Spatial distribution of "tissue-specific" antigens in the developing human heart and skeletal muscle. I. An immunohistochemical analysis of creatine kinase isoenzyme expression patterns

A disrupted compartment boundary underlies abnormal cardiac patterning and congenital heart defects

Failure of septation of the interventricular septum (IVS) is the most common congenital heart defect (CHD), but mechanisms for patterning the IVS are largely unknown. We show that a Tbx5+/Mef2cAHF+ progenitor lineage forms a compartment boundary bisecting the IVS. This coordinated population originates at a first- and second heart field interface, subsequently forming a morphogenetic nexus. Ablation of Tbx5+/Mef2cAHF+ progenitors cause IVS disorganization, right ventricular hypoplasia and mixing of IVS lineages. Reduced dosage of the CHD transcription factor TBX5 disrupts boundary position and integrity, resulting in ventricular septation defects (VSDs) and patterning defects, including Slit2 and Ntn1 misexpression. Reducing NTN1 dosage partly rescues cardiac defects in Tbx5 mutant embryos. Loss of Slit2 or Ntn1 causes VSDs and perturbed septal lineage distributions. Thus, we identify essential cues that direct progenitors to pattern a compartment boundary for proper cardiac septation, revealing new mechanisms for cardiac birth defects.

Human Cardiac Development

Many aspects of heart development are topographically complex and require three-dimensional (3D) reconstruction to understand the pertinent morphology. We have recently completed a comprehensive primer of human cardiac development that is based on firsthand segmentation of structures of interest in histological sections. We visualized the hearts of 12 human embryos between their first appearance at 3.5 weeks and the end of the embryonic period at 8 weeks. The models were presented as calibrated, interactive, 3D portable document format (PDF) files. We used them to describe the appearance and the subsequent remodeling of around 70 different structures incrementally for each of the reconstructed stages. In this chapter, we begin our account by describing the formation of the single heart tube, which occurs at the end of the fourth week subsequent to conception. We describe its looping in the fifth week, the formation of the cardiac compartments in the sixth week, and, finally, the septation of these compartments into the physically separated left- and right-sided circulations in the seventh and eighth weeks. The phases are successive, albeit partially overlapping. Thus, the basic cardiac layout is established between 26 and 32 days after fertilization and is described as Carnegie stages (CSs) 9 through 14, with development in the outlet component trailing that in the inlet parts. Septation at the venous pole is completed at CS17, equivalent to almost 6 weeks of development. During Carnegie stages 17 and 18, in the seventh week, the outflow tract and arterial pole undergo major remodeling, including incorporation of the proximal portion of the outflow tract into the ventricles and transfer of the spiraling course of the subaortic and subpulmonary channels to the intrapericardial arterial trunks. Remodeling of the interventricular foramen, with its eventual closure, is complete at CS20, which occurs at the end of the seventh week. We provide quantitative correlations between the age of human and mouse embryos as well as the Carnegie stages of development. We have also set our descriptions in the context of variations in the timing of developmental features.

Molecular Pathways and Animal Models of Ebstein's Anomaly

Ebstein's anomaly is a congenital malformation of the tricuspid valve characterized by abnormal attachment of the valve leaflets, resulting in varying degrees of valve dysfunction. The anatomic hallmarks of this entity are the downward displacement of the attachment of the septal and posterior leaflets of the tricuspid valve. Additional intracardiac malformations are common. From an embryological point of view, the cavity of the future right atrium does not have a direct orifice connected to the developing right ventricle. This chapter provides an overview of current insight into how this connection is formed and how malformations of the tricuspid valve arise from dysregulation of molecular and morphological events involved in this process. Furthermore, mouse models that show features of Ebstein's anomaly and the naturally occurring model of canine tricuspid valve malformation are described and compared to the human model. Although Ebstein's anomaly remains one of the least understood cardiac malformations to date, the studies summarized here provide, in aggregate, evidence for monogenic and oligogenic factors driving pathogenesis.

A pictorial account of the human embryonic heart between 3.5 and 8 weeks of development

Heart development is topographically complex and requires visualization to understand its progression. No comprehensive 3-dimensional primer of human cardiac development is currently available. We prepared detailed reconstructions of 12 hearts between 3.5 and 8 weeks post fertilization, using Amira® 3D-reconstruction and Cinema4D®-remodeling software. The models were visualized as calibrated interactive 3D-PDFs. We describe the developmental appearance and subsequent remodeling of 70 different structures incrementally, using sequential segmental analysis. Pictorial timelines of structures highlight age-dependent events, while graphs visualize growth and spiraling of the wall of the heart tube. The basic cardiac layout is established between 3.5 and 4.5 weeks. Septation at the venous pole is completed at 6 weeks. Between 5.5 and 6.5 weeks, as the outflow tract becomes incorporated in the ventricles, the spiraling course of its subaortic and subpulmonary channels is transferred to the intrapericardial arterial trunks. The remodeling of the interventricular foramen is complete at 7 weeks.

Sinus venosus incorporation: contentious issues and operational criteria for developmental and evolutionary studies

The sinus venosus is a cardiac chamber upstream of the right atrium that harbours the dominant cardiac pacemaker. During human heart development, the sinus venosus becomes incorporated into the right atrium. However, from the literature it is not possible to deduce the characteristics and importance of this process of incorporation, due to inconsistent terminology and definitions in the description of multiple lines of evidence. We reviewed the literature regarding the incorporation of the sinus venosus and included novel electrophysiological data. Most mammals that have an incorporated sinus venosus show a loss of a functional valve guard of the superior caval vein together with a loss of the electrical sinuatrial delay between the sinus venosus and the right atrium. However, these processes are not necessarily intertwined and in a few species only the sinuatrial delay may be lost. Sinus venosus incorporation can be characterised as the loss of the sinuatrial delay of which the anatomical and molecular underpinnings are not yet understood.

Development of the ventral body wall in the human embryo

Migratory failure of somitic cells is the commonest explanation for ventral body wall defects. However, the embryo increases ~ 25-fold in volume in the period that the ventral body wall forms, so that differential growth may, instead, account for the observed changes in topography. Human embryos between 4 and 10 weeks of development were studied, using amira reconstruction and cinema 4D remodeling software for visualization. Initially, vertebrae and ribs had formed medially, and primordia of sternum and hypaxial flank muscle primordium laterally in the body wall at Carnegie Stage (CS)15 (5.5 weeks). The next week, ribs and muscle primordium expanded in ventrolateral direction only. At CS18 (6.5 weeks), separate intercostal and abdominal wall muscles differentiated, and ribs, sterna, and muscles began to expand ventromedially and caudally, with the bilateral sternal bars fusing in the midline after CS20 (7 weeks) and the rectus muscles reaching the umbilicus at CS23 (8 weeks). The near-constant absolute distance between both rectus muscles and approximately fivefold decline of this distance relative to body circumference between 6 and 10 weeks identified dorsoventral growth in the dorsal body wall as determinant of the 'closure' of the ventral body wall. Concomitant with the straightening of the embryonic body axis after the 6th week, the abdominal muscles expanded ventrally and caudally to form the infraumbilical body wall. Our data, therefore, show that the ventral body wall is formed by differential dorsoventral growth in the dorsal part of the body.

The use of Piper sarmentosum leaves aqueous extract (Kadukmy™) as antihypertensive agent in spontaneous hypertensive rats

BACKGROUND

The National Health and Morbidity Survey in 2011 estimated that 35.1% (5.7 million) of Malaysian adults aged 18 and older suffer from hypertension. Hypertension is still treated by conventional medicine despite its exact aetiology being unknown. Studies showed that oxidative stress and low availability of nitric oxide (NO) causes an increase in vascular wall tension and increase blood pressure. Piper sarmentosum (PS) a traditional Malay herbal plant is well known for its high antioxidant content. Antioxidant is useful in improving cardiovascular diseases particularly hypertension. Thus, it is beneficial to determine the effect of PS leaves aqueous extract (Kadukmy™) on the blood pressure, NO level, oxidative stress markers and serum cholesterol level of the Spontaneous Hypertensive Rats (SHR).

METHODS

Rats were devided into five groups consisting of three treatment groups and two control groups. Baseline blood investigations were done before and following commencement of treatment. Spontaneous hypertensive rats were treated for 28 consecutive days and the blood pressure was measured weekly.

RESULTS

Kadukmy™ administration showed a significant reduction in systolic blood pressure (SBP), diastolic blood pressure (DBP) and mean arterial pressure (MAP) (P < 0.05), increased serum NO level (P < 0.05), reduced serum malondialdehyde (MDA) level (P < 0.05) and reduction of serum total cholesterol level in groups treated with Kadukmy-1™.

CONCLUSIONS

The result of the present study revealed that Kadukmy™ exerts its antioxidant activity to reduce oxidative stress damage, increase NO production and able to reduce blood pressure and cholesterol level.

Epicardially derived fibroblasts preferentially contribute to the parietal leaflets of the atrioventricular valves in the murine heart

The importance of the epicardium for myocardial and valvuloseptal development has been well established; perturbation of epicardial development results in cardiac abnormalities, including thinning of the ventricular myocardial wall and malformations of the atrioventricular valvuloseptal complex. To determine the spatiotemporal contribution of epicardially derived cells to the developing fibroblast population in the heart, we have used a mWt1/IRES/GFP-Cre mouse to trace the fate of EPDCs from embryonic day (ED)10 until birth. EPDCs begin to populate the compact ventricular myocardium around ED12. The migration of epicardially derived fibroblasts toward the interface between compact and trabecular myocardium is completed around ED14. Remarkably, epicardially derived fibroblasts do not migrate into the trabecular myocardium until after ED17. Migration of EPDCs into the atrioventricular cushion mesenchyme commences around ED12. As development progresses, the number of EPDCs increases significantly, specifically in the leaflets which derive from the lateral atrioventricular cushions. In these developing leaflets the epicardially derived fibroblasts eventually largely replace the endocardially derived cells. Importantly, the contribution of EPDCs to the leaflets derived from the major AV cushions is very limited. The differential contribution of EPDCs to the various leaflets of the atrioventricular valves provides a new paradigm in valve development and could lead to new insights into the pathogenesis of abnormalities that preferentially affect individual components of this region of the heart. The notion that there is a significant difference in the contribution of epicardially and endocardially derived cells to the individual leaflets of the atrioventricular valves has also important pragmatic consequences for the use of endocardial and epicardial cre-mouse models in studies of heart development.

Tbx2 and Tbx3 induce atrioventricular myocardial development and endocardial cushion formation

A key step in heart development is the coordinated development of the atrioventricular canal (AVC), the constriction between the atria and ventricles that electrically and physically separates the chambers, and the development of the atrioventricular valves that ensure unidirectional blood flow. Using knock-out and inducible overexpression mouse models, we provide evidence that the developmentally important T-box factors Tbx2 and Tbx3, in a functionally redundant manner, maintain the AVC myocardium phenotype during the process of chamber differentiation. Expression profiling and ChIP-sequencing analysis of Tbx3 revealed that it directly interacts with and represses chamber myocardial genes, and induces the atrioventricular pacemaker-like phenotype by activating relevant genes. Moreover, mutant mice lacking 3 or 4 functional alleles of Tbx2 and Tbx3 failed to form atrioventricular cushions, precursors of the valves and septa. Tbx2 and Tbx3 trigger development of the cushions through a regulatory feed-forward loop with Bmp2, thus providing a mechanism for the co-localization and coordination of these important processes in heart development.

Origin and development of the atrioventricular myocardial lineage: insight into the development of accessory pathways

Defects originating from the atrioventricular canal region are part of a wide spectrum of congenital cardiovascular malformations that frequently affect newborns. These defects include partial or complete atrioventricular septal defects, atrioventricular valve defects, and arrhythmias, such as atrioventricular re-entry tachycardia, atrioventricular nodal block, and ventricular preexcitation. Insight into the cellular origin of the atrioventricular canal myocardium and the molecular mechanisms that control its development will aid in the understanding of the etiology of the atrioventricular defects. This review discusses current knowledge concerning the origin and fate of the atrioventricular canal myocardium, the molecular mechanisms that determine its specification and differentiation, and its role in the development of certain malformations such as those that underlie ventricular preexcitation.

Developmental anatomy of the heart: a tale of mice and man

Because of the increasing availability of tools for genetic manipulation, the mouse has become the most popular animal model for studying normal and abnormal cardiac development. However, despite the enormous advances in mouse genetics, which have led to the production of numerous mutants with cardiac abnormalities resembling those seen in human congenital heart disease, relatively little comparative work has been published to demonstrate the similarities and differences in the developmental cardiac anatomy in both species. In this review we discuss some aspects of the comparative anatomy, with emphasis on the atrial anatomy, the valvuloseptal complex, and ventricular myocardial development. From the data presented it can be concluded that, apart from the obvious differences in size, the mouse and human heart are anatomically remarkably similar throughout development. The partitioning of the cardiac chambers (septation) follows the same sequence of events, while also the maturation of the cardiac valves and myocardium is quite similar in both species. The major anatomical differences are seen in the venous pole of the heart. We conclude that, taking note of the few anatomical “variations,” the use of the mouse as a model system for the human heart is warranted. Thus the analysis of mouse mutants with impaired septation will provide valuable information on cellular mechanisms involved in valvuloseptal morphogenesis (a process often disrupted in congenital heart disease), while the study of embryonic lethal mouse mutants that present with lack of compaction of ventricular trabeculae will ultimately provide clues on the etiology of this abnormality in humans.

Cardiac muscle cell formation after development of the linear heart tube

After the development of the linear heart tube, additional myocardium is formed leading to the muscular mantle around the caval and pulmonary veins and the muscular septa in the embryonic heart. Here, we report the results of our in vivo and in vitro studies of this late myocardium-generating process in the mouse. By using an immunohistochemical approach, we determined that myocardium formation starts around embryonic day 12 in the dorsal mesocardium. In subsequent stages of development, the process extends downstream into the intracardiac mesenchymal tissues of the atrioventricular canal and outflow tract and upstream into the extracardiac mediastinal mesenchyme embedding the pulmonary and caval veins. Given the spatiotemporal pattern of myocardium formation, we applied a three-dimensional in vitro explant culture assay to investigate the myocardium-generating potential of the different cardiac compartments. We determined that this potential is stage- and mesenchyme-dependent. This latter finding suggests an important role for mesenchyme in myocardium formation after the development of the linear heart tube.

Recruitment of intra- and extracardiac cells into the myocardial lineage during mouse development

The tubular heart differentiates from the bilateral cardiac fields in the splanchnic mesoderm. The expression of smooth muscle proteins has been shown to accompany the early phases of cardiac muscle formation. In this study we show that during elongation of the arterial pole of the mouse linear heart tube, alpha-smooth muscle actin (alpha-Sma) expression extends in the area that has been shown to become recruited into the myocardial lineage, but does not yet express myocardial markers. These data suggest that alpha-Sma identifies mesodermal cells that during subsequent development will be recruited into the myocardial lineage. Myocardium formation is not only observed at the arterial pole, but also at the venous pole and in the intracardiac mesenchyme. This results in the formation of the caval and pulmonary myocardium, the smooth-walled atrial myocardium, the myocardial atrioventricular septum, and the myocardial outlet septum. To determine whether recruitment into the myocardial lineage also takes place in these regions, the spatiotemporal pattern of expression of alpha-Sma and of the myocardial markers sarcoplasmatic reticulum calcium ATPase (Serca2a), alpha-myosin heavy chain (Mhc), and beta-Mhc were examined. We show that prior to the expression of myocardial markers, alpha-Sma is expressed in these regions, which suggests that these mesodermal cells become recruited into the cardiac lineage after formation of the linear heart tube.

Cardiac septation: a late contribution of the embryonic primary myocardium to heart morphogenesis

Heart morphogenesis comprises 2 major consecutive steps, viz. chamber formation followed by septation. Septation is the remodeling of the heart from a single-channel peristaltic pump to a dual-channel, synchronously contracting device with 1-way valves. In the human heart, septation occurs between 4 and 7 weeks of development. Cardiac looping and chamber formation bring the contributing structures into position to engage in septation. Cardiomyocytes that participate in chamber formation do not materially contribute to septation. The (re)discovery of the role of extracardiac mesenchymal tissue in atrioventricular septation, the appreciation that the formation of the right atrioventricular connection is more than a mere rightward expansion of the atrioventricular canal, the awareness that myocardium originating from the so-called anterior heart field regresses after its function as outflow-tract sphincter ceases, and the recent finding that the myocardialized proximal portion of the outflow-tract septum becomes the supraventricular crest have all significantly enhanced our understanding of the morphogenetic processes that contribute to septation. The bifurcation of the ventricular conduction system is the landmark that separates the contribution of the atrioventricular cushions and the outflow-tract ridges to septation and that divides the muscular ventricular septum in inlet, trabecular, and outlet portions.

Development of the myocardium of the atrioventricular canal and the vestibular spine in the human heart

To establish the morphogenetic mechanisms underlying formation and separation of the atrioventricular connections, we studied the remodeling of the myocardium of the atrioventricular canal and the extracardiac mesenchymal tissue of the vestibular spine in human embryonic hearts from 4.5 to 10 weeks of development. Septation of the atrioventricular junction is brought about by downgrowth of the primary atrial septum, fusion of the endocardial cushions, and forward expansion of the vestibular spine between atrial septum and cushions. The vestibular spine subsequently myocardializes to form the ventral rim of the oval fossa. The connection of the atrioventricular canal with the atria expands evenly. In contrast, the expression patterns of creatine kinase M and GlN2, markers for the atrioventricular and interventricular junctions, respectively, show that the junction of the canal with the right ventricle forms by local growth in the inner curvature of the heart. Growth of the caudal portion of the muscular ventricular septum to make contact with the inferior endocardial cushion occurs only after the canal has expanded rightward. The atrioventricular node develops from that part of the canal myocardium that retains its continuity with the ventricular myocardium.

Multiple transcriptional domains, with distinct left and right components, in the atrial chambers of the developing heart

During heart development, 2 fast-conducting regions of working myocardium balloon out from the slow-conducting primary myocardium of the tubular heart. Three regions of primary myocardium persist: the outflow tract, atrioventricular canal, and inflow tract, which are contiguous throughout the inner curvature of the heart. The contribution of the inflow tract to the definitive atrial chambers has remained enigmatic largely because of the lack of molecular markers that permit unambiguous identification of this myocardial domain. We now report that the genes encoding atrial natriuretic factor, myosin light chain (MLC) 3F, MLC2V, and Pitx-2, and transgenic mouse lines expressing nlacZ under the control of regulatory sequences of the mouse MLC1F/3F gene, display regionalized patterns of expression in the atrial component of the developing mouse heart. These data distinguish 4 broad transcriptional domains in the atrial myocardium: (1) the atrioventricular canal that will form the smooth-walled lower atrial rim proximal to the ventricles; (2) the atrial appendages; (3) the caval vein myocardium (systemic inlet); and (4) the mediastinal myocardium (pulmonary inlet), including the atrial septa. The pattern of expression of Pitx-2 reveals that each of these transcriptional domains has a distinct left and right component. This study reveals for the first time differential gene expression in the systemic and pulmonary inlets, which is not shared by the contiguous atrial appendages and provides evidence for multiple molecular compartments within the atrial chambers. Furthermore, this work will allow the contribution of each of these myocardial components to be studied in congenitally malformed hearts, such as those with abnormal venous return.

Atrial development in the human heart: an immunohistochemical study with emphasis on the role of mesenchymal tissues

The development of the atrial chambers in the human heart was investigated immunohistochemically using a set of previously described antibodies. This set included the monoclonal antibody 249-9G9, which enabled us to discriminate the endocardial cushion-derived mesenchymal tissues from those derived from extracardiac splanchnic mesoderm, and a monoclonal antibody recognizing the B isoform of creatine kinase, which allowed us to distinguish the right atrial myocardium from the left. The expression patterns obtained with these antibodies, combined with additional histological information derived from the serial sections, permitted us to describe in detail the morphogenetic events involved in the development of the primary atrial septum (septum primum) and the pulmonary vein in human embryos from Carnegie stage 14 onward. The level of expression of creatine kinase B (CK-B) was found to be consistently higher in the left atrial myocardium than in the right, with a sharp boundary between high and low expression located between the primary septum and the left venous valve indicating that the primary septum is part of the left atrial gene-expression domain. This expression pattern of CK-B is reminiscent of that of the homeobox gene Pitx2, which has recently been shown to be important for atrial septation in the mouse. This study also demonstrates a poorly appreciated role of the dorsal mesocardium in cardiac development. From the earliest stage investigated onward, the mesenchyme of the dorsal mesocardium protrudes into the dorsal wall of the primary atrial segment. This dorsal mesenchymal protrusion is continuous with a mesenchymal cap on the leading edge of the primary atrial septum. Neither the mesenchymal tissues of the dorsal protrusion nor the mesenchymal cap on the edge of the primary septum expressed the endocardial tissue antigen recognized by 249-9G9 at any of the stages investigated. The developing pulmonary vein uses the dorsal mesocardium as a conduit to reach the primary atrial segment. Initially, the pulmonary pit, which will becomes the portal of entry for the pulmonary vein, is located along the midline, flanked by two myocardial ridges. As development progresses, tissue remodeling results in the incorporation of the portal of entry of the pulmonary vein in left atrial myocardium, which is recognized because of its high level of creatine. Closure of the primary atrial foramen by the primary atrial septum occurs as a consequence of the fusion of these mesenchymal structures.

An Nkx-dependent enhancer regulates cGATA-6 gene expression during early stages of heart development

The evolutionarily conserved GATA-6 transcription factor is an early and persistent marker of heart development in diverse vertebrate species. We previously found evidence for a functionally conserved heart-specific enhancer upstream of the chicken GATA-6 (cGATA-6) gene and in the present study we used transgenic mouse assays to further characterize this regulatory module. We show that this enhancer is activated in committed precursor cells within the cardiac crescent and that it remains active in essentially all cardiogenic cells through the linear heart stage. Although this enhancer can account for cGATA-6 gene expression early in the cardiogenic program, it is not able to maintain expression throughout the heart later in development. In particular, the enhancer is sequentially downregulated along the posterior to anterior axis, with activity becoming confined to outflow tract myocardium. Enhancers with similar properties have been shown to regulate the early heart-restricted expression of the mouse Nkx2.5 transcription factor gene. Whereas these Nkx2.5 enhancers are GATA-dependent, we show that the cGATA-6 enhancer is Nkx-dependent. We speculate that these enhancers are silenced to allow GATA-6 and Nkx2.5 gene expression to be governed by region-specific enhancers in the multichambered heart.

Modular regulation of cGATA-5 gene expression in the developing heart and gut

The evolutionarily conserved GATA-5 transcription factor is an early and persistent marker of heart and gut development in diverse vertebrate species. To search for control regions that might regulate the chicken GATA-5 (cGATA-5) gene, we assayed a set of cGATA-5/lacZ constructs in transgenic mice and found evidence for two functionally conserved control regions that regulate different facets of cGATA-5 gene expression. The more distal control region is activated in embryonic endoderm at the head-fold stage, whereas the other control region contains a regulatory module that is activated in a restricted region of endoderm following closure of the gut tube. Remarkably, the latter control region also contains a complex regulatory module that is activated in the cardiac crescent at the head-fold stage and subsequently functions in several mesodermal components of the developing heart, including the outer (epicardial) layer. We discuss these results in terms of possible contributions of epicardial-derived cells to the formation of heart valves, conduction tissue, and compact myocardium. These transgenes thus reveal, and provide a means to further analyze, transcriptional programs for several facets of heart morphogenesis and gut development.

Characterization of two types of mitochondrial creatine kinase isolated from normal human cardiac muscle and brain tissue

Two types of mitochondrial creatine kinase (Mi-CK), sarcomeric (sMi-) and ubiquitous (uMi-)CKs, were isolated from normal human cardiac muscle and brain tissue, respectively, and their heterogeneity was characterized by means of isoelectric focusing (IEF). Octameric sMi-CK and uMi-CK were electrophoresed cathodic to cytoplasmic muscle-type creatine kinase isoenzyme (CK-MM) and dimeric Mi-CKs were found at the position of CK-MM on a cellulose acetate membrane. The electrophoretic mobilities of sMi-CK were similar to those of uMi-CK. Octameric sMi-CK was focused at pI 7.1-8.0 and dimeric forms at pI 6.55, 6.75, 6.85, and 6.95. New bands appearing at pI 6.65 and 6.75 after treatment of sMi-CK with carboxypeptidase B were found to be delysined forms. sMi-CK reacted with anti-sMi-CK antibodies, and the immune complexes were focused at pI 5.8. The Km value of sMi-CK for creatine phosphate (PCr) was 1.19 +/-0.20 mmol/L (mean +/- standard error), the activation energy (Ea) was 108.3+/-1.2 kJ/ mol, and the residual enzyme activity after heating at 45 degrees C for 20 min was 79.6+/-1.9%. On the other hand, octameric uMi-CK was focused at pI 7.1-7.9 and the dimeric forms were focused at pI 6.6, 6.7, 6.8, 6.9, and 7.0. Delysined forms were focused around pI 6.3, 6.4, 6.8, and 6.9. uMi-CK reacted with anti-sMi-CK antibodies, and the immune complexes were focused at pI 5.8. The Km value of uMi-CK for PCr was 1.07+/-0.03 mmol/L, Ea of uMi-CK was 110.0+/-0.9 kJ/mol, and the residual enzyme activity after heating at 45 degrees C for 20 min was 90.3+/-0.4%. The sMi-CK and uMi-CK were hybridized and the hybrid Mi-CK appeared at pI 6.78, 6.98, and 7.1-7.95. The pIs of the hybrid Mi-CK were between those of sMi-CK and uMi-CK. As described above, sMi-CK and uMi-CK were slightly different from each other with respect to the pI and some enzyme characteristics.

Myocardialization of the cardiac outflow tract

During development, the single-circuited cardiac tube transforms into a double-circuited four-chambered heart by a complex process of remodeling, differential growth, and septation. In this process the endocardial cushion tissues of the atrioventricular junction and outflow tract (OFT) play a crucial role as they contribute to the mesenchymal components of the developing septa and valves in the developing heart. After fusion, the endocardial ridges in the proximal portion of the OFT initially form a mesenchymal outlet septum. In the adult heart, however, this outlet septum is basically a muscular structure. Hence, the mesenchyme of the proximal outlet septum has to be replaced by cardiomyocytes. We have dubbed this process "myocardialization." Our immunohistochemical analysis of staged chicken hearts demonstrates that myocardialization takes place by ingrowth of existing myocardium into the mesenchymal outlet septum. Compared to other events in cardiac septation, it is a relatively late process, being initialized around stage H/H28 and being basically completed around stage H/H38. To unravel the molecular mechanisms that are responsible for the induction and regulation of myocardialization, an in vitro culture system in which myocardialization could be mimicked and manipulated was developed. Using this in vitro myocardialization assay it was observed that under the standard culture conditions (i) whole OFT explants from stage H/H20 and younger did not spontaneously myocardialize the collagen matrix, (ii) explants from stage H/H21 and older spontaneously formed extensive myocardial networks, (iii) the myocardium of the OFT could be induced to myocardialize and was therefore "myocardialization-competent" at all stages tested (H/H16-30), (iv) myocardialization was induced by factors produced by, most likely, the nonmyocardial component of the outflow tract, (v) at none of the embryonic stages analyzed was ventricular myocardium myocardialization-competent, and finally, (vi) ventricular myocardium did not produce factors capable of supporting myocardialization.

Replacement by homologous recombination of the minK gene with lacZ reveals restriction of minK expression to the mouse cardiac conduction system

The minK gene encodes a 129-amino acid peptide the expression of which modulates function of cardiac delayed rectifier currents (IKr and IKs), and mutations in minK are now recognized as one cause of the congenital long-QT syndrome. We have generated minK-deficient mice in which the bacterial lacZ gene has been substituted for the minK coding region such that beta-galactosidase expression is controlled by endogenous minK regulatory elements. In cardiac myocytes isolated from wild-type neonatal mice, IKs is rarely recorded, while IKr is common. In minK (-/-) myocytes, IKs is absent and IKr is significantly reduced and its deactivation slowed; these results further support a role for minK in modulating both IKs and IKr. Despite these changes, ECGs in (+/+) and (-/-) animals are no different at adult and at neonatal stages. ECG responses to isoproterenol are also similar in the 2 groups. beta-Galactosidase staining in postnatal minK (-/-) hearts is highly restricted, to the sinus-node region, caudal atrial septum, and proximal conducting system. Moreover, as early as embryonal day 11, segmentally restricted beta-galactosidase expression is observed in the portions of the sinoatrial and atrioventricular junctions that are thought to give rise to the conducting system, thereby implicating minK expression as an early event in conduction system development. More generally, the restricted nature of minK expression in the mouse heart suggests species-specific roles of this gene product in mediating the electrophysiological properties of the heart.

Evaluation of regional differences in right ventricular systolic function by acoustic quantification echocardiography and cine magnetic resonance imaging

BACKGROUND

Accurate quantitative evaluation of right ventricular (RV) function has been limited by its complex structural geometry. Although embryological and anatomic observations suggest that the RV is composed of 2 distinct components, the RV sinus and infundibulum, most studies on RV dimensions and function viewed it as a single chamber. This study was designed to determine the volumes, relative contribution to global systolic function, and temporal course of contraction and relaxation of the RV sinus and infundibulum.

METHODS AND RESULTS

Thirty-one individuals without heart disease (aged 1 month to 17 years, 16 boys and 15 girls) participated in this study. Instantaneous area over time, its derivatives, and the temporal course of contraction and relaxation were studied by acoustic quantification echocardiography and phonocardiography in 20 individuals. Global and regional RV volumes and ejection fraction were determined by cine MRI in 11 individuals. The RV sinus made up 81+/-6% of the combined RV end-diastolic volume and 87+/-4% of the combined stroke volume. The infundibulum accounted for the remaining 19+/-6% and 13+/-4%, respectively (P<0.0001). Compared with the infundibulum, the extent of RV sinus fiber shortening was significantly greater: for ejection fraction (56+/-11% versus 38+/-13%, P<0.001), fractional area change (42+/-14% versus 28+/-9%, P<0.0001), and dA/dt (27+/-17% versus 13+/-6%, P<0.0001). Analysis of temporal course of contraction and relaxation (expressed as percentage of the cardiac cycle to adjust for differences in heart rate) showed that the infundibulum follows the RV sinus: onset of contraction 53%+/-14 versus 19+/-11% of systole, time to peak systole 115+/-16% versus 97+/-19% (P< or =0.01), indicating a peristalsis-like pattern of contraction and relaxation.

CONCLUSIONS

The results of this study demonstrate significant regional differences between the sinus and infundibulum components of the RV with regard to contribution to stroke volume, extent of fiber shortening, and sequence of mechanical activation. These data from normal individuals can be used in future research on RV function in pathological conditions.

Isolated congenitally complete heart block attributable to combined nodoventricular and intraventricular discontinuity

Intraventricular together with atrial-axis and nodoventricular discontinuity, in which various parts of the conduction system are replaced by fibrous or fatty tissue, constitute the three major pathological categories of isolated congenitally complete heart block. Intraventricular discontinuity is distinctly rare, with only two previous cases reported in the literature, one of which was associated with a familial history of heart block. The cardiac conduction systems of two cases of isolated congenitally complete heart block were serially sectioned and analyzed histopathologically. The findings were correlated with the clinical features, in particular, the family histories and maternal serum anti-Ro antibodies. Both cases, a 9-day-old neonate and an 8-year-old schoolgirl, showed a combination of nodoventricular and intraventricular discontinuity, with absence of the atrioventricular penetrating bundle, the entire right, and the proximal portion of the left bundle branch. The branching bundle was absent in the first case and replaced by fatty tissue in the second. In contrast to the commoner atrial-axis discontinuity in which the atrioventricular node itself is usually replaced by fibrous or fatty tissue with variable involvement distally, the sinus node, and in particular, the atrioventricular node were normal in both of our cases. There was no family history in either case, whereas tests for the maternal serum anti-Ro antibody were positive in the first but negative in the second case. Intraventricular discontinuity as a cause of isolated congenitally complete heart block is very rare. In our cases, it co-existed with nodoventricular discontinuity. It can be sporadic, familial, or associated with positive maternal serum anti-Ro antibodies.

Heart and neural tube defects in transgenic mice overexpressing the Cx43 gap junction gene

Transgenic mice were generated containing a cytomegaloviral promoter driven construct (CMV43) expressing the gap junction polylpeptide connexin 43. RNA and protein analysis confirmed that the transgene was being expressed. In situ hybridization analysis of embryo sections revealed that transgene expression was targeted to the dorsal neural tube and in subpopulations of neural crest cells. This expression pattern was identical to that seen in transgenic mice harboring other constructs driven by the cytomegaloviral promoter (Kothary, R., Barton, S. C., Franz, T., Norris, M. L., Hettle, S. and Surani, M. A. H. (1991) Mech. Develop. 35, 25–31; Koedood, M., Fitchel, A., Meier, P. and Mitchell, P. (1995) J. Virol. 69, 2194–2207), and corresponded to a subset of the endogenous Cx43 expression domains. Significantly, dye injection studies showed that transgene expression resulted in an increase in gap junctional communication. Though viable and fertile, these transgenic mice exhibited reduced postnatal viability. Examination of embryos at various stages of development revealed developmental perturbations consisting of cranial neural tube defects (NTD) and heart malformations. Interestingly, breeding of the CMV43 transgene into the Cx43 knockout mice extended postnatal viability of mice homozygote for the Cx43 knockout allele, indicating that the CMV43 trangsene may partially complement the Cx43 deletion. Both the Cx43 knockout and the CMV43 transgenic mice exhibit heart defects associated with malformations in the conotruncus, a region of the heart in which neural crest derivatives are known to have important roles during development. Together with our results indicating neural-crest-specific expression of the transgene in our CMV-based constructs, these observations strongly suggest a role for Cx43-mediated gap junctional communication in neural crest development. Furthermore, these observations indicate that the precise level of Cx43 function may be of critical importance in downstream events involving these migratory cell populations. As such, the CMV43 mouse may represent a powerful new model system for examining the role of extracardiac cell populations in cardiac morphogenesis and other developmental processes.

The development of the atrioventricular junction in the human heart

The histogenesis of the separation between atrial and ventricular myocardium at the atrioventricular junction in the developing human heart has been investigated immunohistochemically by using monoclonal antibodies specific for atrioventricular cushion tissue, mesenchymal cells, atrial and ventricular myocardium, and myocardium of the primary ring. It was found that the insulation between the muscle masses of atrium and ventricle is established by the fusion of the tissues of the atrioventricular sulcus (located at the epicardial side of the junctional myocardium) with those of the atrioventricular cushions (located at the endocardial side of the junctional myocardium). This process takes place at the ventricular margin of the myocardium of the atrioventricular canal. The separation of atrial and ventricular myocardium starts at approximately 7 weeks of development in the anteromedial portion of the right atrioventricular junction and is largely completed around the 12th week of development. The only remaining myocardial continuity between atrial and ventricular myocardium is the atrioventricular axis of conduction. Our findings show that the nonmuscular part of the developing leaflets of the atrioventricular valves derives from the atrioventricular cushions and that the tissues of the atrioventricular groove do not contribute to the development of these leaflets.

The immunohistochemical localization of the non-specific lipid transfer protein (sterol carrier protein-2) in rat small intestine enterocytes

A 13 kDa protein was isolated from rabbit small intestine brush-border membrane vesicles that was postulated to be involved in intestinal phosphatidylcholine (PC) and cholesterol uptake. This protein has cholesterol and PC-transfer activity in vitro (Turnhofer, H. et al. (1991) Biochim. Biophys. Acta 1064, 275-286) and has a molecular mass and isoelectric point similar to that of the non-specific lipid transfer protein (nsL-TP, identical to sterol carrier protein-2). In addition, the first 28 N-terminal amino acid residues of the 13 kDa protein are nearly identical to nsL-TP from different species (Lipka, G. et al. (1995) J. Biol. Chem. 270, 5917-5925). In view of its possible role in intestinal lipid absorption, the localization of nsL-TP in rat small intestine was investigated using immunohistochemistry and immunoblotting. It is shown that nsLTP is predominantly localized in a subapical zone of the enterocyte but not in the brush-border membrane, thereby excluding a role in lipid uptake of this protein at the level of the plasma membrane. nsL-TP co-localized with the peroxisomal marker PMP70, underscoring earlier observations that nsL-TP is a peroxisomal protein. nsL-TP was found to be present along the entire length of the small intestine. The 58 kDa cross-reactive protein that was recently identified as a peroxisomal thiolase was shown to be present only in a small segment approximately halfway down the jejunum. The close apposition of the peroxisomes with the apical membrane and the discrete distribution of the 58 kDa protein may indicate that these organelles play a role in the intracellular processing of absorbed lipids.

Quantitative morphometric analysis of progressive infundibular obstruction in tetralogy of Fallot. A prospective longitudinal echocardiographic study

BACKGROUND

The morphological hallmark of tetralogy of Fallot is controversial, with much disagreement as to whether the subpulmonary infundibulum in this lesion is hypoplastic. In addition, few quantitative data are available regarding the morphometry of the subpulmonary infundibulum, what anatomic characteristics are acquired in the postnatal period, and at what rate they progress. We also sought to determine whether echocardiographic morphometric analysis of the infundibulum can predict clinical course in infants with tetralogy of Fallot.

METHODS AND RESULTS

Twenty-one infants with tetralogy of Fallot (median age at initial study, 1.6 months) were prospectively followed with serial echocardiograms until the time of first surgical intervention (median age at surgery, 10 months). Selected video still frames were digitized off-line with a computerized system. Compared with age-matched normal control infants (n = 37), the following indexed infundibular dimensions in patients with tetralogy of Fallot were significantly smaller: length (1.86 +/- 0.54 versus 2.7 +/- 0.56 cm/BSA0.5, P < .0001), cross-sectional area (1.6 +/- 0.49 versus 4.7 +/- 1.3 cm2/BSA, P < .0001), and volume (1.24 +/- 0.62 versus 7.2 +/- 3 mL/BSA1.5, P < .0001). The following measurements were increased in tetralogy patients: infundibular septal thickness (0.83 +/- 0.21 versus 0.54 +/- 0.06 cm/BSA0.5, P = .0002) and infundibular free-wall thickness (0.62 +/- 0.13 versus 0.49 +/- 0.06 cm/BSA0.5, P = .006). The angle between infundibular septum and ventricular septum had a greater degree of anterosuperior deviation in tetralogy patients, resulting in a larger infundibuloventricular septal angle (77 +/- 8.2 degrees versus 31 +/- 6.5 degrees, P < .0001). During follow-up, infundibular volume in tetralogy patients decreased from 1.24 +/- 0.62 to 0.81 +/- 0.47 mL/BSA1.5 (P = .002), correlating with infundibular septal thickness (r = -.63, P < .003). The mean rate of decrease of indexed infundibular volume was 0.1 +/- 0.13 mL.BSA-15.mo-1. Correlation analysis revealed a nonlinear correlation between the degree of infundibular septal malalignment and indexed infundibular volume (r = .93, P < .0001). Tetralogy patients who required early surgical intervention (4.8 +/- 0.9 versus 10.7 +/- 1.7 months, P < .0001) had a smaller infundibulum at presentation (0.92 +/- 0.35 versus 1.41 +/- 0.67 mL/BSA1.5, P = .04) and an accelerated rate of infundibular narrowing (0.17 +/- 0.18 versus 0.06 +/- 0.08 mL.BSA-1.5.mo-1, P = .04).

CONCLUSIONS

Compared with normal infants, the subpulmonary infundibulum in tetralogy of Fallot is characterized by a smaller volume, shorter and thicker infundibular septum, and anterosuperior deviation of the infundibular septum. Infundibular obstruction in tetralogy patients is progressive, with an average rate of decrease in indexed infundibular volume of 0.1 +/- 0.13 mL.BSA-1.5.mo-1. Infants who are likely to require early therapeutic intervention may be identified on their initial echocardiogram as having an infundibular volume of < 0.9 to 1.0 mL/BSA1.5.

Dystrophin expression in the developing conduction system of the human heart

Duchenne muscular dystrophy (DMD) is frequently associated with myocardial involvement. Dystrophin, the DMD protein, is found at the plasmamembrane of striated muscle fibers. Although dystrophin is missing in most or all muscle fibers of DMD patients, cardiac muscle is not as severely affected as skeletal muscle. Therefore it is of great importance to study the expression of dystrophin in normal cardiac muscle. We performed immunohistochemical studies and examined cardiac muscle of fetuses of 8 to 13 weeks of development on dystrophin expression. At these stages dystrophin is observed in the myocytes of the developing ventricular conduction system and in the atrial cardiomyocytes. Dystrophin was absent from the heart of a 12-week-old DMD fetus.

Section directed cryosectioning of specimens for scanning electron microscopy: a new method to study cardiac development

A new method to study the developing heart was developed. Using this "section directed" cryosectioning method, appropriate fixed embryos can be trimmed optimally to obtain sectional planes that, if necessary, can be matched with histologically treated sections. As a result, the morphological information obtained with the scanning electron microscope can be compared in detail with the information on the molecular phenotypes of the subpopulations of cells as deducted from staining patterns of the sections. This method allows combination of the specific advantages of sophisticated histological techniques, such as immunohistochemistry and in situ hybridisation, with those of the scanning electron microscope.

Capillary distribution in the ventricles of hearts with pulmonary atresia and intact ventricular septum

BACKGROUND

Pulmonary atresia and intact ventricular septum (PA-IVS) can be complicated by the presence of a severely hypoplastic thick-walled right ventricle with or without ventriculo-coronary arterial communications. A variable amount of myocardial pathology has been described in these hearts, probably the result of ischemic conditions and a high pressure in the right ventricle. We studied whether the capillary network is still intact, allowing a sufficient perfusion of the myocardium, which will be important for the success of palliative surgery.

METHODS AND RESULTS

We studied the distribution of capillaries in the myocardium of hearts with PA-IVS and compared the results with normal hearts. The capillaries were detected by immunohistochemistry using a monoclonal antibody (408) against endothelium. Remarkable abnormalities in capillary distribution were found in the right ventricle of hearts with PA-IVS and reflect the arrangement of the myocytes. Thus, disorganization of capillaries, which is found to be the most common pattern, always paralleled the myocardial disarray. A low density of capillaries is always found in areas with a low density of myocytes, ie, with hypertrophied myocytes, compact fibrotic tissue, or diffuse fibrosis. Disarray and other disturbances in orientation of capillaries and myocytes are present in hearts with PA-IVS, a hypoplastic right ventricle, and ventriculo-coronary arterial communications. These disturbances are more extensive when interruptions of the coronary arteries are also present. In hearts with PA-IVS and a hypoplastic right ventricle only, extensive regions with low capillary densities and severe myocyte pathology are observed. On the contrary, hearts with PA-IVS and a normal-size right ventricle show minor abnormalities in capillary and myocyte organization.

CONCLUSIONS

In hearts with PA-IVS, various abnormal capillary distribution patterns are found. Our findings correlate well with clinical data that reported the best surgical results in hearts in which the major part of the myocardium showed a normal capillary distribution and myocyte morphology. This suggests that the capillary distribution may be an important parameter for the function of the heart. Because the distribution of the capillaries is found to be a good reflection of the arrangement of the myocytes, antibody 408 is also a useful tool in detecting abnormalities of the myocardium in a fast and easy way.

Formation of the tricuspid valve in the human heart

BACKGROUND

Some of the problems concerning the origin of the inlet component of the definitive right ventricle were resolved in a previous study in which we showed it to be derived exclusively from the embryonic right ventricle. Questions remain, however, concerning the relative contributions of endocardial cushion tissue and myocardium to the definitive valvar apparatus guarding the right atrioventricular orifice and the origin of the valvar leaflets.

METHODS AND RESULTS

The formation of the tricuspid valve was studied by scanning electron microscopic and immunohistochemical techniques. Concurrent with the development of the right atrioventricular connection, a myocardial ridge forms at the boundary between the atrioventricular canal and the embryonic right ventricle. It grows to become a myocardial gully that funnels atrial blood beneath the lesser curvature of the initial heart tube toward the middle of the right ventricle. Fenestrations in the floor of the gully create an additional inferior opening in the funnel, transforming its initial anterior rim into the septomarginal trabeculation. The septum formed by the fusion of the endocardial ridges of the outflow tract becomes myocardialized in its inferior portion to form, in part, the outlet septum and, in part, the supraventricular crest. The smooth atrial surface of the tricuspid valvar leaflets develops from endocardial cushion tissue. The leaflets become freely movable, however, only after delamination of the tension apparatus within the myocardium. The inferior and septal leaflets derive from the gully and the ventricular septum, their delamination being a single, continuous process. The antero-superior leaflet forms by delamination from the developing supraventricular crest.

CONCLUSIONS

The leaflets of the tricuspid valve develop equally from the endocardial cushion tissues and the myocardium. The myocardium contributing to the valve comes from two sources, the tricuspid gully complex and the developing supraventricular crest. These findings facilitate the understanding of several congenital malformations.

Immunohistochemical delineation of the conduction system. I: The sinoatrial node

We have raised a mouse monoclonal antibody that reacts specifically with the myocytes of the sinoatrial node of the bovine heart. By use of this antibody (445-6E10) and antibodies against the gap junction protein connexin43, the periphery of the sinoatrial node and the distribution of gap junctions in the nodal region were studied. The reaction patterns of 445-6E10 and anti-connexin43 are exactly complementary; ie, connexin43 was not detected in the nodal myocytes but was clearly present in the atrial myocytes. Both reaction patterns demonstrate that nodal myocytes and atrial myocytes can unambiguously be distinguished by their characteristic molecular phenotype. The transitional nodal myocytes at the periphery of the node that have intermediate morphological and electrophysiological characteristics could now clearly be defined as nodal by our immunohistochemical criteria. The center of the node is surrounded by a region of interdigitating nodal and atrial bundles. Nodal bundles, coming from the center of the node, penetrate the atrial myocardium aligned at atrial bundles, forming histological connections between nodal and atrial myocytes at regular distances. This interdigitating arrangement of bundles of connexin43-negative nodal and connexin43-positive atrial myocytes is also found in the human and rat heart. We hypothesize that the architecture of the periphery of the node is important to prevent silencing of the pacemaking nodal myocytes by the atrium while ensuring a sufficient source loading of the nodal myocytes.

Expression of alpha-tropomyosin during cardiac development in the chick embryo

A new monoclonal antibody (mAb) that recognizes alpha-tropomyosin in cardiac muscle cells was used in a qualitative (polyacrylamide gel electrophoresis and indirect immunofluorescence) and quantitative (fluorescence-activated cell sorting) study of the expression of this protein during heart development. alpha-Tropomyosin expression was weak in early stages of chick embryo development (Hamburger and Hamilton stage 18), and increased steadily until Hamburger Hamilton stage 40. In early stages, the protein was found mainly in cytoplasm, whereas by the final stages, it was more abundant in the cytoskeletal compartment. The mAb cross-reacted with alpha-tropomyosin in smooth and striated muscle cells from chickens, mice, and humans, but did not cross-react with nonmuscle tropomyosin.

Lactase gene expression during early development of rat small intestine

Expression of lactase messenger (m) RNA and protein in rat small intestine during fetal and postnatal development was analyzed using in situ hybridization and immunohistochemistry. Lactase mRNA was first identified at 18 days of development, and lactase protein was first detected at day 20. Lactase mRNA and protein were present along the entire villus. Lactase mRNA increased, reaching a maximum at day 20. Just before birth a decrease in lactase mRNA was observed. In newborn intestine, lactase mRNA was present only from the base of the villus up to the mid-villus region and was undetectable up to the villus tips. Lactase protein continued to be expressed along the entire villus. These data show that expression of lactase mRNA and protein do not parallel, indicating a posttranscriptional control in fetal development. Lactase gene transcription is initiated late in gestation concomitant with villus formation and is exclusively seen in villus epithelial cells. The restriction after birth of lactase mRNA expression to cells at the villus base suggests the occurrence of a previously unknown step in postnatal differentiation of the enterocyte.

New findings concerning ventricular septation in the human heart. Implications for maldevelopment

BACKGROUND

The mechanics involved in development of the inlet component of the morphologically right ventricle are, as yet, undecided. Some argue that this component is derived from the descending limb of the ventricular loop, and that the inlet and apical trabecular components of the muscular ventricular septum have separate developmental origins. Others state that the entirety of the right ventricle grows from the ascending limb of the loop, and that the muscular septum, apart from its outer component, has a unitary origin. We now have material from human embryos at our disposal, which, we believe, solves this conundrum.

METHODS AND RESULTS

We used a monoclonal antibody against an antigen to neural tissue from the chick to demarcate a ring of cells separating the descending (inlet) and ascending (outlet) limbs of the developing ventricular loop of the human heart. Preparation of serial sections of graded human embryos enabled us to trace the fate of this ring, and hence the formation of the inlet of the right ventricle, to the completion of cardiac septation. Eight embryos were studied, encompassing stages 14-23 of the Carnegie classification. The ring of cells initially separating the ascending and descending limbs of the ventricular loop were, at the conclusion of ventricular septation, located within the atrioventricular junction, sequestrated for the most part in the terminal segment of atrial myocardium.

CONCLUSIONS

Our study conclusively shows that the inlet component of the morphologically right ventricle is derived from the ascending limb of the embryonic ventricular loop, and that the inlet and apical trabecular components of the muscular septum are derived from the same primary ventricular septum.

Maturation of mitochondrial and other isoenzymes of creatine kinase in skeletal muscle of preterm born infants

We studied pre- and postnatal changes in total creatine kinase (CK) activity, mitochondrial creatine kinase (Mi-CK) activity and immunochemical reactivity with anti-Mi-CK antibodies in skeletal muscle specimens from 12 infants, 10 of them preterm born, after a pregnancy varying between 28 and 40 weeks. Our results demonstrate that Mi-CK is present in fetal human quadriceps muscle and that the specific activity of Mi-CK increases during prenatal development from week 28 to 40 by a factor about two. Generally, adult levels have not been reached at birth, indicating a further postnatal increase of the activity of the enzyme. The Mi-CK protein content also increases during prenatal development. These results suggest that in human skeletal muscle the expression and accumulation of Mi-CK starts at mid-gestation, later than is known to occur for cytosolic CK.

Messenger RNA sorting in enterocytes. Co-localization with encoded proteins

This study describes the intracellular compartmentalization of three different mRNAs in the polarized rat fetal enterocyte. They encode proteins that are known to be localized within different regions of the epithelial cell namely (i) the apical, membrane-bound glycoprotein, lactase-phlorizin hydrolase (lactase), (ii) the mitochondrially localized enzyme, carbamoylphosphate synthetase (CPS), and (iii) the cytoplasmically localized enzyme, phosphoenolpyruvate carboxykinase (PEPCK). These mRNAs are found in close proximity to their respective protein products, i.e. the apical membrane, mitochondria and cytoplasm, respectively. The significance of these observations is twofold; (i) they indicate that mRNAs are sorted into specific domains of the cytosol of intestinal epithelial cells; and (ii) they imply the presence of two distinct pathways of mRNA targeting one that allows transport of mRNAs that are translated on ribosomes associated with the rough endoplasmic reticulum (lactase mRNA), and the other that allows sorting of mRNAs that are translated on free polysomes (CPS and PEPCK mRNA).

Spatial distribution of "tissue-specific" antigens in the developing human heart and skeletal muscle. III. An immunohistochemical analysis of the distribution of the neural tissue antigen G1N2 in the embryonic heart; implications for the development of the atrioventricular conduction system

A monoclonal antibody raised against an extract from the Ganglion Nodosum of the chick and designated G1N2 proves to bind specifically to a subpopulation of cardiomyocytes in the embryonic human heart. In the youngest stage examined (Carnegie stage 14, i.e., 4 1/2 weeks of development) these G1N2-expressing cells are localized in the myocardium that surrounds the foramen between the embryonic left and right ventricle. In the lesser curvature of the cardiac loop this "primary" ring occupies the lower part of the wall of the atrioventricular canal. During subsequent development, G1N2-expressing cells continue to identify the entrance to the right ventricle, but the shape of the ring changes as a result of the tissue remodelling that underlies cardiac septation. During the initial phases of this process the staining remains recognizable as a continuous band of cells in the myocardium that surrounds the developing right portion of the atrioventricular canal, subendocardially in the developing interventricular septum and around the junction of the embryonic left ventricle with the subaortic portion of the outflow tract. During the later stages of cardiac septation, the latter part of the ring discontinues to express G1N2, while upon the completion of septation, no G1N2-expressing cardiomyocytes can be detected anymore. The topographic distribution pattern of G1N suggests that the definitive ventricular conduction system derives from a ring of cells that initially surrounds the "primary" interventricular foramen. The results indicate that the atrioventricular bundle and bundle branches develop from G1N2-expressing myocytes in the interventricular septum, while the "compact" atrioventricular node develops at the junction of the band of G1N2-positive cells in the right atrioventricular junction (the right atrioventricular ring bundle) and the ("penetrating") atrioventricular bundle. A "dead-end tract" represents remnants of conductive tissue in the anterior part of the top of the interventricular septum. The location of the various components of the avian conduction system is topographically homologous with that of the G1N2-ring in the human embryonic heart, indicating a phylogenetically conserved origin of the conduction system in vertebrates.

Demonstration of 'cardiac-specific' myosin heavy chain in masticatory muscles of human and rabbit

Human and rabbit masticatory muscles were analyzed immuno- and enzyme-histochemically using antibodies specific to 'cardiac' alpha, slow and fast myosin heavy chain isoforms. In human masseter, temporalis, and lateral pterygoid muscle 'cardiac' alpha myosin heavy chain is found in fibres that contain either fast, or fast and slow myosin heavy chain. In rabbit masseter, temporalis and digastric muscles, fibres are present that express 'cardiac' alpha myosin heavy chain either exclusively, or concomitantly with slow myosin heavy chain or fast myosin heavy chain. Our results demonstrate a much broader distribution of 'cardiac' alpha myosin heavy chain than hitherto recognized and these might explain in part the specific characteristics of masticatory muscles. The 'cardiac' alpha myosin heavy chain is only found in skeletal muscles originating from the cranial part of the embryo (including the heart muscle), suggesting that its expression might be determined by the developmental history of these muscles.

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.