptal development (reviewed by Little and Rongish, Experentia, 51:873-882, 1995). The aim of this study was to identify connective tissue ECM proteins present at sites of valvuloseptal morphogenesis, and to determine how their patterns of expression change during the developmental process.

METHODS

Immunofluorescence microscopy was used to examine the distribution of fibulin-1, vitronectin, and fibronectin in the embryonic chicken heart over a broad developmental time frame (Hamburger and Hamilton stages 14 to 44), emphasizing stages that illustrate endocardial cushion formation, growth, fusion, and development into valvuloseptal components.

RESULTS AND CONCLUSIONS

Fibulin-1 immunolabeling was concentrated in endocardial cushions, notably at boundaries with the myocardium, during stages when the cushions are differentiating into valvular and septal components. Fibulin-1 was detected in the endocardial cushions prior to their seeding with cushion cells, but became undetectable by early midgestation. Vitronectin expression was similar to fibulin-1, but less restricted in its distribution. Vitronectin was observed before endocardial cushion cell migration commenced and persisted until the formation of prevalvular structures (early midgestation) in the atrioventricular cushions. Vitronectin remained detectable in the semilunar valves until late midgestation. Fibronectin was present in the endocardial cushion region and in portions of the endocardium and myocardium throughout the stages presented. Our data suggests that the ECM of the endocardial cushions undergoes remodelling in a regionally and temporally specific manner which corresponds with morphogenetic changes during valvuloseptal development.

Morphological observations on the pathogenetic process of transposition of the great arteries induced by retinoic acid in mice

BACKGROUND

The pathogenesis of complete transposition of the great arteries (TGA) is still controversial because useful animal models have not been established. We previously reported that all-trans retinoic acid induced complete TGA at a high proportion in mice. The aim of the present study was to clarify the morphogenesis of the cardiac outflow tract in the retinoic acid-treated embryos destined to develop TGA.

METHODS AND RESULTS

We first examined the morphology of TGA in mouse fetuses treated with retinoic acid to establish an animal model of TGA (experiment 1) and then examined the retinoic acid-treated embryonic hearts by means of ink injection and histology (experiment 2). All mouse fetuses and embryos showed visceroatrial situs solitus and d-ventricular loop. In experiment 1, among 45 embryos treated with retinoic acid 70 mg/kg at day 8.5 of gestation, 35 (78%) had TGA and 3 (6.7%) had a double-outlet right ventricle with a subpulmonary ventricular septal defect. In experiment 2, all hearts already exhibited d-loop at gestation day 8.5. At gestation day 9.5, conus swellings, composed of acellular cardiac jelly, where hypoplastic, and the conotruncal cavity was nonspiral or tubular. At gestation day 11.0, aberrant conus swellings located anteroposteriorly to give a straight orientation to the conotruncal cavity. At gestation day 12.0, side-by-side great arteries were transposed in that the aorta arose from the right ventricle and the pulmonary artery arose above the interventricular foramen.

CONCLUSIONS

These results suggest that a reproducible animal model of TGA can be produced in mice by treatment with retinoic acid; that there was no loop anomaly, such as an A-loop or L-loop, in our model; and that hypoplasia of the conus swellings appears to be the primary event leading to TGA.

Effects of retinoic acid on embryonic development of mice in culture

The effects of all-trans-retinoic acid (RA) (tretinoin) on the craniofacial development of mouse embryos were examined using whole embryo culture. In day 8 embryos cultured for 48 h, embryonic growth was inhibited concentration-dependently by all-trans-RA treatment. Most of the treated embryos exhibited hypoplasia of the primary palatal processes and a reduction in the development of the first visceral arches. In day 10 embryos cultured for 48 h, although embryonic growth was not inhibited at any concentrations of all-trans-RA, median cleft lip (93%), hypoplasia of the primary palatal processes (37%) and limb reduction deformities (48%) occurred commonly. Furthermore, RA treatment greatly reduced the size of the secondary palatal processes. The incorporation of 3H-thymidine in the treated maxillary processes was decreased to 65% of the control value at 1.0 x 10(-7) M all-trans-RA. These findings indicate that all-trans-RA is teratogenic in mouse whole embryo culture.

Joint action of t-retinoic and valproic acids on Xenopus embryo development

The joint action of all-trans retinoic acid (RA) with valproic acid (VPA) was examined at malformation-inducing concentrations for Xenopus embryos. The compounds were selected for testing to evaluate malformation as an endpoint for the developmental toxicity of chemical mixtures and to help assess whether joint action types can be related to modes or mechanisms of chemical toxicity for Xenopus. Three mixtures (3:1, 1:1, and 1:3 RA:VPA) were tested in 96 hr static-renewal exposures. Three separate tests were conducted on each mixture. Positive controls were also tested (i.e., 1:0 and 0:1 solutions). Using toxic unit analysis, the joint action for induction of malformations of all types, craniofacial malformations, and microcephaly was response addition, indicating the chemicals are dissimilar and non-interactive. The results support recent joint action studies in providing evidence that similar acting chemicals might be defined as chemicals with the same biochemical/molecular mechanism of action in inducing malformations in Xenopus embryos.

Avian model for 13-cis-retinoic acid embryopathy: demonstration of neural crest related defects

The effects of 13-cis-retinoic acid on the developing chick embryo were investigated. Fertilized eggs were injected via the yolk sac with single 50 microliters doses of either 1.5 micrograms, 15 micrograms, or 150 micrograms of 13-cis-retinoic acid in dimethyl sulfoxide on varying days of incubation (embryonic days 2, 3, 4, 5, or 6). Control embryos were given solvent alone or a mock injection. The embryos were examined on day 14 of incubation. The effects of retinoic acid on mortality and total malformations were both dose and developmental-stage responsive. The defects caused by 13-cis-retinoic acid occurred in mesenchymal tissues derived in part from the cranial neural crest ectomesenchyme. The craniofacial and cardiovascular malformations produced in the chick are analogous to those seen in animal models of retinoid teratogenesis and in human fetuses exposed to 13-cis-retinoic acid during maternal therapy for cystic acne. Following 13-cis-retinoic acid treatment, craniofacial and specific cardiovascular malformations were increased significantly compared to those in matched solvent and mock treated controls. The greatest number of malformations occurred when 13-cis-retinoic acid was given after cranial neural crest cell migration was complete. We propose that the primary effect of 13-cis-retinoic acid is on region-specific localization and differentiation of the mesenchymal subpopulation of cranial neural crest cells.

Normal stages of cardiac organogenesis in the mouse: I. Development of the external shape of the heart

Normal development of the mouse embryonic heart was studied at the organ level using microdissection and scanning electron microscopy (SEM). Altogether 225 embryos, sampled at 8-hour intervals between 11ed (ed = embryonic day; day of vaginal plug = 1ed) and 15ed were collected. Their hearts were fixed by high flow-low pressure perfusion, microdissected, and observed in SEM. Standardized frontal, right profile, and left profile SEM micrographs were obtained and analyzed. The main purpose of this study was to create a series of normal stages of mouse cardiac development as a reference for ongoing studies in experimental cardiac teratology (e.g., in fetal mouse trisomies). Comparisons with chick, human, and dog embryonic hearts, prepared using the same technique, show that the mouse embryonic heart is characterized by a relatively deep interventricular sulcus. The absence of a conoventricular sulcus in the mouse results in poor definition of the boundary between the conus and the right ventricle. The external separation of the aorta and the pulmonary artery is evident from 13ed onward. The respective positions of the great arteries (aorta dextroposterior, pulmonary artery sinistroanterior) does not change until the end of cardiac organogenesis (15ed in the mouse).

Interaction between the splotch mutation and retinoic acid in mouse neural tube defects in vitro

The interaction between the splotch gene (Sp) and all-trans retinoic acid (RA) was investigated using cytogenetically marked Sp/+ and +/+ mouse embryos cultured in the presence of RA. Retinoic acid retarded the development of and had a teratogenic effect on mouse embryos in culture. In particular, RA had seemingly opposite effects on the posterior neural tube, inducing abnormally early fusion in some embryos and causing a dose-dependent delay in others. When the effects of RA on identified Sp/+ and +/+ embryos were compared, the only observed difference in their responses was in the degree of the delay in posterior neuropore (PNP) closure. At the end of the culture period, among the untreated control embryos, the Sp heterozygotes showed retardation of PNP closure compared to +/+ embryos. In addition, the RA treatment was found to have induced a greater delay in posterior neural tube closure in Sp/+ than in +/+ embryos. The basis for this difference in response to RA is presumed to be the retardation of PNP closure that is caused by the Sp gene in heterozygous form. The effects of the gene and the teratogen are additive and the gene carriers thus have greater mean PNP lengths at the end of culture. Since the length of the PNP is an indication of an embryo's likelihood of developing spina bifida, this provides an explanation for the observation that Sp/+ embryos are more sensitive to the spina bifida-causing effects of RA than are +/+ embryos.

Teratogenicity of benzoic acid derivatives of retinoic acid in cultured mouse embryos

Isotretinoin (13-cis-RA) is a human teratogen and mouse embryos exposed to 13-cis-RA in vivo exhibit many of the same defects as humans. Early postimplantation mouse embryos exposed to 13-cis-RA in culture exhibit developmental alterations of the visceral arches, similar to those seen after in vivo exposure. Certain benzoic acid derivatives of retinoic acid have been shown to possess activity equal to or greater than retinoic acid in several in vitro systems. This study examines the teratogenic effects of some of these retinoids on mouse embryos in vitro. Day 8 CD-1 mouse embryos were cultured for 48 hours in the presence of these benzoic acid derivatives. With the exception of Ro-15-0778, all compounds produced visceral arch malformations similar to those seen in embryos exposed to 13-cis-RA, but at dramatically different effective concentrations. Extremely low concentrations of the retinoic acid-related compounds tested appear to have detrimental effects on embryonic development and these compounds may be poor candidates for therapeutic use.

The DiGeorge anomaly as a developmental field defect

The DiGeorge "syndrome" is a characteristic malformation pattern involving craniofacial, cardiac, thymic, and parathyroid structures. Evidence is accumulating that the DiGeorge "syndrome" is actually not a syndrome, but a polytopic developmental field defect. We present evidence of causal heterogeneity of the DiGeorge anomaly. This heterogeneity will be discussed in the light of recent findings that indicate that the dysmorphogenetically reactive unit responsible for the phenotype of the DiGeorge anomaly is a population of cephalic neural crest cells.

Retinoic acid embryopathy

Retinoic acid, an analogue of vitamin A, is known to be teratogenic in laboratory animals and has recently been implicated in a few clinical case reports. To study the human teratogenicity of this agent, we investigated 154 human pregnancies with fetal exposure to isotretinoin, a retinoid prescribed for severe recalcitrant cystic acne. The outcomes were 95 elective abortions, 26 infants without major malformations, 12 spontaneous abortions, and 21 malformed infants. A subset of 36 of the 154 pregnancies was observed prospectively. The outcomes in this cohort were 8 spontaneous abortions, 23 normal infants, and 5 malformed infants. Exposure to isotretinoin was associated with an unusually high relative risk for a group of selected major malformations (relative risk = 25.6; 95 per cent confidence interval, 11.4 to 57.5). Among the 21 malformed infants we found a characteristic pattern of malformation involving craniofacial, cardiac, thymic, and central nervous system structures. The malformations included microtia/anotia (15 infants), micrognathia (6), cleft palate (3), conotruncal heart defects and aortic-arch abnormalities (8), thymic defects (7), retinal or optic-nerve abnormalities (4), and central nervous system malformations (18). The pattern of malformation closely resembled that produced in animal studies of retinoid teratogenesis. It is possible that a major mechanism of isotretinoin teratogenesis is a deleterious effect on cephalic neural-crest cell activity that results in the observed craniofacial, cardiac, and thymic malformations.

The honeybee syndrome - implications of the teratogenicity of mannose in rat-embryo culture

Lethal effects of D-mannose in the honeybee have been recognized for more than a half a century. We observed another toxic effect of D- mannose during culture of rat embryos from the early head-fold stage to the 26-to-29-somite stage (Days 9-1/2 through 11-1 of gestation). The addition to culture mediums of 1.5 mg of D-mannose per milliliter caused growth retardation and faulty neural-tube closure in approximately two thirds of the embryos. Mannose effects occurred during the first 24 hours of culture and were attended by modes inhibition of the glycolysis that constitutes the principal energy pathway at this stage of development. Adding more glucose to preserve glycolytic flux or increasing atmospheric oxygen to promote oxidative metabolism offset the mannose teratogenesis. Our findings highlight the metabolic vulnerabilities that exist during early organogenesis, before oxidative flexibility is established. They may serve as a model to explain the teratogenicity of many other seemingly unrelated agents that could act by perturbing glycolysis at this vulnerable stage.

Whole embryo culture: a screening technique for teratogens?

Head-fold and early somite stages of mouse and rat embryos maintained in whole-embryo culture throughout much of the period of organogenesis demonstrate normal growth and morphogenesis. Embryos directly exposed to teratogens in the culture system and embryos cultured in serum from adult animals treated with toxic compounds develop congenital abnormalities that resemble malformations induced in vivo by these same agents. Furthermore, the defects are dose- and stage-dependent, such that higher doses produce a greater percentage of malformed embryos, and younger embryos are more susceptible than older ones. These results--together with the observations that 1) data are rapidly produced, 2) quantifiable endpoints can be measured in mammalian systems at costs considerably below those inherent in in vivo analyses, and 3) the potential exists for monitoring serum toxicity in humans and primates--suggest that the whole-embryo culture system may be useful as a screening technique for potentially teratogenic substances.