The local expression of adult chicken heart myosins during development. II. Ventricular conducting tissue

An Appreciation of Anatomy in the Molecular World

Robert H. Anderson is one of the most important and accomplished cardiac anatomists of the last decades, having made major contributions to our understanding of the anatomy of normal hearts and the pathologies of acquired and congenital heart diseases. While cardiac anatomy as a research discipline has become largely subservient to molecular biology, anatomists like Professor Anderson demonstrate anatomy has much to offer. Here, we provide cases of early anatomical insights on the heart that were rediscovered, and expanded on, by molecular techniques: migration of neural crest cells to the heart was deduced from histological observations (1908) and independently shown again with experimental interventions; pharyngeal mesoderm is added to the embryonic heart (1973) in what is now defined as the molecularly distinguishable second heart field; chambers develop from the heart tube as regional pouches in what is now considered the ballooning model by the molecular identification of regional differentiation and proliferation. The anatomical discovery of the conduction system by Purkinje, His, Tawara, Keith, and Flack is a special case because the main findings were never neglected in later molecular studies. Professor Anderson has successfully demonstrated that sound knowledge of anatomy is indispensable for proper understanding of cardiac development.

The Anatomy, Development, and Evolution of the Atrioventricular Conduction Axis

It is now well over 100 years since Sunao Tawara clarified the location of the axis of the specialised myocardium responsible for producing coordinated ventricular activation. Prior to that stellar publication, controversies had raged as to how many bundles crossed the place of the atrioventricular insulation as found in mammalian hearts, as well as the very existence of the bundle initially described by Wilhelm His Junior. It is, perhaps surprising that controversies continue, despite the multiple investigations that have taken place since the publication of Tawara’s monograph. For example, we are still unsure as to the precise substrates for the so-called slow and fast pathways into the atrioventricular node. Much has been done, nonetheless, to characterise the molecular make-up of the specialised pathways, and to clarify their mechanisms of development. Of this work itself, a significant part has emanated from the laboratory coordinated for a quarter of a century by Antoon FM Moorman. In this review, which joins the others in recognising the value of his contributions and collaborations, we review our current understanding of the anatomy, development, and evolution of the atrioventricular conduction axis.

The "Dead-End Tract" and Its Role in Arrhythmogenesis

Idiopathic outflow tract ventricular arrhythmias (VAs) represent a significant proportion of all VAs. The mechanism is thought to be catecholamine-mediated delayed after depolarizations and triggered activity, although other etiologies should be considered. In the adult cardiac conduction system it has been demonstrated that sometimes an embryonic branch, the so-called "dead-end tract", persists beyond the bifurcation of the right and left bundle branch (LBB). Several findings suggest an involvement of this tract in idiopathic VAs (IVAs). The aim of this review is to summarize our current knowledge and the possible clinical significance of this tract.

The effects of an environmentally relevant 58-congener polychlorinated biphenyl (PCB) mixture on cardiac development in the chick embryo

Chicken (Gallus domesticus) embryonic exposure in ovo to a 58-congener polychlorinated biphenyl (PCB) mixture resulted in teratogenic heart defects in chick embryos at critical heart developmental stages Hamburger-Hamilton (HH) stages 10, 16, and 20. The 58-congener mixture contained relative proportions of primary congeners measured in belted sandpiper (Megaceryle alcyon) and spotted sandpiper (Actitis macularia) eggs collected along the upper Hudson River, New York, USA, and chicken doses were well below observed environmental exposure levels. Embryos were injected with 0.08 µg PCBs/g egg weight and 0.50 µg PCBs/g egg weight (0.01 and 0.064 ng toxic equivalent/g, respectively) at embryonic day 0, prior to incubation. Mortality of exposed embryos was increased at all developmental stages, with a marked rise in cardiomyopathies at HH16 and HH20 (p < 0.05). Heart abnormalities occurred across all treatments, including abnormal elongation and expansion of the heart tube at HH10, improper looping and orientation, indentations in the emerging ventricular wall (HH16 and HH20), and irregularities in overall heart shape (HH10, HH16, and HH20). Histology was conducted on 2 cardiac proteins critical to embryonic heart development, ventricular myosin heavy chain and titin, to investigate potential mechanistic effects of PCBs on heart development, but no difference was observed in spatiotemporal expression. Similarly, cellular apoptosis in the developing heart was not affected by exposure to the PCB mixture. Conversely, cardiomyocyte proliferation rates dramatically declined (p < 0.01) at HH16 and HH20 as PCB exposure concentrations increased. Early embryonic cardiomyocyte proliferation contributes to proper formation of the morphology and overall thickness of the ventricular wall. Therefore, in ovo exposure to this 58-congener PCB mixture at critical stages adversely affects embryonic heart development.

Embryological development of pacemaker hierarchy and membrane currents related to the function of the adult sinus node: implications for autonomic modulation of biopacemakers

The sinus node is an inhomogeneous structure. In the embryonic heart all myocytes have sinus node type pacemaker channels (I (f)) in their sarcolemma. Shortly before birth, these channels disappear from the ventricular myocytes. The response of the adult sinus node to changes in the interstitium, in particular to (neuro)transmitters, results from the interplay between the responses of all of its constituent cells. The response of the whole sinus node cannot be simply deduced from these cellular responses, because all cells have different responses to specific agonists. A biological pacemaker will be more homogeneous. Therefore it can be anticipated that tuning of cycle length may be problematic. It is discussed that efforts to create a biological pacemaker responsive to vagal stimulation, may be counterproductive, because it may have the potential risk of 'standstill' of the biological pacemaker. A normal sinus node remains spontaneously active at high concentrations of acetylcholine, because it has areas that are unresponsive to acetylcholine. The same is pertinent to other substances with a negative chronotropic effect. Such functional inhomogeneity is lacking in biological pacemakers.

Developmental expression and comparative genomic analysis of Xenopus cardiac myosin heavy chain genes

Myosin heavy chains (MHC) are cytoskeletal motor proteins essential to the process of muscle contraction. We have determined the complete sequences of the Xenopus cardiac MHC genes, alpha-MHC and ventricular MHC (vMHC), and have characterized their developmental expression profiles. Whereas alpha-MHC is expressed from the earliest stages of cardiac differentiation, vMHC transcripts are not detected until the heart has undergone chamber formation. Early expression of vMHC appears to mark the cardiac conduction system, but expression expands to include the ventricle and outflow tract myocardium during subsequent development. Sequence comparisons, transgenic expression analysis, and comparative genomic studies indicate that Xenopus alpha-MHC is the true orthologue of the mammalian alpha-MHC gene. On the other hand, we show that the Xenopus vMHC gene is most closely related to chicken ventricular MHC (vMHC1) not the mammalian beta-MHC. Comparative genomic analysis has allowed the detection of a mammalian MHC gene (MyH15) that appears to be the orthologue of vMHC, but evidence suggests that this gene is no longer active.

Spatiotemporal pattern of commitment to slowed proliferation in the embryonic mouse heart indicates progressive differentiation of the cardiac conduction system

Patterns of DNA synthesis in the developing mouse heart between ED7.5-18.5 were studied by a combination of thymidine and bromodeoxyuridine labeling techniques. From earliest stages, we found zones of slow myocyte proliferation at both the venous and arterial poles of the heart, as well as in the atrioventricular region. The labeling index was distinctly higher in nonmyocardial populations (endocardium, epicardium, and cardiac cushions). Ventricular trabeculae showed lower proliferative activity than the ventricular compact layer after their appearance at ED9.5. Low labeling was observed in the pectinate muscles of the atria from ED11.5. The His bundle, bundle branches, and Purkinje fiber network likewise were distinguished by their lack of labeling. Thymidine birthdating (label dilution) showed that the cells in these emerging components of the cardiac conduction system terminally differentiated between ED8.5-13.5. These patterns of slowed proliferation correlate well with those in other species, and can serve as a useful marker for the forming conduction system.

Hemodynamics is a key epigenetic factor in development of the cardiac conduction system

The His-Purkinje system (HPS) is a network of conduction cells responsible for coordinating the contraction of the ventricles. Earlier studies using bipolar electrodes indicated that the functional maturation of the HPS in the chick embryo is marked by a topological shift in the sequence of activation of the ventricle. Namely, at around the completion of septation, an immature base-to-apex sequence of ventricular activation was reported to convert to the apex-to-base pattern characteristic of the mature heart. Previously, we have proposed that hemodynamics and/or mechanical conditioning may be key epigenetic factors in development of the HPS. We thus hypothesized that the timing of the topological shift marking maturation of the conduction system is sensitive to variation in hemodynamic load. Spatiotemporal patterns of ventricular activation (as revealed by high-speed imaging of fluorescent voltage-sensitive dye) were mapped in chick hearts over normal development, and following procedures previously characterized as causing increased (conotruncal banding, CTB) or reduced (left atrial ligation, LAL) hemodynamic loading of the embryonic heart. The results revealed that the timing of the shift to mature activation displays striking plasticity. CTB led to precocious emergence of mature HPS function relative to controls whereas LAL was associated with delayed conversion to apical initiation. The results from our study indicate a critical role for biophysical factors in differentiation of specialized cardiac tissues and provide the basis of a new model for studies of the molecular mechanisms involved in induction and patterning of the HPS in vivo.

Immunohistochemical demonstration of Leu-7 (HNK-1), Neurone-specific Enolase (NSE) and Protein-Gene Peptide (PGP) 9.5 in the developing camel (Camelus dromedarius) heart

The development of the heart-conducting system has been controversially discussed. The common opinion that these specialized myocytes originate from mesodermal precursors has been challenged when nerve-specific antigens (Leu-7, NF, GIN2) were demonstrated in embryonic hearts of various species, suggesting a neural crest contribution to the embryonic conducting tissue. Anti-Leu-7 (HNK-1) antibodies were reported to reliably mark the conducting system in developing rat, chicken and human hearts. The present investigation was carried out on the hearts of 15 camel fetuses at 35, 45, 60, 75 and 100 cm crown-rump length (three specimens for each stage), in addition to three adult hearts. We investigated the antigenicity of cardiac structures for Leu-7, NSE (Neurone specific Enolase) and PGP (Protein Gene Peptide) 9.5. In all specimens investigated, both NSE and PGP 9.5 were expressed by cardiac nerves and conducting system components. The sinuatrial and atrioventricular nodes, the atrioventricular bundle as well as subendocardial and intramyocardial Purkinje fibers were stained. In contrast, the developing conducting system did not react with anti-Leu-7 antibody, although Leu-7 antigenicity was strongly expressed by the developing cardiac nerves. In adult camel hearts, the same pattern of immunoreactivity for the markers studied was still retained. Our results show that the expression of marker proteins for the developing conducting system is species-specific. Therefore, these markers are of little significance in discussions on the possible neurogenic nature of the heart conducting tissue.

Cell biology of cardiac development

Building a vertebrate heart is a complex task and involves several tissues, including the myocardium, endocardium, neural crest, and epicardium. Interactions between these tissues result in the changes in function and morphology (and also in the extracellular matrix, which serves as a substrate for morphological change) that are requisite for development of the heart. Some of the signaling pathways that mediate these changes have now been identified and several investigators are now filling in the missing pieces in these pathways in hopes of ultimately understanding the molecular mechanisms that govern healthy heart development. In addition, transcription factors that regulate various aspects of heart development have been identified. Transcription factors of the GATA and Nkx2 families are of particular importance for early specification of the heart field and for regulating expression of genes that encode proteins of the contractile apparatus. This chapter highlights some of the most significant discoveries made in the rapidly expanding field of heart development.

Conducting the embryonic heart: orchestrating development of specialized cardiac tissues

The heterogeneous tissues of the pacemaking and conduction system comprise the "smart components" of the heart, responsible for setting, maintaining, and coordinating the rhythmic pumping of cardiac muscle. Over the last few years, a wealth of new information has been collected about the unique genetic and phenotypic characteristics expressed by these tissues during cardiac morphogenesis. More recently, genetically modified viruses, mutational analysis, and targeted transgenesis have enabled even more precise resolution of the relationships between cell fate, gene expression, and differentiation of specialized function within developing myocardium. While some information provided by these newer approaches has supported conventional wisdom, some fresh and unexpected perspectives have also emerged. In particular, there is mounting evidence that extracardiac populations of cells migrating into the tubular heart have important morphogenetic roles in the inductive pattering and functional integration of the developing conduction system.

Differential expression of PSA-NCAM and HNK-1 epitopes in the developing cardiac conduction system of the chick

Although the critical role of the His-Purkinje system (HPS) in the propagation of cardiac action potentials from the atria to the ventricular myocardium in the mature heart is well appreciated, its functional and anatomical development are not well understood. The embryonic heart begins beating early in development devoid of a mature conduction system, and the HPS cannot be identified by conventional histochemical analysis until the seventh embryonic day of chicken development. Although many biochemical markers have been found that apparently identify HPS precursors, little is known about how these biochemical markers function in the maturation of the cardiac conduction system. Using immunohistological techniques, we demonstrated that the maturation of the HPS may be observed by the expression of two distinct populations of conduction system precursors, identified by the expression of cell surface carbohydrates PSA-NCAM (PSA) and HNK-1, both of which are known to participate in cell-cell and cell-substrate interactions in the development of the nervous system. By stage 25, PSA was detected in ventricular trabeculae and the interventricular septum (IVS) in a pattern that resembles bundle branches and Purkinje fibers. Beginning at stage 28, HNK-1 epitope expression in the IVS was observed in myocardium just superior to the PSA-positive bundles in a pattern that resembles the common His bundle. This junctional region was also positive for atrial myosin heavy chain (alpha MHC), another marker for the HPS. These data suggest that a complex, coordinated process of biochemical and morphological change governs the development of the cardiac conduction system.

Cytokeratins as a marker for epicardial formation in the quail embryo

Several techniques have been used to visualize the migration pattern of the epicardial cells from the proepicardial organ over the myocardial surface. As the epicardial cells contain keratin tonofilament bundles, we have incubated 92 whole-mount quail hearts with an anti-keratin antibody. This immunohistochemical method showed that the complete epicardial covering of the embryonic heart is preceded by the formation of three epicardial rings. The epicardial rings are formed on the outer myocardial surface in the grooves that separate the cardiac segments from each other. We have also documented timing and patterning of isolated epicardial islands. They are not encountered at random over the myocardial surface, but only along the edge of the advancing epicardial front border and in two defined future epicardial ring areas on the ventral side of the outflow tract. The epicardial islands suggest that in the quail free-floating parts of epicardium can attach to the myocardium. Characteristics of the surface of the myocardium at the transitional zones between the cardiac segments, as well as the three-dimensional remodelling of the heart during cardiac morphogenesis seem to play a role in the pattern in which the epicardium eventually completely ensheaths the myocardial surface. Congenital heart defects are often related to malpositioned transitional zones that dictate the pattern of epicardial outgrowth. As the embryonic position of the epicardial rings is mirrored in the pattern of the main arterial stems, the coronary vascularization pattern might be altered in congenitally malformed hearts as well.

Terminal diversification of the myocyte lineage generates Purkinje fibers of the cardiac conduction system

The rhythmic contraction of the vertebrate heart is dependent on organized propagation of electrical excitation through the cardiac conduction system. Because both muscle- and neuron-specific genes are co-expressed in cells forming myocardial conduction tissues, two origins, myogenic and neural, have been suggested for this specialized tissue. Using replication-defective retroviruses, encoding recombinant beta-galactosidase (beta-gal), we have analyzed cell lineage for Purkinje fibers (i.e., the peripheral elements of the conduction system) in the chick heart. Functioning myocyte progenitors were virally tagged at embryonic day 3 of incubation (E3). Clonal beta-gal+ populations of cells, derived from myocytes infected at E3 were examined at 14 (E14) and 18 (E18) days of embryonic incubation. Here, we report that a subset of clonally related myocytes differentiates into conductile Purkinje fibers, invariably in close spatial association with forming coronary arterial blood vessels. These beta-gal+ myogenic clones, containing both working myocytes and Purkinje fibers, did not incorporate cells contributing to tissues of the central conduction system (e.g. atrioventricular ring and bundles). In quantitative analyses, we found that whereas the number of beta-gal+ myocyte nuclei per clone more than doubled between E14 and E18, the number of beta-gal+ Purkinje fiber nuclei remained constant.(ABSTRACT TRUNCATED AT 250 WORDS)

Persisting zones of slow impulse conduction in developing chicken hearts

We performed a correlative electrophysiological and immunohistochemical study of embryonic chicken hearts during the septational period (Hamburger and Hamilton stages 13-31 [2-7 days of incubation]). The analyses yield conclusive evidence for slow conduction, up to 7 days of development, in the outflow tract, in the atrioventricular canal, and in the sinoatrial junction. The conduction velocity remains approximately 1 cm/sec in the outflow tract and increases in the ventricle 20-fold to approximately 20 cm/sec between 2 and 7 days of development. Transmembrane potentials of myocytes in the outflow tract and atrioventricular canal slowly rise (less than 5 V/sec), whereas in the atrium and ventricle, the upstroke velocity is eightfold to 13-fold higher. In the outflow tract, repolarization is completed only after the start of the next cycle. Because of the persistence of slow conduction, the myocardium flanking the developing atria and ventricle is thought to represent segments of persisting "primary" myocardium, whereas the more rapidly conducting "working" myocardium of the ventricle and atria is thought to represent more advanced stages of myocardial differentiation. The persisting primary myocardium was characterized by a continued coexpression of both the atrial and ventricular isoforms of myosin heavy chain. The developing atria and ventricle could be demarcated morphologically from the primary myocardium because the free walls of these segments only express their respective isoforms of myosin heavy chain. The slowly conducting myocardial zones appear to be essential for the function of the embryonic heart because 1) they provide the septating heart with alternating segments of slow and relatively fast conduction necessary for consecutive contraction of the atrial and ventricular segments and 2) their sphincterlike prolonged peristaltic contraction pattern can substitute for the adult type of one-way valves that start to develop at the end of septation.

Cardiac expression of polysialylated NCAM in the chicken embryo: correlation with the ventricular conduction system

The neural cell adhesion molecule (NCAM) and its polysialic acid moeity (PSA) affect cellular interactions during the development of the nervous system and skeletal muscle. NCAM has also been identified in the embryonic heart of various species including humans. However, knowledge regarding the role of NCAM and its function-modulating PSA in cardiogenesis is limited. The distribution of NCAM and its PSA in the ventricular myocardium of chicken embryos was determined by indirect immunofluorescence staining. The NCAM polypeptide was found throughout the cardiac myocardium. In contrast PSA was located in discrete regions in stage 20 to 44 embryos (during and after septation). Myocardium at the subendocardial regions of the atrioventricular canal and ventricular trabeculae were PSA positive by stage 20. At later stages, transverse sections of the postseptation heart just below the level of the atrioventricular interface revealed a PSA-positive bundle of myocardium in the septum. This bundle was continuous with two branches at a more apical level which in turn were continuous with the PSA-positive subendocardial myocardium lining the left and right ventricles. This pattern of PSA in the myocardium was similar to that of the ventricular conduction system configuration defined in the adult heart. Electron micrographs of the subendocardium of the ventricular septum revealed PSA positivity on myofibril-containing cells with the ultrastructural location of Purkinje fibers. At later stages (35-44) a subset of cells within PSA-positive regions was stained by an antibody against an isoform of the myosin heavy chain found in adult Purkinje fibers. These cells and surrounding tissue lacked PSA in the adult heart. Thus polysialylated NCAM may be modulating cell-cell interactions during the development of the ventricular conduction system.

Structural and phylogenetic analysis of the chicken ventricular myosin heavy chain rod

We have isolated and characterized five overlapping clones that encompass 3.2 kb and encode a part of the short subfragment 2, the hinge, and the light meromyosin regions of the myosin heavy chain rod as well as 143 bp of the 3' untranslated portion of the mRNA. Northern blot analysis showed expression of this mRNA mainly in ventricular muscle of the adult chicken heart, with trace levels detected in the atrium. Transient expression was seen in skeletal muscle during development and in regenerating skeletal muscle following freeze injury. To our knowledge, this is the first report of an avian ventricular myosin heavy chain sequence. Phylogenetic analysis indicated that this isoform is a distant homolog of other ventricular and skeletal muscle myosin heavy chains and represents a distinct member of the multigene family of sarcomeric myosin heavy chains. The ventricular myosin heavy chain of the chicken is either paralogous to its counterpart in other vertebrates or has diverged at a significantly higher rate.

Spatial distribution of "tissue-specific" antigens in the developing human heart and skeletal muscle. II. An immunohistochemical analysis of myosin heavy chain isoform expression patterns in the embryonic heart

The spatial distribution of alpha- and beta-myosin heavy chain isoforms (MHCs) was investigated immunohistochemically in the embryonic human heart between the 4th and the 8th week of development. The development of the overall MHC isoform expression pattern can be outlined as follows: (1) In all stages examined, beta-MHC is the predominant isoform in the ventricles and outflow tract (OFT), while alpha-MHC is the main isoform in the atria. In addition, alpha-MHC is also expressed in the ventricles at stage 14 and in the OFT from stage 14 to stage 19. This expression pattern is very reminiscent of that found in chicken and rat. (2) In the early embryonic stages the entire atrioventricular canal (AVC) wall expresses alpha-MHC whereas only the lower part expresses beta-MHC. The separation of atria and ventricles by the fibrous annulus takes place at the ventricular margin of the AVC wall. Hence, the beta-MHC expressing part of the AVC wall, including the right atrioventricular ring bundle, is eventually incorporated in the atria. (3) In the late embryonic stages (approx. 8 weeks of development) areas of alpha-MHC reappear in the ventricular myocardium, in particular in the subendocardial region at the top of the interventricular septum. These coexpressing cells are topographically related to the developing ventricular conduction system. (4) In the sinoatrial junction of all hearts examined alpha- and beta-MHC coexpressing cells are observed. In the older stages these cells are characteristically localized at the periphery of the SA node.

Isomyosin expression pattern during formation of the tubular chicken heart: a three-dimensional immunohistochemical analysis

Three-dimensional (3-D) distribution of atrial and ventricular isomyosins is analysed immunohistochemically during the formation of the tubular chicken heart (stage 7 to 12 [H/H]) using antibodies specific for adult chicken atrial and ventricular myosin heavy chains, respectively. This analysis revealed that both types of isomyosins can be first detected at stage 8 (H/H, possessing four pairs of somites), i.e., when the heart primordium still exists as two separate cardiogenic plates. The ventricular type of isomyosin is initially expressed in those areas of cardiogenic plates in the vicinity of the anterior intestinal portal. The atrial type of isomyosin is initially expressed in zones caudal and lateral to the areas of ventricular isomyosin expression. Medial to the atrial isomyosin-expressing areas, cardiogenic plate areas exist that initially lack myosin expression. Those parts of the cardiogenic plates that fuse in front of the anterior intestinal portal, thereby forming the heart tube, are characterized by the expression of both isomyosins; however, the caudolateral parts of the heart primordium maintain their single atrial isomyosin expression during further development. Cardiac contractions are therefore first observed at stage 10 (H/H, possessing ten pairs of somites) in myocardium that coexpresses both isomyosins.

Isomyosin expression patterns during rat heart morphogenesis: an immunohistochemical study

An immunohistochemical study of cardiac alpha and beta myosin heavy chain (MHC) expression during rat heart morphogenesis was performed. In tubular hearts (embryonic days, ED10-11) coexpression of both cardiac alpha and beta MHC was found throughout the heart, except for the left free wall of the atrium, where only cardiac alpha MHC is detected. A transition of coexpression to single expression of either cardiac alpha or beta MHC begins at the same time in both atria and ventricles but requires a longer time for completion in the ventricules; in the atria transition takes place during the period ED 12-13 and in the ventricles during ED12-15. Furthermore, expression of cardiac alpha and beta MHC was detected in the sinus venosus, and cardiac alpha MHC expression was detected in the pulmonary veins. A comparison of the results obtained in chicken embryos revealed that in tubular hearts the expression pattern is similar, whereas in later developmental stages two major differences were observed: 1) transition of coexpression to single expression in rat ventricles appears to take a longer developmental period; 2) the persistence of areas of coexpression in the sinoatrial junction, dorsal mesocardium, atrioventricular junction, and outflow tract, as found in the chicken embryo in later developmental stages, is not found in the rat heart.

Deformation-corrected computer-aided three-dimensional reconstruction of immunohistochemically stained organs: application to the rat heart during early organogenesis

The application of a computer-assisted, three-dimensional reconstruction procedure for serial sections to embryonic rat hearts during the period of cardiac looping and compartmentalization is described. The procedure relies on immunohistochemical staining for the introduction of selective contrast and on episcopic and diascopic images of each of the sections for alignment and correction of compression due to sectioning. Episcopic (reference) images are taken from the embedding block just before the cutting of a slice and are still aligned and undeformed. Diascopic images are taken from the sections after immunohistochemical processing and, hence, contain selective contrast but are deformed and no longer aligned. The three-dimensional images are visualized as shaded voxel models. This approach allowed the unequivocal delineation of the developing myocardium and the inspection of its changing architecture both from the outside and from within. Furthermore, it allowed a quantification of myocardial volume. Because standardized and hence comparable views of three different stages were generated, changes in the shape of the cardiac loop, the atria, and the ventricles as well as changes in the position of the atrioventricular canal and interventricular foramen could be accurately described. Characteristic changes in the position of both the right ventricle and the atrioventricular canal that are essential for the formation of a correctly functioning four-chambered heart could be observed. These changes in shape occur while the myocardial size increases dramatically.

Creatine kinase isozyme expression in embryonic chicken heart

The distribution pattern of creatine kinase (EC 2.7.3.2) isozymes in developing chicken heart was studied by immunohistochemistry. Creatine kinase M, which is absent from adult heart, is transiently expressed between 4 and 11 days of incubation. During that period, numerous muscular cells in the roof and septum of the atrium, in the interventricular septum and on top of the trabeculae cordis and at the rim of the outflow tract stain strongly with a polyclonal antibody that is specific for the M subunit. In the ventricle and outflow tract, the M-positive cells are found mainly subendocardially and in the right half, at the transition of conducting and working myocytes. Creatine kinase B, which is the predominant adult isozyme, is initially expressed to a high concentration in a small group of disperse myocardial cells in upstream part of the inflow tract. When compared to the expression pattern of cardiac myosin heavy chains, the observed creatine kinase expression pattern suggests that M-positive cells are mainly found in areas that participate in the formation of cardiac conductive tissue, whereas B-positive cells are first found in areas that are involved in the generation of cardiac rhythm.

A molecular view of cardiogenesis

Cardiac development involves a complex integration of subcellular processes into multicellular and, finally, whole organ effects. Until recently it has been difficult to investigate the genetic control of this organ level differentiation of the heart. The proliferation of molecular biology methodologies has provided mechanisms to directly investigate the control of these processes. This article focuses on molecular lines of research on two key areas in cardiac development: the regulation of expression of sarcomeric contractile and regulatory proteins, and atrial natriuretic factor. Molecular approaches are described which have allowed investigators to begin to determine the tissue and stage-specific expression of genes, to locate those genes in the genome, determine their sequences, and to directly investigate the mechanisms controlling their expression.

Isomyosin expression patterns in tubular stages of chicken heart development: a 3-D immunohistochemical analysis

The 3-D distribution of atrial and ventricular isomyosins is analysed in tubular chicken hearts (stage 12+ to 17 (H/H)) using antibodies specific for adult chicken atrial and ventricular myosin heavy chains, respectively. At stage 12+ (H/H) all myocytes express the atrial isomyosin; furthermore, all myocytes except those originally situated in the dorsolateral wall of the sinu-atrium coexpress the ventricular isomyosin as well. Moreover, it appears that recently incorporated myocardial cells at both ends of the heart tube start with a coexpression of both isomyosins. From stage 14 (H/H) onwards a regional loss of expression of one of either isomyosins is observed in the atrial and ventricular compartment. In this way the single isomyosin expression types that are characteristic for the adult working myocardium of the atria and ventricles arise. So, the isomyosin expression patterns are, unexpectedly, hardly useful to discriminate the different heart parts of the tubular heart. The ventricle, defined by its adult type of isomyosin expression, is even not detectable before stage 14 (H/H). Interestingly, interconnected coexpression areas, which may be precursor conductive tissues, are still present at stage 17 (H/H) in the outflow tract, the ventricular trabeculae, the atrio-ventricular transitional zone and in the sinu-atrium. The pattern of isomyosin coexpression was found to correlate with a peristaltoid contraction and a slow conduction velocity, whereas single expression areas correlate with a synchronous contraction and a relatively fast conduction velocity. The possible implications of the changing isomyosin pattern for the differentiation of the tubular myocardium, in particular in relation to the development of the conductive tissues, will be discussed.

Isomyosin expression in developing chicken atria: a marker for the development of conductive tissue?

Isomyosin expression patterns in embryonic chicken atria during the first two weeks of development were analyzed immunohistochemically. In the 3-days embryonic chicken heart (HH19-20), strong coexpression of both isomyosins can be found as band-like zones at the lateral sides of the sinoatrial junction. The zones converge on the bottom of the atrium and continue as a band around the atrioventricular canal. In the 5-days heart (HH27-28) the coexpression area encompasses the entire sinoatrial junction and extends into parts of the sinus venosus and into the dorsocaudal atrial wall. In the 7-days heart (HH 32-33) the relative extension of coexpression areas reaches its maximum. Coexpression is also found in a ring-like band in the ventral (bottom) wall of the atria peripheral to the ring-like band in the atrioventricular junction. The latter band has now become continuous with the coexpression area in the bottom of the interatrial septum. Caudally coexpression extends behind the atrioventricular cushions towards the interventricular septum and cranially coexpression of the atrioventricular junction has become continuous with that of the ring around the outflow tract (cf Sanders et al. 1986). In the second week of incubation a decrease of coexpression is observed. The isomyosin expression pattern described in this study has put forward additional arguments that the conductive tissue originates from areas that continue to express both isomyosins relatively late in development.

Acetylcholinesterase in prenatal rat heart: a marker for the early development of the cardiac conductive tissue?

In rat embryos, acetylcholinesterase (AChE, EC 3.1.1.7) activity is present in a continuous sleeve of myocytes that extends from the myocardium that is adjacent to the atrioventricular endocardial cushions via the ventricular trabeculae to the outflow tract. No activity is found in the atrial roof, in the ventricular walls and in the interventricular septum except for its subendocardial surface. AChE-positive cells are first identified in 11-day rat embryos, while the prototypical distribution is best demonstrable in 13-day embryos. Part of the AChE-positive cell system is identifiable as a precursor of the adult conduction system by topographical criteria in 16-day fetuses and by morphological criteria in 20-day fetuses. At birth (2 days later), AChE activity has disappeared from the cardiac myocytes except for a ring of tissue at the atrial side of the atrioventricular junction. These findings suggest that the embryonic heart can be divided into an upstream myocardium that has no AChE activity and a downstream myocardium that is characterized by the presence of AChE. Furthermore they suggest that an acetylcholine-dependent mechanism may be responsible for the retardation of the depolarization wave in the downstream parts of the heart. Finally they show that the adult conduction system is formed by a transdifferentiation of part of a far more extensive embryonic precursor system.