An ultrastructural topographical study on myofibrillogenesis in the heart of the chick embryo during pulsation onset period

When Does the Human Embryonic Heart Start Beating? A Review of Contemporary and Historical Sources of Knowledge about the Onset of Blood Circulation in Man

The onset of embryonic heart beating may be regarded as the defining feature for the beginning of personal human life. Clarifying the timing of the first human heartbeat, therefore, has religious, philosophical, ethical, and medicolegal implications. This article reviews the historical and contemporary sources of knowledge on the beginning of human heart activity. Special attention is given to the problem of the determination of the true age of human embryos and to the problem of visualization of the human embryonic heart activity. It is shown that historical and current textbook statements about the onset of blood circulation in man do not derive from observations on living human embryos but derive from the extrapolation of observations on animal embryos to the human species. This fact does not preclude the existence of documented observations on human embryonic heart activity: Modern diagnostic (ultrasound) and therapeutic (IVF) procedures facilitate the visualization of early embryonic heart activity in precisely dated pregnancies. Such studies showed that the human heart started its pumping action during the fourth post-fertilization week. A small number of direct observations on the heart activity of aborted human embryos were reported since the 19th century, but did not receive much recognition by embryologists.

Dose-dependent effect of cannabinoid WIN-55,212-2 on myelin repair following a demyelinating insult

Dysfunctions in the endocannabinoid system have been associated with experimental animal models and multiple sclerosis patients. Interestingly, the endocannabinoid system has been reported to confer neuroprotection against demyelination. The present study aims to assess the effects of the cannabinoid agonist WIN-55,212-2 in cuprizone fed animals on myelin repair capacity. Animals exposed to cuprizone were simultaneously treated withWIN-55,212-2, behaviorally tested and finally the corpus callosum was exhaustively studied by Western blotting, qRT-PCR and a myelin staining procedure. We report that the long-term administration of WIN-55,212-2 reduced the global amount of CB₁ protein. Histological analysis revealed clear demyelination after being fed cuprizone for three weeks. However, cuprizone-fed mice subjected to 0.5 mg/Kg of WIN-55,212-2 displayed no differences when compared to controls during demyelination, although there was a robust increase in the myelinated axons during the remyelination phase. These animals displayed better performance on contextual fear conditioning which was in turn non-attributable to an antinociceptive effect. In contrast, a 1 mg/Kg dosage caused a remarkable demyelination accompanied by limited potential for myelin repair. Upon drug administration while mice ongoing demyeliniation, the expression of Aif1 (microglia) and Gfap (astrocytes) followed a dose-dependent manner whereas the expression of both markers was apparently attenuated during remyelination. Treatment with vehicle or 0.5 mg/Kg of the drug during demyelination increased the expression of Pdgfra (oligodendrocyte precursor cells) but this did not occur when 1 mg/Kg was administered. In conclusion, the drug at 0.5 mg/Kg did not alter myelin architecture while 1 mg/Kg had a deleterious effect in this model.

Functional Morphology of the Cardiac Jelly in the Tubular Heart of Vertebrate Embryos

The early embryonic heart is a multi-layered tube consisting of (1) an outer myocardial tube; (2) an inner endocardial tube; and (3) an extracellular matrix layer interposed between the myocardium and endocardium, called "cardiac jelly" (CJ). During the past decades, research on CJ has mainly focused on its molecular and cellular biological aspects. This review focuses on the morphological and biomechanical aspects of CJ. Special attention is given to (1) the spatial distribution and fiber architecture of CJ; (2) the morphological dynamics of CJ during the cardiac cycle; and (3) the removal/remodeling of CJ during advanced heart looping stages, which leads to the formation of ventricular trabeculations and endocardial cushions. CJ acts as a hydraulic skeleton, displaying striking structural and functional similarities with the mesoglea of jellyfish. CJ not only represents a filler substance, facilitating end-systolic occlusion of the embryonic heart lumen. Its elastic components antagonize the systolic deformations of the heart wall and thereby power the refilling phase of the ventricular tube. Non-uniform spatial distribution of CJ generates non-circular cross sections of the opened endocardial tube (initially elliptic, later deltoid), which seem to be advantageous for valveless pumping. Endocardial cushions/ridges are cellularized remnants of non-removed CJ.

The Memory of the Heart

The embryological development of the heart is one of the most fascinating phenomena in nature and so is its final structure and function. The various ontogenetic passages form the evolutive basis of the final configuration of the heart. Each key step can be recognized in the final features, as the heart maintains a kind of “memory” of these passages. We can identify the major lines of development of the heart and trace these lines up to the mature organ. The aim of this review is to identify these key parameters of cardiac structure and function as essential elements of the heart’s proper functioning and bases for its treatment. We aim to track key steps of heart development to identify what it “remembers” and maintains in its final form as positively selected. A new vision based on the whole acquired knowledge must guide an in-depth scientific approach in future papers and guidelines on the topic and a complete, farsighted therapeutic conduct able to ensure the physiological correction of cardiac pathologies. The application of this modern, functional vision of the heart could improve the clinical treatment of heart disease, filling the gaps still present.

Functiogenesis of cardiac pacemaker activity

Throughout our investigations on the ontogenesis of the electrophysiological events in early embryonic chick hearts, using optical techniques to record membrane potential probed with voltage-sensitive dyes, we have introduced a novel concept of "functiogenesis" corresponding to "morphogenesis". This article gives an account of the framework of "functiogenesis", focusing on the cardiac pacemaker function and the functional organization of the pacemaking area.

The chick embryo as an expanding experimental model for cancer and cardiovascular research

A long and productive history in biomedical research defines the chick as a model for human biology. Fundamental discoveries, including the description of directional circulation propelled by the heart and the link between oncogenes and the formation of cancer, indicate its utility in cardiac biology and cancer. Despite the more recent arrival of several vertebrate and invertebrate animal models during the last century, the chick embryo remains a commonly used model for vertebrate biology and provides a tractable biological template. With new molecular and genetic tools applied to the avian genome, the chick embryo is accelerating the discovery of normal development and elusive disease processes. Moreover, progress in imaging and chick culture technologies is advancing real-time visualization of dynamic biological events, such as tissue morphogenesis, angiogenesis, and cancer metastasis. A rich background of information, coupled with new technologies and relative ease of maintenance, suggest an expanding utility for the chick embryo in cardiac biology and cancer research.

Tropomyosin expression and dynamics in developing avian embryonic muscles

The expression of striated muscle proteins occurs early in the developing embryo in the somites and forming heart. A major component of the assembling myofibrils is the actin-binding protein tropomyosin. In vertebrates, there are four genes for tropomyosin (TM), each of which can be alternatively spliced. TPM1 can generate at least 10 different isoforms including the striated muscle-specific TPM1alpha and TPM1kappa. We have undertaken a detailed study of the expression of various TM isoforms in 2-day-old (stage HH 10-12; 33 h) heart and somites, the progenitor of future skeletal muscles. Both TPM1alpha and TPM1kappa are expressed transiently in embryonic heart while TPM1alpha is expressed in somites. Both RT-PCR and in situ hybridization data suggest that TPM1kappa is expressed in embryonic heart whereas TPM1alpha is expressed in embryonic heart, and also in the branchial arch region of somites, and in the somites. Photobleaching studies of Yellow Fluorescent Protein-TPM1alpha and -TPM1kappa expressed in cultured avian cardiomyocytes revealed that the dynamics of the two probes was the same in both premyofibrils and in mature myofibrils. This was in sharp contrast to skeletal muscle cells in which the fluorescent proteins were more dynamic in premyofibrils. We speculate that the differences in the two muscles is due to the appearance of nebulin in the skeletal myocytes premyofibrils transform into mature myofibrils.

Genomic analyses and cloning of novel chicken natriuretic peptide genes reveal new insights into natriuretic peptide evolution

The natriuretic peptide (NP) family consists of multiple subtypes in teleosts, including atrial, B-type, ventricular, and C-type NPs (ANP, BNP, VNP, CNP-1-4, respectively), but only ANP, BNP, CNP-3, and CNP-4 have been identified in tetrapods. As part of understanding the molecular evolution of NPs in the tetrapod lineage, we identified NP genes in the chicken genome. Previously, only BNP and CNP-3 have been identified in birds, but we characterized two new chicken NP genes by cDNA cloning, synteny and phylogenetic analyses. One gene is an orthologue of CNP-1, which has only ever been reported in teleostei and bichir. The second gene could not be assigned to a particular NP subtype because of high sequence divergence and was named renal NP (RNP) due to its predominant expression in the kidney. CNP-1 mRNA was only detected in brain, while CNP-3 mRNA was expressed in kidney, heart, and brain. In the developing embryo, BNP and RNP transcripts were most abundant 24h post-fertilization, while CNP mRNA increased in a stage-dependent manner. Synthetic chicken RNP stimulated an increase in cGMP production above basal level in chicken kidney membrane preparations and caused a potent dose-dependent vasodilation of pre-constricted dorsal aortic rings. From conserved chromosomal synteny, we propose that the CNP-4 and ANP genes have been lost in chicken, and that RNP may have evolved from a VNP-like gene. Furthermore, we have demonstrated for the first time that CNP-1 is retained in the tetrapod lineage.

Heart myofibrillogenesis occurs in isolated chick posterior blastoderm: a culture model

Early cardiogenesis including myofibrillogenesis is a critical event during development. ­Recently we showed that prospective cardiomyocytes reside in the posterior lateral blastoderm in the chick embryo. Here we cultured the posterior region of the chick blastoderm in serum-free medium and observed the process of myofibrillogenesis by immunohistochem­istry. After 48 hours, explants expressed sarcomeric proteins (sarcomeric α-actinin, 61%; smooth muscle α-actin, 95%; Z-line titin, 56%; sarcomeric myosin, 48%); however, they did not yet show a mature striation. After 72 hours, more than 92% of explants expressed I-Z-I proteins, which were incorporated into the striation in 75% of explants or more (sarcomeric α-actinin, 75%; smooth muscle α-actin, 81%; Z-line titin, 83%). Sarcomeric myosin was ­expressed in 63% of explants and incorporated into A-bands in 37%. The percentage incidence of expression or striation of I-Z-I proteins was significantly higher than that of sarcomeric myosin. Results suggested that the nascent I-Z-I components appeared to be gener­ated independently of A-bands in the cultured posterior blastoderm, and that the process of myofibrillogenesis observed in our culture model faithfully reflected that in vivo. Our blastoderm culture model appeared to be useful to investigate the mechanisms regulating the early cardiogenesis.

Computational model for early cardiac looping

Looping is a vital event during early cardiac morphogenesis, as the initially straight heart tube bends and twists into a curved tube, laying out the basic pattern of the future four-chambered heart. Despite intensive study for almost a century, the biophysical mechanisms that drive this process are not well understood. To explore a recently proposed hypothesis for looping, we constructed a finite element model for the embryonic chick heart during the first phase of looping, called c-looping. The model includes the main structures of the early heart (heart tube, omphalomesenteric veins, and dorsal mesocardium), and the analysis features realistic three-dimensional geometry, nonlinear passive and active material properties, and anisotropic growth. As per our earlier hypothesis for c-looping, actin-based morpho-genetic processes (active cell shape change, cytoskeletal contraction, and cell migration) are simulated in specific regions of the model. The model correctly predicts the initial gross morphological shape changes of the heart, as well as distributions of morphogenetic stresses and strains measured in embryonic chick hearts. The model was tested further in studies that perturbed normal cardiac morphogenesis. The model, taken together with the new experimental data, supports our hypothesis for the mechanisms that drive early looping.

BMP is an important regulator of proepicardial identity in the chick embryo

The proepicardium (PE) is a transient structure formed by pericardial coelomic mesothelium at the venous pole of the embryonic heart and gives rise to several cell types of the mature heart. In order to study PE development in chick embryos, we have analyzed the expression pattern of the marker genes Tbx18, Wt1, and Cfc. During PE induction, the three marker genes displayed a left-right asymmetric expression pattern. In each case, expression on the right side was stronger than on the left side. The left-right asymmetric gene expression observed here is in accord with the asymmetric formation of the proepicardium in the chick embryo. While initially the marker genes were expressed in the primitive sinus horn, subsequently, expression became confined to the PE mesothelium. In order to search for signaling factors involved in PE development, we studied Bmp2 and Bmp4 expression. Bmp2 was bilaterally expressed in the sinus venosus. In contrast, Bmp4 expression was initially expressed unilaterally in the right sinus horn and subsequently in the PE. In order to assess its functional role, BMP signaling was experimentally modulated by supplying exogenous BMP2 and by inhibiting endogenous BMP signaling through the addition of Noggin. Both supplying BMP and blocking BMP signaling resulted in a loss of PE marker gene expression. Surprisingly, both experimental situations lead to cardiac myocyte formation in the PE cultures. Careful titration experiments with exogenously added BMP2 or Noggin revealed that PE-specific marker gene expression depends on a low level of BMP signaling. Implantation of BMP2-secreting cells or beads filled with Noggin protein into the right sinus horn of HH stage 11 embryos resulted in downregulation of Tbx18 expression, corresponding to the results of the explant assay. Thus, a distinct level of BMP signaling is required for PE formation in the chick embryo.

Understanding heart development and congenital heart defects through developmental biology: a segmental approach

ABSTRACT The heart is the first organ to form and function during development. In the pregastrula chick embryo, cells contributing to the heart are found in the postero-lateral epiblast. During the pregastrula stages, interaction between the posterior epiblast and hypoblast is required for the anterior lateral plate mesoderm (ALM) to form, from which the heart will later develop. This tissue interaction is replaced by an Activin-like signal in culture. During gastrulation, the ALM is committed to the heart lineage by endoderm-secreted BMP and subsequently differentiates into cardiomyocyte. The right and left precardiac mesoderms migrate toward the ventral midline to form the beating primitive heart tube. Then, the heart tube generates a right-side bend, and the d-loop and presumptive heart segments begin to appear segmentally: outflow tract (OT), right ventricle, left ventricle, atrioventricular (AV) canal, atrium and sinus venosus. T-box transcription factors are involved in the formation of the heart segments: Tbx5 identifies the left ventricle and Tbx20 the right ventricle. After the formation of the heart segments, endothelial cells in the OT and AV regions transform into mesenchyme and generate valvuloseptal endocardial cushion tissue. This phenomenon is called endocardial EMT (epithelial-mesenchymal transformation) and is regulated mainly by BMP and TGFbeta. Finally, heart septa that have developed in the OT, ventricle, AV canal and atrium come into alignment and fuse, resulting in the completion of the four-chambered heart. Altered development seen in the cardiogenetic process is involved in the pathogenesis of congenital heart defects. Therefore, understanding the molecular nature regulating the 'nodal point' during heart development is important in order to understand the etiology of congenital heart defects, as well as normal heart development.

N-cadherin is required for the differentiation and initial myofibrillogenesis of chick cardiomyocytes

To investigate initial stages of cardiac myofibrillogenesis, heart-forming mesoderm was excised from stage 6 chick embryos and explanted on fibronectin-coated coverglasses. The explants were fixed at various times and immunofluorescently stained with antibodies to N-cadherin, alpha-catenin, beta-catenin, sarcomeric myosin, pan and sarcomeric alpha-actinins, or rhodamine phalloidin. After 7 hours in culture the cells appeared epithelial. N-cadherin, alpha- and beta-catenin, pan alpha-actinin, and F-actin showed circumferential localization at cell borders. No cells in the explant were positive for sarcomeric alpha-actinin or sarcomeric myosin at this stage. Sarcomeric alpha-actinin and sarcomeric myosin were detected around 10 hours after plating. Sarcomeric alpha-actinin initially appeared as small beads along thin actin filaments. Mature Z-lines began to be organized at 20 hours, at the same time the cells started to contract. When the rat monoclonal antibody NCD-2, which inhibits N-cadherin function, was added to the culture at early time-points, cells lost cell-cell contacts, became spherical in shape, and contained tangled actin fibers. The expression of sarcomeric alpha-actinin and sarcomeric myosin was suppressed. These results indicate that 1) the precardiac mesoderm explant cells differentiate and form well-organized myofibrils in culture, 2) N-cadherin-mediated cell-cell interactions are necessary for early differentiation of cardiomyocytes and organization of myofibrils.

Mechanisms involved in valvuloseptal endocardial cushion formation in early cardiogenesis: roles of transforming growth factor (TGF)-beta and bone morphogenetic protein (BMP)

Endothelial-mesenchymal transformation (EMT) is a critical event in the generation of the endocardial cushion, the primordia of the valves and septa of the adult heart. This embryonic phenomenon occurs in the outflow tract (OT) and atrioventricular (AV) canal of the embryonic heart in a spatiotemporally restricted manner, and is initiated by putative myocardially derived inductive signals (adherons) which are transferred to the endocardium across the cardiac jelly. Abnormal development of endocardial cushion tissue is linked to many congenital heart diseases. At the onset of EMT in chick cardiogenesis, transforming growth factor (TGFbeta)-3 is expressed in transforming endothelial and invading mesenchymal cells, while bone morphogenetic protein (BMP)-2 is expressed in the subjacent myocardium. Three-dimensional collagen gel culture experiments of the AV endocardium show that 1) myocardially derived inductive signals upregulate the expression of AV endothelial TGFbeta3 at the onset of EMT, 2) TGFbeta3 needs to be expressed by these endothelial cells to trigger the initial phenotypic changes of EMT, and 3) myocardial BMP2 acts synergistically with TGFbeta3 in the initiation of EMT.

A novel role for cardiac neural crest in heart development

Ablation of premigratory cardiac neural crest results in defective development of the cardiac outflow tract. The purpose of the present study was to correlate the earliest functional and morphological changes in heart development after cardiac neural crest ablation. Within 24 hours after neural crest ablation, the external morphology of the hearts showed straight outflow limbs, tighter heart loops, and variable dilations. Incorporation of bromodeoxyuridine in myocytes, an indication of proliferation, was doubled after cardiac neural crest ablation. The myocardial calcium transients, which are a measure of excitation-contraction coupling, were depressed by 50% in both the inflow and outflow portions of the looped heart tube. The myocardial transients could be rescued by replacing the cardiac neural crest. The cardiac jelly produced by the myocardium was distributed in an uneven, rather than uniform, pattern. An extreme variability in external morphology could be attributed to the uneven distribution of cardiac jelly. In the absence of cardiac neural crest, the myocardium was characterized by somewhat disorganized myofibrils that may be a result of abnormally elevated proliferation. In contrast, endocardial development appeared normal, as evidenced by normal expression of fibrillin-2 protein (JB3 antigen) and normal formation of cushion mesenchyme and trabeculae. The signs of abnormal myocardial development coincident with normal endocardium suggest that the presence of cardiac neural crest cells is necessary for normal differentiation and function of the myocardium during early heart development. These results indicate a novel role for neural crest cells in myocardial maturation.

Development of mechanisms regulating intracellular Ca2+ concentration in cardiac muscle cells of early chick embryos

The development of mechanisms for the regulation of intracellular-free calcium ion concentration ([Ca2+]i) was investigated in precardiac mesodermal cells (PMC) and cardiac muscle cells (CMC) from early chick embryos by microfluorometry using a Ca2+-sensitive fluorescent probe, fura-2, and transmission electron microscopy. Microfluorometry indicated that two types of regulatory mechanisms, involving the dihydropyridine receptor (DHPR) and the ryanodine receptor (RYR), are present in CMC when the heartbeat begins at the 8-9 somite stages. Nifedipine completely suppressed the beating of hearts isolated from embryos on Days 1.5 and 2. Ryanodine had no effect on the beating of hearts isolated from embryos on Day 1.5, though it completely suppressed beating in hearts from Embryonic Day 2. Microfluorometry revealed that a change occurred in the Ca2+-regulating mechanisms of CMC on Day 2. Transmission electron microscopy showed the appearance in CMC, also on Day 2, of peripheral couplings with feet structures, and SR adjacent to the Z-line of myofibrils. These findings suggest that the calcium-induced calcium-release (CICR) mechanism appears in the CMC of the chick on the second day of embryonic development.

Local and regional variations in myofibrillar patterns in looping rat hearts

BACKGROUND

In chickens, cytodifferentiation, right side dominance in myofibril development, and variations in myofibrillar patterns in different areas and layers of the myocardial wall exist which have been implicated in the process of heart looping. Little comparable information is available for developing myofibrillar patterns in the early development of mammalian hearts.

METHODS

We have used transmission electron microscopy (TEM), confocal scanning laser microscopy (CSLM), and 3-D reconstruction techniques also present in the looping hearts of embryonic day (ED) 9.5 to 11.5 rat hearts.

RESULTS

Local and regional variations and right side dominance in myofibrillar patterns were shown during looping in 9.5 through 11.5 days of development in embryonic rat heart. At 9.5 days of development, myofibrils near the lumen of the myocardial wall were primarily in circumferential bands while near the pericardial surface they were primarily in longitudinal bands. In older embryos, regional variations in myofibrillar organization was found in areas associated with the cardiac cushions, trabeculae, and myocardial wall of the developing heart chambers. Based on sarcomeric structure, myofibrils in the ventricle and outflow tract were more advanced than those found in the atrial wall.

CONCLUSIONS

The local and regional patterns of myofibrils in looping rat hearts are similar to those which have been found in developing chicken hearts. This study and others indicate cytodifferentiation and development of the contractile apparatus has a crucial role in the process of heart looping.

Myofibrillogenesis in rodent skeletal muscle in vitro: two pathways involving thick filament aggregates

Thick filament aggregates play an important role in myofibrillogenesis in rodent skeletal muscle in vitro. This ultrastructural study describes these aggregates, shows their involvement in the process of myofibril formation, and correlates their appearance and function with current models of myofibrillogenesis. Initially, following myoblast fusion in normal mouse skeletal muscle in vitro, abundant stress fiber-like structures (SFLS) are found near the periphery of early myotubes. These undergo internal rearrangements, forming subcortical sarcomeres and early myofibrils. However, additional thick filaments are synthesized, and some join appositionally to the nascent myofibrils, increasing their diameter. More interiorly, this thick filament synthesis accelerates, with filaments aligning into aggregates resembling discrete A-bands, usually with M-lines and M-regions. The ends of these 'A-band' aggregates are infiltrated with ribosomes and capped by flocculent material. Ultimately, aggregates are incorporated into preexisting myofibrils or associate end-to-end to form new, parallel myofibrils, the flocculent material forming putative I-bands with diminished Z-lines and few thin filaments. As differentiation continues, Z-lines and thin filaments appear, forming true myofibrils. Dysgenic mouse skeletal muscle develops similarly, but when this non-contractile cell matures (i.e., generates action potentials), filaments and their organization break down. Cloned myogenic rat L5/A10 cells also follow this developmental pattern, but in mature, contracting myotubes, Z-lines remain irregular and thin filaments are reduced. In all three types of muscle developing in vitro, thick filament aggregates are a common and predominant feature and as such appear to constitute an additional or alternate pathway to previously described models of myofibrillogenesis.

Different temporal patterns of expression result in the same type, amount, and distribution of filamin (ABP) in cardiac and skeletal myofibrils

The morphogenesis of functional myofibrils in chick skeletal and cardiac muscle occurs in greatly different time spans, in about 7 and 2 days, respectively. In chick skeletal myogenic cells, one isoform of the 250 kD actin-binding protein (ABP) filamin is associated with stress fiber-like structures of myoblasts and early myotubes, then disappears for approximately 4 days, whereupon a second filamin isoform reappears at the Z-disc periphery. We sought to determine if cardiac myogenesis involves this sequence of appearance, disappearance, and reappearance of a new filamin isoform in a compressed time scale. It was known that in mature heart, filamin is localized at the Z-disc periphery as in mature (fast) skeletal muscle, and is also associated with intercalated discs. We find that myocardial filamin has an apparent molecular weight similar to that of adult skeletal muscle filamin and lower than that of smooth muscle filamin, and that both skeletal and cardiac muscle contain roughly 200 filamin monomers per sarcomere. Two-dimensional peptide mapping shows that myocardial filamin is very similar to skeletal muscle filamin. Myocardial, slow skeletal, and fast skeletal muscle filamins are all phosphorylated, as previously shown for filamin of non-striated muscle. Using immunofluorescence, we found that filamin could not be detected in the developing heart until the 14-somite stage, when functional myofibrils exist and the heart has been beating for 3 to 4 hours. We conclude that in cardiac and skeletal myogenesis, different sequences of filamin gene expression result in myofibrils with similar filamin distributions and isoforms.

Expression of phospholamban mRNA during early avian muscle morphogenesis is distinct from that of alpha-actin

We have studied the expression of phospholamban during the early development of chick embryos by in situ hybridization and have compared it to that of alpha-cardiac and alpha-skeletal actin. In adult cross-striated muscles there is only one phospholamban gene and it is expressed exclusively in the heart and slow muscles. In the heart phospholamban transcripts were first detected at stage 14 in the region of presumptive ventricle and at stage 20 in the atrium. In the myotomal portion of the somites phospholamban mRNA was first detected at stage 20, which lagged behind the appearance of the alpha-actins. In the limb rudiments all three mRNAs were barely detectable through stage 24, but increased by stage 28+. However, quantitative analysis of signal intensity at stage 28+ indicated that less phospholamban mRNA is present in the limb bud than in the myotome since for phospholamban the ratio of the signal density in the myotome to that in the limb rudiments was about twice the value of the ratio determined for the alpha-actins. Northern blot analysis of embryonic day 11 chick fast pectoralis muscle showed that phospholamban mRNA was not detected in vivo while alpha-cardiac actin mRNA was. Moreover, no phospholamban mRNA was detected in primary cultures derived from pectoralis muscle of the same age. In concert with previous observations that phospholamban is not detectable at stage 30-32 in wing or thigh muscle, these results suggest that phospholamban mRNA is expressed independently of the alpha-actins in the limb buds during early myogenesis.

Clonal analysis of cardiac morphogenesis in the chicken embryo using a replication-defective retrovirus. III: Polyclonal origin of adjacent ventricular myocytes

Replication-incompetent variants of the avian spleen necrosis virus (SNV) encoding cytoplasmic or nuclear-directed beta-galactosidase (beta-gal) have been used to trace the clonal growth of myocytes during left ventricular free-wall formation. Tubular-stage hearts were infected with a mixed suspension of both retroviruses and, after hatching, the progeny of marked cells in the ventricular wall were examined by X-gal histochemistry. When a small number of virions was introduced individual blue patches contained myocytes with only one label type (cytoplasmic or nuclear). These results confirmed our previous conclusion that each cluster or patch represents a single clone (Mikawa et al., 1992, Dev. Dynamics, 193:11-23). Each of these clones formed a clone-shaped patch which often extended through the entire thickness of the ventricular myocardium, but typically each patch was heterogeneous, containing a mixture of labeled and unlabeled cells. We then asked whether the two populations of myocytes in each patch were clonally related or generated from more than one progenitor. When hearts were infected with high titer viral suspensions many patches were observed in which cytoplasmic-tagged myocytes were intermingled with nuclear-tagged myocytes. Thus, the cone-shaped myocyte patches in the ventricular wall are polyclones derived from separate progenitors in the precardiac mesoderm. This finding led us to examine the separation of clonally related ventricular myocytes in the developing hearts. Embryos were infected with retroviral suspensions at varying stages of development and the resulting colonies examined after hatching.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of sarcomeric myosin in the presumptive myocardium of chicken embryos occurs within six hours of myocyte commitment

The distribution of sarcomeric myosin heavy chain (MyHC) has been examined immunocytochemically in the presumptive myocardial cells of chicken embryos (stages 6-10) prior to the onset of the heart beat. Embryos were stained with monoclonal antibody MF20, a reagent which recognizes all chicken sarcomeric MyHCs (Bader et al., 1982), and then examined both in whole mount by immunofluorescence and in semithin, plastic-embedded sections following immunoperoxidase labeling. We observed that myosin could be detected as early as stage 7 (0-2 pairs of somites) in 29% of the 31 embryos examined, and by stage 8 (4 pairs of somites) more than 80% of the embryos were MF20+. Every embryo with 5 pairs of somites (stage 8+) labeled strongly with MF20. Labeling was first detected at stage 7 to 7+ as a diffuse fluorescent signal within pleomorphic cells of the splanchnic mesoderm located in two crescent-shaped regions bordering each side of the anterior intestinal portal (AIP). With progressive development, the two crescent-shaped regions merged at the apex of the AIP, and as the two heart tubes began fusion at stage 9, the MyHC+ regions extended cranially and medially. By somite stages 9-10, the myosin-positive cells completely encircled the heart tube. From stages 7 to 9 the myosin signal had no sarcomeric distribution; i.e., there were no MyHC striations nor periodic repeats evident in the presumptive myocytes until late stage 9 and stage 10. Semithin sections revealed that myosin was first distributed in apical regions of the myocytes, adjacent to the pericardial coelom. The implications of these findings for myocyte determination, differentiation and morphogenesis are discussed.

Onset of expression and regional deposition of alpha-smooth and sarcomeric actin during avian heart development

The sequential appearance of mRNAs for smooth, cardiac, and skeletal alpha-actin has been described during development of the chicken heart (Ruzicka, D.L., and R.J. Schwartz 1988 J. Cell Biol., 107:2575-2586). To assess whether this reflects the deposition of corresponding isoproteins, we have immunocytochemically localized smooth and sarcomeric (cardiac and skeletal) alpha-actin in Hamburger-Hamilton (H-H) stage 7-18 embryos using monoclonal antibodies. Within the developing embryo at stage 9-, smooth muscle alpha-actin was exclusively detected in the developing heart, upon fusion of the endocardial tubes; sarcomeric alpha-actin was observed later (stage 9). By the onset of contraction at stage 10+, intense immunostaining of both smooth and sarcomeric isoproteins was observed in the ventricle; at this time smooth muscle alpha-actin was also detected in splanchnic mesoderm of the pre-vitelline area, in a cellular layer adjacent to the only embryonic cells that exhibited factor VIII (von Willebrand factor) antigens. Double immunostaining of the myocardium at stage 11, at which time striations were first detected, revealed the co-existence of smooth and sarcomeric actin in developing sarcomeres. Intense expression of sarcomeric actin continued in the heart after stage 11, whereas smooth muscle alpha-actin was down-regulated in the ventricle and became regionalized to the inflow and outflow tracts. As expected, smooth muscle alpha-actin was detected around intra- and extra-embryonic vascular structures at later developmental stages, while sarcomeric actin was observed in somites.

Cardiac looping in the chick embryo: the role of the posterior precardiac mesoderm

Grafts of mesoderm taken from the precardiac region of quail embryos of stages 5-7 were inserted into the precardiac mesoderm of chick embryos of stages 5-7. The experiments were of four types and were code named to indicate the origin and the destination of the graft. QACP: tissue from the anterior end of the quail precardiac area was inserted into the posterior end of the chick precardiac mesoderm; QPCA: tissue from the posterior end of the quail precardiac area was inserted into the anterior end of the chick precardiac mesoderm; QACA: tissue from the anterior end of the quail precardiac area was inserted into the anterior end of the chick precardiac mesoderm; QPCP: tissue from the posterior end of the quail precardiac area was inserted into the posterior end of the chick precardiac mesoderm. In no case was precardiac tissue removed from the host. Three main-types of anomaly were obtained: inverted hearts, in which looping took place to the left rather than to the right; compact hearts, in which no looping occurred, and hearts in which extra tissues or regions were apparent. The incidence of compact hearts was significantly greater with QPCA than with any other category of experiment. When older donors were used (stages 8-9), the incidence of compact hearts fell. No variations in the origin of the graft, nor in its ultimate destination in the host, were found to affect the frequency of any of the anomalies. Sections showed that quail hearts tended to have thicker walls than chick hearts; although quail tissues were often incorporated into the host chick hearts, they retained the histological characteristics of the donors. The fact that no compact hearts resulted from the experiment QACA, or from the mock operations, leads us to conclude that failure to loop in the compact hearts was not due to mechanical trauma caused by the operation, but to some specific difference between grafts taken from the anterior and posterior precardiac mesoderm. The fact that compact hearts were obtained when chick donors were used instead of quails, shows that the effect is not species-specific. We propose that a morphogen is secreted by the posterior end of the precardiac mesoderm and this plays a role in controlling the cessation of looping.

Clonal analysis of cardiac morphogenesis in the chicken embryo using a replication-defective retrovirus: I. Formation of the ventricular myocardium

Cells of the precardiac mesoderm (stages 4-6) and dividing myocytes of early hearts (stages 10-15) were tagged with a replication-incompetent retrovirus (CXL) (Mikawa et al., 1991b) encoding bacterial beta-galactosidase (beta-gal). Two protocols were used to infect the cardiogenic cells. (1) Small blocks (approximately 50 micron 2) of anterolateral mesoderm were dissected from gastrula-stage embryos (stages 4-6) and incubated in liquid medium containing the retrovirus. After removal of CXL, the tissues were dispersed into single-cell suspensions and pressure injected into the precardiac areas of recipient embryos (stages 4-6). Such embryos were then incubated in vitro at 37 degrees C for 2 days (New, 1968), and those embryos with beating hearts were fixed for X-gal histochemistry and paraffin serial sectioning. (2) CXL was pressure injected in ovo (embryonic stages 4-15) into cardiogenic tissues and the eggs subsequently returned to an incubator. At selected stages of development embryos or whole hearts were fixed, stained with X-gal, and serially sectioned after paraffin embedding. The first method showed that (1) cells of the precardiac mesoderm could be infected with the retrovirus, (2) the transplanted cells would differentiate into beating myocytes, and (3) beta-gal expression was sufficiently high to be detected histochemically. With the second procedure we could show that (1) beta-gal-tagged cells formed colonies in the myocardium, (2) the labeled cells were exclusively myocytes, (3) the number of cells per colony increased with increasing age of embryonic development, (4) the size of colonies was larger in the left than the right ventricle, (5) many of the colonies were transmural, i.e., they extended from epicardial to endocardial layers of the myocardium and generally exhibited a cone or funnel-shape with the base of the cone nearest the epicardium, (6) the orientation of myocytes within each colony changed at different layers of the myocardium, and (7) the cones contained both beta-gal+ and beta-gal- myocytes. DNA labeling studies with [3H]thymidine indicated that cardiogenic cells divided every 16-18 hr during the first week of development and that the CXL-labeled cells divided indistinguishably from unlabeled myocytes. Based on these observations a model for the growth of the myocardium is presented.

Myogenic cytodifferentiation of the precardiac mesoderm in the rat

The contractile cells of the primitive heart are derived from a subpopulation of the lateral plate splanchnic mesoderm. While the formation of the cardiac primordia has been studied in the avian embryo, little is known about this cell population in the mammal. To investigate the distribution and cellular differentiation of the myocardial precursors in the early mammalian embryo, we studied the sequential immunohistochemical appearance of desmin and myosin in whole mounts of rat embryos from the presomite (gestational day 9) through the 6-8 somite, straight heart tube (gestational day 10) stages of early cardiac morphogenesis. In contrast to the chicken, and previous reports in the mouse, our results show that myogenic differentiation of the muscle precursor cells of the heart begins in the presomite embryo prior to formation of the anterior intestinal portal or foregut. In addition, this cell population of the precardiac mesoderm appears as a single crescent-shaped population of cells in continuity across the midline which extends caudally during development and then fuses in the midline to form the primitive heart tube. Unlike skeletal myogenesis, desmin and myosin appear simultaneously and are codistributed throughout this initial period of heart development. These results suggest that myocardial differentiation in the rat is precocious when compared to the chicken and precedes the morphogenetic processes involved in formation of the primitive heart tube. Furthermore, this study provides the first description in the mammal of the spatial distribution of the myogenic precardiac mesoderm.

Cytoskeletal filaments in embryonic chick myocardial cells as revealed by the quick-freeze deep-etch method combined with immunocytochemistry

The three-dimensional organization of cytoskeletal filaments associated with the myofibrils and sarcolemma of the myocardial cells of early chick embryos was studied by the rapid-freeze deep-etch method combined with immunocytochemistry. In the endoplasmic region of saponin-treated myocardial cells, 12-14 nm filaments formed a loose network surrounding nascent myofibrils. These 12-14 nm filaments attached to the myofibrils and some of them converged into Z disc regions. In the non-junctional cytocortical region thinner 8-11 nm filaments composed a dense network just beneath the sarcolemma. In myofibril terminating regions at the sarcolemma, i.e., the fascia adherens, 3-5 nm cross-bridges were observed among the thin filaments. In Triton-permeabilized and myosin subfragment 1 (S1)- treated samples, subsarcolemmal 8-11 nm filaments proved to be S1-decorated actin filaments under which there was a loose network of S1-undecorated filaments. Subsarcolemmal S1-decorated actin filaments had mixed polarity and attached to the sarcolemma at one end. A loose network of S1-undecorated filaments among myofibrils in the endoplasmic region was revealed to consist of desmin-containing intermediate filaments after immuno-gold staining for desmin. These networks connecting myofibrils with sarcolemma were assumed to play an important role in integrating and transmitting the contractile force of individual myofibrils within early embryonic myocardial cells.

Actin bundles on the right side in the caudal part of the heart tube play a role in dextro-looping in the embryonic chick heart

The role of actin bundles on the heart looping of chick embryos was examined by using cytochalasin B, which binds to the barbed end of actin filaments and inhibits association of the subunits. It was applied to embryos cultured according to New's method. Looping did not occur when cytochalasin B was applied diffusely in the medium. Further, we disorganized actin bundles in a limited part of the heart tube to examine the role of actin bundles in each part in asymmetry formation. A small crystal of cytochalasin B was applied to the caudal part of the heart tube on either the left or right side. The disorganization of actin bundles on the left side resulted in the right-bending of the heart, an initial sign of dextro-looping (normal pattern), and right side disorganization resulted in left-bending. We suggest that actin bundles on the right side of the caudal part of a heart tube generate tension and cause dextro-looping. Embryos whose hearts bent to the right rotated their heads to the right, and embryos with left-bent-hearts rotated their heads to the left. The rotation of the heart tube may therefore decide in which direction the body axis rotates.

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.

Changes in the arrangement of actin bundles during heart looping in the chick embryo

We assessed the arrangement of actin bundles in the looping chick heart. Actin filaments were stained with rhodamine-labeled phalloidin, and their total arrangement was observed in whole mount specimens. Before the straight heart tube was formed, actin bundles were in a net-like arrangement as if to indicate the cell borders. With progress of the heart tube formation, actin bundles were gradually arranged in a circumferential direction. In the looped heart, regional differences in actin arrangements were observed. In the truncus arteriosus, actin bundles ran in a net-like arrangement. In the bulbus cordis, actin bundles ran in random directions. In the ventricle, actin bundles were roughly arranged in a circumferential direction. Between these three regions, actin bundles ran in a circumferential direction especially on the concave side. Near the right contour on the ventral face, some actin bundles ran in a longitudinal direction along the axis of the tubular heart. In the bulbus cordis and the ventricle at the looped stage, there was another group of actin bundles in the inner layer of the myocardium which ran in a circumferential direction. We presume that the arrangement of actin bundles is related to heart looping.

Myogenesis in the mouse embryo: differential onset of expression of myogenic proteins and the involvement of titin in myofibril assembly

Antibodies to muscle-specific proteins were used in immunofluorescence to monitor the development of skeletal muscle during mouse embryogenesis. At gestation day (g.d.) 9 a single layer of vimentin filament containing cells in the myotome domain of cervical somites begins to stain positively for myogenic proteins. The muscle-specific proteins are expressed in a specific order between g.d. 9 and 9.5. Desmin is detected first, then titin, then the muscle specific actin and myosin heavy chains, and finally nebulin. At g.d. 9.5 fibrous desmin structures are already present, while for the other myogenic proteins no structure can be detected. Some prefusion myoblasts display at g.d. 11 and 12 tiny and immature myofibrils. These reveal a periodic pattern of myosin, nebulin, and those titin epitopes known to occur at and close to the Z line. In contrast titin epitopes, which are present in mature myofibrils along the A band and at the A-I junction, are still randomly distributed. We propose, that the Z line connected structures and the A bands (myosin filaments) assemble independently, and that the known interaction of the I-Z-I brushes with the A bands occurs at a later developmental stage. After fusion of myoblasts to myotubes at g.d. 13 and 14 all titin epitopes show the myofibrillar banding pattern. The predominantly longitudinal orientation of desmin filaments seen in myoblasts and in early myotubes is transformed at g.d. 17 and 18 to distinct Z line connected striations. Vimentin, still present together with desmin in the myoblasts, is lost from the myotubes. Our results indicate that the putative elastic titin filaments act as integrators during skeletal muscle development. Some developmental aspects of eye and limb muscles are also described.

Epicardial formation in embryonic chick heart: computer-aided reconstruction, scanning, and transmission electron microscopic studies

Epicardial formation in the embryonic chick heart from initial to final stages was revealed by means of computer-aided reconstructions based on serial resin sections for light microscopy, with further detailed observations using scanning and transmission electron microscopy. The origin of the epicardium was recognized as protrusions of mesothelial cell clusters on the right side of the external surface of the sinus venosus at 23 somites (stage 14+). These protrusions elongated to give rise to several villous processes, the tips of which eventually touched the dorsal wall of the embryonic heart at 30 somites (stage 17). Originating from these adhesion sites, mesothelial cells spread gradually onto myocardial cells in all directions to form a monolayered sheetlike cover. Thus, by stage 23, the ventricle was completely overlaid with epicardium, and blood-island-like structures appeared within the subepicardial layer. The atrium was not enveloped by epicardium until stage 25, and the extreme distal end of the bulbus cordis was reached by the advancing epicardium at stage 27. A chronological table of epicardial formation in the chick heart is presented.

Immunocytochemical studies of cardiac myofibrillogenesis in early chick embryos. III. Generation of fasciae adherentes and costameres

To study whether the first myofibrils are separate from or firmly bound to the myocytic cell membranes, whole mount preparations of 6-12-somite-stage chick embryonic hearts were examined by fluorescence microscopy after double labeling with antibodies to vinculin (fluorescein-conjugated) and rhodamine-phalloidin, or with antibodies to titin (rhodamine-conjugated) and nitrobenz-oxadiazole-phallacidin. When a small number of myofibrils appeared for the first time at the nine somite stage, most of them were already bound to the cell membranes through zonulae adherentes, fasciae adherentes, or costameres. In the outer of the two myocardial cell layers, in which the myocytes were closely in contact with each other along polygonal boundaries, fasciae adherentes and costameres developed at the boundaries, apparently by conversion of preexisting zonulae adherentes. On the other hand, in the inner cell layer, in which myocytes were more loosely associated with each other, both costameres and fasciae adherentes appeared to develop de novo, the former in association with the inner surface of the myocardial wall and the latter at the intercellular boundaries. The myofibrillar tracks in the inner layer followed long and smooth courses and were as a whole aligned in the circumferential direction of the tubular heart wall from the earliest stage of myofibril formation. Those in the outer layer were arranged in a pattern of two- or three-dimensional networks in the 9-10 somite stage, although many myofibrils were also circumferentially directed. The fact that the majority of the first myofibrils were already bound to the cell membranes in a directed manner suggests that myocytes at the earliest stage of myofibril formation are endowed with spatial information that directs the organization of nascent myofibrils. It is proposed that the myocyte cell membranes perform an essential role in cardiac myofibrillogenesis.

Sequential activation of alpha-actin genes during avian cardiogenesis: vascular smooth muscle alpha-actin gene transcripts mark the onset of cardiomyocyte differentiation

The expression of cytoplasmic beta-actin and cardiac, skeletal, and smooth muscle alpha-actins during early avian cardiogenesis was analyzed by in situ hybridization with mRNA-specific single-stranded DNA probes. The cytoplasmic beta-actin gene was ubiquitously expressed in the early chicken embryo. In contrast, the alpha-actin genes were sequentially activated in avian cardiac tissue during the early stages of heart tube formation. The accumulation of large quantities of smooth muscle alpha-actin transcripts in epimyocardial cells preceded the expression of the sarcomeric alpha-actin genes. The accumulation of skeletal alpha-actin mRNAs in the developing heart lagged behind that of cardiac alpha-actin by several embryonic stages. At Hamburger-Hamilton stage 12, the smooth muscle alpha-actin gene was selectively down-regulated in the heart such that only the conus, which subsequently participates in the formation of the vascular trunks, continued to express this gene. This modulation in smooth muscle alpha-actin gene expression correlated with the beginning of coexpression of sarcomeric alpha-actin transcripts in the epimyocardium and the onset of circulation in the embryo. The specific expression of the vascular smooth muscle alpha-actin gene marks the onset of differentiation of cardiac cells and represents the first demonstration of coexpression of both smooth muscle and striated alpha-actin genes within myogenic cells.

Regional gradient of pacemaker activity in the early embryonic chick heart monitored by multisite optical recording

1. Regional gradient of pacemaker activity in the early embryonic precontractile chick heart was quantitatively assessed by means of simultaneous multiple-site optical recordings of changes in membrane potential, using a measuring system with a 10 X 10-element photodiode array which had a spatial resolution of 30 microns. 2. Absorption changes related to spontaneous electrical activity were recorded simultaneously from many contiguous regions in the area in which the pacemaker site was located in seven- to nine-somite embryonic hearts stained with a voltage-sensitive merocyanine-rhodanine dye (NK 2761). 3. The absorption changes related to slow diastolic depolarization were detected, and they were concentrated in and near the pacemaking area. The area in which the absorption changes related to slow diastolic depolarization were detected increased in size as development proceeded. 4. The slope of the absorption change related to diastolic depolarization was measured as an indicator of the pacemaker activity. It was largest in the pacemaking area, and gradually decreased towards the periphery. 5. The maximum slope of the optical change related to slow diastolic depolarization also increased as development proceeded and was related to early development of the heart rate. Thus, these results suggest that formation of a regional gradient of pacemaker activity results in the functional architecture of the pacemaking area in the early phases of cardiogenesis.

Functional pacemaking area in the early embryonic chick heart assessed by simultaneous multiple-site optical recording of spontaneous action potentials

Pacemaking areas in the early embryonic chick hearts were quantitatively assessed using simultaneous multiple-site optical recordings of spontaneous action potentials. The measuring system with a 10- X 10- or a 12 X 12-element photodiode array had a spatial resolution of 15-30 microns. Spontaneous action potential-related optical signals were recorded simultaneously from multiple contiguous regions in the area in which the pacemaker site was located in seven- to nine-somite embryonic hearts stained with a voltage-sensitive merocyanine-rhodanine dye (NK 2761). In the seven- to early eight-somite embryonic hearts, the location of the pacemaking area is not uniquely determined, and as development proceeds to the nine-somite stage, the pacemaking area becomes confined to the left pre-atrial tissue. Analysis of the simultaneous multiple-site optical recordings showed that the pacemaking area was basically circular in shape in the later eight- to nine-somite embryonic hearts. An elliptical shape also was observed at the seven- to early eight-somite stages of development. The size of the pacemaking area was estimated to be approximately 1,200-3,000 micron2. We suggest that the pacemaking area is composed of approximately 60-150 cells, and that the pacemaking area remains at a relatively constant size throughout the seven- to nine-somite stages. It is thus proposed that a population of pacemaking cells, rather than a single cell, serves as a rhythm generator in the embryonic chick heart.

Immunocytochemical studies of cardiac myofibrillogenesis in early chick embryos. II. Generation of alpha-actinin dots within titin spots at the time of the first myofibril formation

In whole mount preparations of the 9 somite stage chick embryonic hearts that were immunofluorescently double labeled for titin and alpha-actinin, presumptive myofibrils were recognized as rows of several periodically aligned titin spots. Within these titin spots, smaller alpha-actinin dots were observed. These periodical arrangements of titin spots and alpha-actinin dots were not found in the 7 somite stage hearts. In wide myofibrils in the 10 somite stage hearts, the alpha-actinin dots and titin spots simultaneously became 'lines.' To study the ultrastructural features of the titin-positive regions in the 6-9 somite stage hearts, the thoracic portions of the embryos were immunofluorescently labeled for titin and embedded in resin. Ultrathin sections were mounted on electron microscopic grids and examined in immunofluorescence optics. The titin-positive regions thus identified were then examined in the electron microscope. No readily discernable specific ultrastructural features were found in titin-positive regions of the 6 somite stage cardiac primodia. Examination of the sections of the 9 somite stage hearts, on the other hand, revealed the occasional presence of small dense bodies, Z bodies, in the titin-positive regions. These observations strongly suggest that these Z bodies are the ultrastructural counterparts of the alpha-actinin dots seen by immunofluorescence optics and that they are formed nearly at the time of the formation of the first myofibrils. In some of the nascent myofibrils the Z bodies were found to be considerably narrower than the myofibrils, implying that the Z bodies are required not for the assembly of myofibrils per se but for their stabilization. Immunofluorescent labeling for titin and alpha-actinin revealed that the length of the shortest sarcomeres in the first myofibrils is approximately 1.5 micron, approximately the width of the A bands of mature myofibrils. The possibility that the A bands might define the initial length of nascent sarcomeres was indicated.

Immunocytochemical studies of cardiac myofibrillogenesis in early chick embryos. I. Presence of immunofluorescent titin spots in premyofibril stages

Our initial attempts to immunolabel intact myocardial walls of 4-12 somite stage chick embryos were hindered by the presence of the cardiac jelly that covers the inner myocardial wall surface and prevents the access of antibodies to that surface. We overcame this difficulty by treating the specimens with hyaluronidase, which made the cardiac jelly permeable to the antibodies. An additional nonionic detergent treatment made the two or more cell layers of the myocardial wall accessible to the antibodies from both surfaces of the wall. Specimens treated in this manner were fluorescently labeled with antibodies to titin, myosin, or actin or with NBD-phallacidin for F-actin and examined as whole mount preparations or cut into semithin sections after resin embedding. These preparations and sections revealed that titin, a putative scaffolding protein of sarcomeres, is present in a punctate state and also in a diffuse form throughout the cytoplasm of cardiac myocytes in the premyofibril stages (4-7 somite stages) as well as in the early stages of myofibril formation. We interpreted the punctate and diffuse states to represent an aggregated state of several titin molecules and a dispersed state of individual titin molecules, respectively. In the 4-7 somite cardiac primodia, myosin and actin show only a uniform labeling throughout the cytoplasm of the myocytes. These observations are in contrast to a previous report that titin and myosin are tightly linked during in vitro skeletal myofibrillogenesis (Hill, C. S., S. Duran, Z. Ling, K. Weber, and H. Holtzer, 1986, J. Cell Biol., 103:2185-2196). In the 8-11 somite stage hearts, the number of individual titin spots rapidly reduces, while the number of myofibrils with periodically aligned titin spots increases, which strongly suggests that the titin spots are incorporated into the newly arising myofibrils. Titin spots were seen as doublets only after titin spots were incorporated into the first myofibrils. However, the fact that the distance between the components of the narrowest doublet was close to the resolution limit of the light microscope left open the possibility that undiscernible doublets of submicroscopic separations might exist in the premyofibril stages. The myosin labeling revealed the sarcomeric periodicity in an earlier stage of myofibril development than the F-actin labeling. In addition, we made two morphogenic observations.(ABSTRACT TRUNCATED AT 400 WORDS)

Heart tumors specifically induced in young avian embryos by the v-myc oncogene

To determine if expression of the v-myc oncogene had any effect during ontogeny, we injected avian myelocytomatosis virus strain MC29 into avian embryos at various stages of development. The injection of MC29 at embryonic day 2 (E2) or 3 (E3) caused, about 10 days later, rhabdomyosarcomas of the heart and, in some cases, skin muscle hypertrophy. When the injection was performed at E4 or E5, the number of heart tumors declined, whereas the number of skin muscle tumors increased significantly. The p110gag-myc protein was found in all tumors analyzed. When the virus was injected intravenously into E10 embryos, no tumors appeared during embryonic life, in striking contrast to the results obtained from injections at earlier stages. The monoclonal antibody 13F4, which is specific for the myogenic lineage, bound strongly to tumoral heart tissue, whereas it bound weakly to normal cardiac cells. Comparison of the peaks of tumor incidence in relationship to the timing of injection suggests that the v-myc product could interfere in vivo with an early step of the muscle lineage differentiation program. In addition, we show that the p58c-myc protein, which is supposed to play an important role in the control of cell proliferation, is only faintly detected in the heart of normal E3 embryos, in contrast to limb and tail buds, which readily express detectable levels of p58c-myc.

A monoclonal antibody specific for avian early myogenic cells and differentiated muscle

A monoclonal antibody raised in mouse in response to homogenates of Remark ganglia and dorsal mesentery of chicken embryos was found to exhibit a unique reactivity towards myogenic cells, heart, striated muscles, and smooth muscles in chicken and quail. Indirect immunofluorescence assays were performed at different stages of chicken and quail embryonic development and, after hatching, on tissue sections and cultured cells. They revealed that the cytoplasmic marker recognized by 13F4 is expressed in early embryonic heart, in somitic myotome (from stage 14 onward), in the skeletal muscles in limbs and trunk, in all muscles in the head and the branchial arches, in the smooth muscles of the digestive tract and blood vessels. In myofibrils of striated muscles, the antigen is localized in the Z lines. The antigenicity of the molecule recognized by 13F4 is not associated with a glycolipid or a glycoprotein. It is of peptidic nature and its molecular weight is 54 kDa. We stress the value of this cell-type-specific marker in studies on ontogenesis and differentiation of all muscular structures, namely, of myocardium and striated muscles, which express 13F4 antigenicity from an early developmental stage.

Mapping of early development of electrical activity in the embryonic chick heart using multiple-site optical recording

1. Using a multiple-site optical recording method with a 100-element photodiode array and a voltage-sensitive merocyanine-rhodanine dye, we have been able to monitor, for the first time, spontaneous electrical activity in pre-fused cardiac primordia in the 6- and 7-somite chick embryos. 2. To study the regional development of spontaneous electrical activity in the early embryonic pre-contractile chick heart at the later 7-9-somite stages, we have also recorded optically action potentials simultaneously from the entire heart, and constructed maps of the early development of electrical activity. 3. The data show that during the 6-9-somite stages, the size of the active area gradually increases, and that the development of electrical activity was spatially non-uniform: two peaks of activity were found in the right and left sides of the cono-ventricular region at the 7-8-somite stages. As development proceeded to the 9-somite stage, several peak areas of activity appeared. 4. The results are discussed in relation to the spatial pattern of proliferation of electrically active cells in the early phases of cardiogenesis.