Heart anatomy and developmental biology

Embryonic Development of Heart in Indian Buffalo (Bubalus bubalis)

The present study was conducted on 35 buffalo foetuses from 0.9 cm CVRL (32 days) to 99.5 cm CVRL (298 days) to observe the morphogenesis and histogenesis of heart. The study revealed that, in 0.9 cm CVRL buffalo foetus, heart was unseptated and tubular which was clearly divided into common atrial chamber dorsally, primitive ventricles ventrally, primitive outflow tract with bulbous cordis region proximally, and aortic sac distally at 1.2 cm CVRL. Septum primum appeared at 1.9 cm CVRL whereas the truncal swellings and fold of interventricular septum appeared at 2.5 cm CVRL foetus. At 3.0 cm CVRL septum primum, endocardial cushions, septum secundum, and foramen ovale were observed. At 7.6 cm CVRL the endocardial cushions fused to form right and left atrioventricular openings and ventricular apex became pointed. Interventricular canal was obliterated and four-chambered heart was recognised along with atrioventricular valve, chordae tendineae, and papillary muscles in 8.7 cm CVRL (66 days) buffalo foetus. The endocardium as well as epicardium of the atria was thicker as compared to ventricle, whereas the myocardium of atria was thin as compared to ventricles in all the age groups. All the internal structures of heart were well differentiated from 50 cm CVRL onwards. The detailed structural components of buffalo heart during prenatal period have been discussed in the present paper.

Molecular mechanism of ventricular trabeculation/compaction and the pathogenesis of the left ventricular noncompaction cardiomyopathy (LVNC)

Ventricular trabeculation and compaction are two of the many essential steps for generating a functionally competent ventricular wall. A significant reduction in trabeculation is usually associated with ventricular compact zone deficiencies (hypoplastic wall), which commonly leads to embryonic heart failure and early embryonic lethality. In contrast, hypertrabeculation and lack of ventricular wall compaction (noncompaction) are closely related defects in cardiac embryogenesis associated with left ventricular noncompaction (LVNC), a genetically heterogenous disorder. Here we review recent findings through summarizing several genetically engineered mouse models that have defects in cardiac trabeculation and compaction.

Computational modeling of cardiac growth in the post-natal rat with a strain-based growth law

INTRODUCTION

The postnatal heart grows mostly in response to increased hemodynamic load. However, the specific biomechanical stimuli that stimulate cardiac growth as a reaction to increased hemodynamic load are still poorly understood. It has been shown that isolated neonatal rat cardiac myocytes normalize resting sarcomere length by adding sarcomeres in series when subjected to uniaxial static strain. Because there is experimental evidence that myocytes can distinguish the direction of stretch, it was postulated that myocytes also may normalize interfilament lattice spacing as a response to cross-fiber stretch.

METHODS

A growth law was proposed in which fiber axial growth was stimulated by fiber strain deviating from zero and fiber radial growth by cross-fiber strain (parallel to the wall surface) deviating from zero. Fiber radial growth rate constant was 1/3 of the fiber axial growth rate constant. The growth law was implemented in a finite element model of the newborn Sprague-Dawley rat residually stressed left ventricle (LV). The LV was subjected to an end-diastolic pressure of 1 kPa and about 25 weeks of normal growth was simulated.

RESULTS

Most cellular and chamber dimension changes in the model matched experimentally measured ones: LV cavity and wall volume increased from 2.3 and 54 μl, respectively, in the newborn to 276 μl and 1.1 ml, respectively, in the adult rat; LV shape became more spherical; internal LV radius increased faster than wall thickness; and unloaded sarcomere lengths exhibited a transmural gradient. The major discrepancy with experiments included a reversed transmural gradient of cell length in the older rat.

CONCLUSION

A novel strain-based growth law has been presented that reproduced physiological postnatal growth in the rat LV.

Analysis of ventricular hypertrabeculation and noncompaction using genetically engineered mouse models

Ventricular trabeculation and compaction are two of the many essential steps for generating a functionally competent ventricular wall. A significant reduction in trabeculation is usually associated with ventricular compact zone deficiencies (hypoplastic wall), which commonly lead to embryonic heart failure and early embryonic lethality. In contrast, hypertrabeculation and lack of ventricular wall compaction (noncompaction) are closely related defects in cardiac embryogenesis associated with left ventricular noncompaction, a genetically heterogeneous disorder. Here we summarize our recent findings through the analyses of several genetically engineered mouse models that have defects in cardiac trabeculation and compaction. Our data indicate that cellular growth and differentiation signaling pathways are keys in these ventricular morphogenetic events.

Subdivision of the cardiac Nkx2.5 expression domain into myogenic and nonmyogenic compartments

Nkx2.5 is expressed in the cardiogenic mesoderm of avian, mouse, and amphibian embryos. To understand how various cardiac fates within this domain are apportioned, we fate mapped the mesodermal XNkx2.5 domain of neural tube stage Xenopus embryos. The lateral portions of the XNkx2.5 expression domain in the neural tube stage embryo (stage 22) form the dorsal mesocardium and roof of the pericardial cavity while the intervening ventral region closes to form the myocardial tube. XNkx2.5 expression is maintained throughout the period of heart tube morphogenesis and differentiation of myocardial, mesocardial, and pericardial tissues. A series of microsurgical experiments showed that myocardial differentiation in the lateral portion of the field is suppressed during normal development by signals from the prospective myocardium and by tissues located more dorsally in the embryo, in particular the neural tube. These signals combine to block myogenesis downstream of XNkx2.5 and at or above the level of contractile protein gene expression. We propose that the entire XNkx2.5/heart field is transiently specified as cardiomyogenic. Suppression of this program redirects lateral cells to adopt dorsal mesocardial and dorsal pericardial fates and subdivides the field into distinct myogenic and nonmyogenic compartments.

Dickkopf genes are co-ordinately expressed in mesodermal lineages

Dickkopf-1 (dkk-1) is member of a novel family of secreted proteins and functions in head induction during Xenopus embryogenesis, acting as a potent inhibitor of Wnt signalling. Here we report: (1) the isolation of two additional murine members of the dkk family, dkk-2 and dkk-3; and (2) analysis of adult and embryonic gene expression of mouse dkk-1,-2, and -3, Xenopus dkk-1 as well as chicken dkk-3. Comparative developmental analyses of the dkk-1, dkk-2 and dkk-3 in mice indicate that these genes are both temporally and spatially regulated. They define overlapping deep domains in mesenchymal lineages suggesting a co-ordinated mode of action. All dkks show distinct and elevated expression patterns in tissues that mediate epithelial- mesenchyme transformations suggesting that they may participate in heart, tooth, hair and whisker follicle, limb and bone induction. In the limb buds expression of these genes are found in regions of programmed cell death. In a given organ, dkk-1 tends to be the earliest member expressed. Comparison with Xenopus dkk-1 and chicken dkk-3 shows evolutionarily conserved expression patterns. Our observations indicate that dkk genes constitute a new family of secreted proteins that may mediate inductive interactions between epithelial and mesenchymal cells.

Immediate postnatal rat heart development modified by abdominal aortic banding: analysis of gene expression

Proliferative growth of the ventricular myocyte (cardiomyocyte) is primarily limited to embryonic, fetal and very early neonatal periods of heart development. In contrast, cardiomyocyte maturation, as evidenced by cellular hypertrophy, is a long-term process that can occupy the bulk of the life-span of the mature organism. As the newborn myocyte undergoes a 'transition' from proliferative to hypertrophic growth, ventricular remodeling of the non-myocyte compartment is characterized by increased extracellular matrix (ECM) formation and coronary capillary angiogenesis. A role for ventricular-derived growth factors (GFs) in these inter-related processes are examined in an animal model of altered heart development produced by neonatal aortic banding. The suprarenal abdominal aorta of five day old rat pups were banded (B), sham operated (S), or untreated (C) and ventricular tissue (left ventricular free wall and septum) obtained at 7-, 14-, and 21-days post-intervention. Using Northern blot RNA hybridizations, expression of growth factors (GFs) and/or GF-receptors (GFR's) temporally associated with heart development were evaluated. Transcript levels for TGF-beta 1, IGF-II, and their associated cell surface receptors were increased in B animals. Concomitant changes in extracellular matrix (ECM) genes (as evaluated by Collagens Type I, III, and IV) were also increased in B animals. In addition, transcript levels for the vascular morphogenesis and remodeling-related protein SPARC (Secreted Protein, Acidic and Rich in Cysteine) was also elevated in the B animals. In several instances, S animals demonstrated changes in steady state transcript levels for genes which may influence myocyte maturation during the postnatal period. This suggests that normal autocrine/paracrine growth regulatory stimuli and responses can be modified (by surgical intervention and/or abdominal aortic banding) and these perturbations in gene expression may be related to previously documented changes in myocyte cell number, vascular composition, and ventricular architecture of the banded, neonatal heart. Future studies using this model will provide an opportunity to evaluate and possibly identify the stimuli and signal transduction machinery that regulate the final phases of myocyte proliferation, stimulate capillary formation and ECM deposition, and orchestrate the transition to hypertrophic growth during heart development.

Expression of intermediate filament desmin and vimentin in the human fetal heart

BACKGROUND

The intermediate filament (IF) desmin provides support for contractile machinery in muscle cells, and vimentin plays an important role in maintaining the stability of mesenchymal cells and in signal transduction. However, development of IFs in heart tissue during intrauterine life in human is not well established.

METHODS

In the present study, development of desmin and vimentin in human fetal hearts aged 9-28 weeks of gestation (n = 41) were investigated by immunohistochemistry with monoclonal antibodies against desmin and vimentin. Relative density of fluorescence of each sample was determined by densitometry. Left ventricle (LV) tissues from a 1-year-old child (n = 1) were examined by immunohistochemistry for postnatal comparison. Western blot analyses were done with only a few randomly selected LV tissues from fetuses of 9, 20, and 28 weeks gestation to assess trends of desmin and vimentin expression.

RESULTS

By Western blot analyses, 53-kDa desmin and 54-kDa vimentin were present in all fetal heart tissues examined. Desmin intensity was progressively increased with increasing fetal age, whereas vimentin intensity decreased. Desmin was present only in cardiomyocytes. In the earlier period (10-14 weeks gestation), desmin was localized along the cardiomyocyte membrane and/or Z lines in regular intervals, and later (25-28 weeks gestation) it was structurally well integrated; however, its network was incomplete. Only cardiomyocytes from a 1-year-old child revealed highly developed and integrated desmin lattices. However, vimentin was present in the mesenchymal tissue including fibroblasts and surrounding blood vessels. In part, some cardiomyocytes showed a weakly positive reaction with monoclonal antibody against vimentin in 9-14 weeks gestation. Vimentin-positive areas, however, were progressively diminished with increasing fetal age. Vimentin was present only in the connective tissue and coverings of the 1-year-old child's heart. Relative density of fluorescence of desmin was increased with increasing fetal age, whereas that of vimentin decreased.

CONCLUSIONS

These results indicate that there is a fetal age (or gestation)-dependent expression of IFs in human fetal heart: desmin increases with increasing fetal age, whereas vimentin decreases.

Developmental analysis of tropomyosin gene expression in embryonic stem cells and mouse embryos

Tropomyosins (TMs) comprise a family of actin-binding proteins which play an important role in the regulation of contractility in muscle (cardiac, skeletal, and smooth) and nonmuscle cells. Although they are present in all cells, different isoforms are characteristic of specific cell types. In vertebrates, there are four different TM genes (alpha-TM, beta-TM, TM30, and TM4), three of which generate alternatively spliced isoforms. This study defines the expression patterns of these isoforms during murine embryogenesis, using both in vivo and in vitro conditions. The embryonic stem cell culture system, which has been shown to mimic different stages of mouse embryonic development, including the differentiation of primitive organ systems such as the myocardium, is used for our in vitro analysis. Our results demonstrate that several TM isoforms are expressed in specific developmental patterns, often correlated with the differentiation of particular tissues or organs. Surprisingly, other TMs, such as the striated muscle beta-TM and smooth muscle alpha-TM, are expressed constitutively. This study also demonstrates that there is an excellent correlation between the expression patterns of the TM isoforms observed in developing embryonic stem cells and mouse embryos. In addition, a quantitative molecular analysis of TM isoforms was conducted in embryonic, neonatal, and adult cardiac tissue. Our results show for the first time that the alpha- and beta-TM striated muscle transcripts are present in the earliest functional stages of the heart, and these TM isoforms are identical to those present throughout cardiac development.

Developmental analysis of tropomyosin gene expression in embryonic stem cells and mouse embryos

Tropomyosins (TMs) comprise a family of actin-binding proteins which play an important role in the regulation of contractility in muscle (cardiac, skeletal, and smooth) and nonmuscle cells. Although they are present in all cells, different isoforms are characteristic of specific cell types. In vertebrates, there are four different TM genes (alpha-TM, beta-TM, TM30, and TM4), three of which generate alternatively spliced isoforms. This study defines the expression patterns of these isoforms during murine embryogenesis, using both in vivo and in vitro conditions. The embryonic stem cell culture system, which has been shown to mimic different stages of mouse embryonic development, including the differentiation of primitive organ systems such as the myocardium, is used for our in vitro analysis. Our results demonstrate that several TM isoforms are expressed in specific developmental patterns, often correlated with the differentiation of particular tissues or organs. Surprisingly, other TMs, such as the striated muscle beta-TM and smooth muscle alpha-TM, are expressed constitutively. This study also demonstrates that there is an excellent correlation between the expression patterns of the TM isoforms observed in developing embryonic stem cells and mouse embryos. In addition, a quantitative molecular analysis of TM isoforms was conducted in embryonic, neonatal, and adult cardiac tissue. Our results show for the first time that the alpha- and beta-TM striated muscle transcripts are present in the earliest functional stages of the heart, and these TM isoforms are identical to those present throughout cardiac development.

Ontogenic activities and interactions of the lactate dehydrogenase isozymes with cellular structure in the guinea pig

The LDH activities and isozyme distributions associated with soluble and particulate fractions of five major tissues have been followed during the development of the guinea pig. Evidence has been provided for an appreciable degree of interaction between LDH and cellular structure in all these tissues (liver, kidney, skeletal muscle, brain and heart) at all stages of development, but particularly in the early foetal stages. These data have been discussed in relation to the nature and extent of this binding and the correlations with the metabolic emphases in these tissue situations during development.

Impaired elastic matrix development in the great arteries after ablation of the cardiac neural crest

The cells that form the aorticopulmonary septum in the avian embryo have been shown to be similar to the cells that form the walls of the great vessels in two ways: both are derived from the cardiac neural crest and both are able to synthesize an elastogenic matrix in the early embryo. Because of these similarities, and because ablation of the cardiac neural crest causes congenital defects of the outflow tract that are related to failure of proper septation, it was hypothesized that such an ablation also would cause the walls of the great vessels to be defective. The purpose of this study was to compare the elastic matrix in the mediae of the great vessels of normal embryos with those from which the cardiac neural crest had been ablated. The results show that the elastic matrix in the great vessels of the experimental embryos was impaired 1) in the rate of downstream propagation of the initiation of elastogenesis among younger embryos, incubation days 4-8 and 2) in the spatial configuration of the elastic matrix among the older embryos, incubation days 16-20. These results may provide a biological explanation for the elastin defect that affects the pulmonary artery of many patients with cyanotic congenital heart defects.

Age-related methylation changes in DNA may reflect the proliferative potential of organs

The percentage of 5-methylcytosine in DNA was measured in brain, liver, heart and skeletal muscle of the rat at various ages. Age-related hypomethylation occurred rapidly shortly after birth and then declined to eventually stabilize in brain, heart and skeletal muscle. Hypomethylation in liver DNA continued throughout the period studied (6 months). Our hypothesis that the age-related decline of 5-methylcytosine content in DNA is related to the proliferative potential of organs is discussed.

From the head to the heart: some thoughts on similarities between brain function and morphogenesis, and on their significance for research methodology and biological theory

A broad review of the phenomena of morphogenesis and of brain function, and of the history of research in these two areas, suggests that there are quite striking similarities between the two sets of biological phenomena. Among other things, both reflect the interaction of internally complex components at several levels of organization, display variance as an essential characteristic, and incorporate information from the environment. It is argued that reductionist approaches are inadequate to deal with fundamental problems of either morphogenesis or brain function, and alternative foundations for research strategy and tactics are discussed. Attention is also given to the question of why morphogenesis and brain function are so similar, and it is suggested that this may reflect the existence of rules of information acquisition, transmission, and storage to which both are subject. Variance, it is argued, is an essential component of information acquisition processes, and hence of biological integrity, at all levels of organization.