Relations between development of the capillary wall and myoarchitecture of the rat heart

Sixty Years of Heart Research in the Institute of Physiology of the Czech Academy of Sciences

In 2023, six decades have elapsed since the first experimental work on the heart muscle was published, in which a member of the Institute of Physiology of the Czech Academy of Sciences participated as an author; Professor Otakar Poupa was the founder and protagonist of this research domain. Sixty years - more than half of the century - is certainly significant enough anniversary that is worth looking back and reflecting on what was achieved during sometimes very complicated periods of life. It represents the history of an entire generation of experimental cardiologists; it is possible to learn from its successes and mistakes. The objective of this review is to succinctly illuminate the scientific trajectory of an experimental cardiological department over a 60-year span, from its inaugural publication to the present. The old truth - historia magistra vitae - is still valid. Keywords: Heart, Adaptation, Development, Hypoxia, Protection.

Developmental Aspects of Cardiac Adaptation to Increased Workload

The heart is capable of extensive adaptive growth in response to the demands of the body. When the heart is confronted with an increased workload over a prolonged period, it tends to cope with the situation by increasing its muscle mass. The adaptive growth response of the cardiac muscle changes significantly during phylogenetic and ontogenetic development. Cold-blooded animals maintain the ability for cardiomyocyte proliferation even in adults. On the other hand, the extent of proliferation during ontogenetic development in warm-blooded species shows significant temporal limitations: whereas fetal and neonatal cardiac myocytes express proliferative potential (hyperplasia), after birth proliferation declines and the heart grows almost exclusively by hypertrophy. It is, therefore, understandable that the regulation of the cardiac growth response to the increased workload also differs significantly during development. The pressure overload (aortic constriction) induced in animals before the switch from hyperplastic to hypertrophic growth leads to a specific type of left ventricular hypertrophy which, in contrast with the same stimulus applied in adulthood, is characterized by hyperplasia of cardiomyocytes, capillary angiogenesis and biogenesis of collagenous structures, proportional to the growth of myocytes. These studies suggest that timing may be of crucial importance in neonatal cardiac interventions in humans: early definitive repairs of selected congenital heart disease may be more beneficial for the long-term results of surgical treatment.

Adult spiny mice (Acomys) exhibit endogenous cardiac recovery in response to myocardial infarction

Complex tissue regeneration is extremely rare among adult mammals. An exception, however, is the superior tissue healing of multiple organs in spiny mice (Acomys). While Acomys species exhibit the remarkable ability to heal complex tissue with minimal scarring, little is known about their cardiac structure and response to cardiac injury. In this study, we first examined baseline Acomys cardiac anatomy and function in comparison with commonly used inbred and outbred laboratory Mus strains (C57BL6 and CFW). While our results demonstrated comparable cardiac anatomy and function between Acomys and Mus, Acomys exhibited a higher percentage of cardiomyocytes displaying distinct characteristics. In response to myocardial infarction, all animals experienced a comparable level of initial cardiac damage. However, Acomys demonstrated superior ischemic tolerance and cytoprotection in response to injury as evidenced by cardiac functional stabilization, higher survival rate, and smaller scar size 50 days after injury compared to the inbred and outbred mouse strains. This phenomenon correlated with enhanced endothelial cell proliferation, increased angiogenesis, and medium vessel maturation in the peri-infarct and infarct regions. Overall, these findings demonstrate augmented myocardial preservation in spiny mice post-MI and establish Acomys as a new adult mammalian model for cardiac research.

Angiogenic Endothelial Cell Signaling in Cardiac Hypertrophy and Heart Failure

Endothelial cells are, by number, one of the most abundant cell types in the heart and active players in cardiac physiology and pathology. Coronary angiogenesis plays a vital role in maintaining cardiac vascularization and perfusion during physiological and pathological hypertrophy. On the other hand, a reduction in cardiac capillary density with subsequent tissue hypoxia, cell death and interstitial fibrosis contributes to the development of contractile dysfunction and heart failure, as suggested by clinical as well as experimental evidence. Although the molecular causes underlying the inadequate (with respect to the increased oxygen and energy demands of the hypertrophied cardiomyocyte) cardiac vascularization developing during pathological hypertrophy are incompletely understood. Research efforts over the past years have discovered interesting mediators and potential candidates involved in this process. In this review article, we will focus on the vascular changes occurring during cardiac hypertrophy and the transition toward heart failure both in human disease and preclinical models. We will summarize recent findings in transgenic mice and experimental models of cardiac hypertrophy on factors expressed and released from cardiomyocytes, pericytes and inflammatory cells involved in the paracrine (dys)regulation of cardiac angiogenesis. Moreover, we will discuss major signaling events of critical angiogenic ligands in endothelial cells and their possible disturbance by hypoxia or oxidative stress. In this regard, we will particularly highlight findings on negative regulators of angiogenesis, including protein tyrosine phosphatase-1B and tumor suppressor p53, and how they link signaling involved in cell growth and metabolic control to cardiac angiogenesis. Besides endothelial cell death, phenotypic conversion and acquisition of myofibroblast-like characteristics may also contribute to the development of cardiac fibrosis, the structural correlate of cardiac dysfunction. Factors secreted by (dysfunctional) endothelial cells and their effects on cardiomyocytes including hypertrophy, contractility and fibrosis, close the vicious circle of reciprocal cell-cell interactions within the heart during pathological hypertrophy remodeling.

The neuropeptide galanin promotes an anti-thrombotic phenotype on endocardial endothelial cells from heart failure patients

Thromboembolic complications are a significant cause of mortality and re-hospitalization in heart failure (HF) patients. One source of thrombi is the ventricular endocardial surface that becomes increasingly pro-thrombotic as HF progresses. Anticoagulation comes with bleeding risks so identifying therapeutic agents for improving cardiac endothelial health are of critical clinical importance. Endocardial endothelial cells are closely apposed to cardiac sympathetic nerves. In HF, cardiac sympathetic nerves are dysregulated and promote disease progression. Whether endocardial endothelial health and function is impacted by sympathetic dysregulation in HF is unknown. Also unexplored is the impact of neuropeptides, such as galanin and neuropeptide Y (NPY), co-released from sympathetic nerve terminals, on endothelial health. In this study we examined the effect of sympathetic nerve-released neurotransmitters and neuropeptides on the procoagulant phenotype of cultured human endocardial endothelial cells from HF patients. As a functional readout of procoagulant state we examined thrombin-mediated von Willebrand factor (vWF) extrusion and multimer expression. We demonstrate that vWF extrusion and multimer expression is promoted by thrombin, that isoproterenol (a beta-adrenergic receptor agonist) augments this effect, whereas co-treatment with the beta-blockers propranolol and carvedilol blocks this effect. We also show that vWF extrusion and multimer expression is attenuated by treatment with the neuropeptide galanin, but not with NPY. Our results are consistent with a protective role of beta-blockers and galanin on endocardial endothelial health in heart failure. Improving endothelial health through galanin therapy is a future clinical application of this study.

Vascularizing the heart

As the developing heart grows and the chamber walls thicken, passive diffusion of oxygen and nutrients is replaced by a vascular plexus which remodels and expands to form a mature coronary vascular system. The coronary arteries and veins ensure the continued development of the heart and facilitate cardiac output with progression towards birth. Many aspects of coronary vessel development are surprisingly not well understood and recently there has been much debate surrounding both the developmental origin and tissue contribution of cardiovascular cells alongside the specific signals that determine their fate and function. What is clear is that an understanding of the cellular and molecular cues to vascularize the heart of the embryo has significant implications for adult heart disease and regeneration, as we move towards targeted cell-based therapies for neovascularization and coronary bypass engraftment. This review will focus on the proposed cellular origins for the coronary endothelium with due consideration to the pro-epicardial organ/epicardium, sinus venosus and endocardium as potential sources, and we will explore the outstanding questions and technical limitations with respect to accurate labelling and lineage tracing of the developing coronaries. We will briefly document canonical vascular signalling that induces vessels in the heart alongside a focus on the potential for developmental reprogramming and putative mechanisms underpinning venous vs. arterial cell fate. Finally, we will extrapolate directly from development to address adult maintenance of the coronaries, vascular homeostasis and remodelling in response to pathology, aligned with the potential for revascularizing the injured adult heart.

Three-dimensional myoarchitecture of the bovine tongue demonstrated by diffusion spectrum magnetic resonance imaging with tractography

The anatomy of the mammalian tongue consists of an intricate array of variably aligned and extensively interwoven muscle fibers. As a result, it is particularly difficult to resolve the relationship between the tongue's microscopic anatomy and tissue-scale mechanical function. In order to address this question, we employed a method, diffusion spectrum imaging (DSI) with tractography, for displaying the macroscopic orientational properties of the tissue's constituting myofibers. DSI measures spatially variant proton displacement for a given 3D imaging segment (voxel), reflecting the principal orientation(s) of its myofibers. Tractography uses the angular similarity displayed by the principal fiber populations of multiple adjacent voxels to generate tract-like structures. DSI with tractography thus defines a unique set of tracts based on the net orientational behavior of the myofiber populations at different positions in the tissue. By this approach, we demonstrate a novel myoarchitectural pattern for the bovine tongue, consisting of short and orthogonally aligned crossing fiber tracts in the intrinsic core region, and longer, parallel-aligned fiber tracts on the tissue margins and in the regions of extrinsic fiber insertion. The identification of locally aligned myofiber populations by DSI with tractography allows us to reconsider lingual anatomy, not in conventional microscopic terms, but as a set of heterogeneously aligned and macroscopically resolved myofiber tracts. We postulate that the properties associated with these myofiber tracts predict the mechanical behavior of the tissue and thus constitute a method to relate structure and function for anatomically complex muscular tissues.

Formation of the coronary vasculature during development

The formation of the coronary vasculature involves a series of carefully regulated temporal events that include vasculogenesis, angiogenesis, arteriogenesis and remodeling. This review explores these events, which begin with the migration of proepicardial cells to form the epicardium and end with postnatal growth and remodeling. Coronary endothelial, smooth muscle and fibroblast cells differentiate via epithelial-mesenchymal transformation; these cells delaminate from the epicardium. Following the formation of a tubular network by endothelial cells, an aortic ring of endothelial cells penetrates the aorta at the left and right aortic cusps to form the two ostia. Smooth muscle cell recruitment occurs rapidly and the coronary artery network begins forming as blood flow is established. Recent studies have identified a number of regulatory molecules that play key roles in epicardial formation and the transformation of its component cells into mesenchyme. Moreover, we are finally gaining some understanding regarding the interplay of angiogenic growth factors in the complex process of establishing the coronary vascular tree. Understanding coronary embryogenesis is important for interventions regarding adult cardiovascular diseases as well as those necessary to correct congenital defects.

Coronary vessel development: the epicardium delivers

Coronary artery disease accounts for 54% of all cardiovascular disease in the United States. Understanding how coronary vessels develop is likely to uncover novel drug targets and therapeutic strategies that will be useful in directing the repair or remodeling of coronary vessels in adults. Recent insights have identified the importance of cells derived from the proepicardium and epicardium in the formation of coronary vessels. This article reviews the basic steps in coronary vessel development, the molecules implicated in these steps, and the pressing questions awaiting answers.

Embryonic development of coronary vasculature in rats: corrosion casting studies

The aim of this study was to analyze the development of coronary vessels at different stages of embryonic life in rats using corrosion casts and scanning electron microscopy (SEM). We studied morphologic details of vessel maturation, expansion, and pattern formation from the stage of development when the coronary system forms patent connections with the aorta and the right atrium (embryonic day 16 (ED16)) to full-term fetus (ED21). The internal surface morphologies of the arterial and venous vessel walls were different and were dependent on the distance from the orifice and the capillary system. They also depended on the maturation state of a given vessel. In various branches of the coronary system we demonstrated round, fusiform or polygonal, endothelial cell imprints. The capillary network was dense, however, at the early stages of development, it formed a thin layer over the myocardium. By ED21 capillaries assumed an orientation parallel to the long axes of the cardiac myocytes. During all stages of development, different forms of angiogenesis by intussusceptive growth were observed. Splitting of the vessel wall occurred in two or three points along the vessel, forming two- or three-link chains. Certain areas of vessels resembled doughnuts, from which several sister vessels originated. The coronary arteries were situated deep within the myocardial wall. The major coronary veins were mostly located on the surface of the capillary plexuses of the myocardial wall. In conclusion, this method of vessel casting enables the detection of angiogenesis by intussusceptive growth, and the visualization of a capillary's position to the myocardial wall, thickness of the capillary plexuses, and the internal surface morphology of major vessels.

Structure and function of the developing zebrafish heart

The combination of optical clarity and large scale of mutants makes the zebrafish vital for developmental biologists. However, there is no comprehensive reference of morphology and function for this animal. Since study of gene expression must be integrated with structure and function, we undertook a longitudinal study to define the cardiac morphology and physiology of the developing zebrafish. Our studies included 48-hr, 5-day, 2-week, 4-week, and 3-month post-fertilization zebrafish. We measured ventricular and body wet weights, and performed morphologic analysis on the heart with H&E and MF-20 antibody sections. Ventricular and dorsal aortic pressures were measured with a servonull system. Ventricular and body weight increased geometrically with development, but at different rates. Ventricle-to-body ratio decreased from 0.11 at 48-hr to 0.02 in adult. The heart is partitioned into sinus venosus, atrium, ventricle, and bulbus arteriosus as identified by the constriction between the segments at 48-hr. Valves were formed at 5-day post-fertilization. Until maturity, the atrium showed extensive pectinate muscles, and the atrial wall increased to two to three cell layers. The ventricular wall and the compact layer increased to three to four cell layers, while the extent and complexity in trabeculation continued. Further thickening of the heart wall was mainly by increase in cell size. The bulbus arteriosus had similar characteristics to the myocardium in early stages, but lost the MF-20 positive staining, and transitioned to smooth muscle layer. All pressures increased geometrically with development, and were linearly related to stage-specific values for body weight (P < 0.05). These data define the parameters of normal cardiac morphology and ventricular function in the developing zebrafish.

Development of cardiac sensitivity to oxygen deficiency: comparative and ontogenetic aspects

Hypoxic states of the cardiovascular system are undoubtedly associated with the most frequent diseases of modern times. They originate as a result of disproportion between the amount of oxygen supplied to the cardiac cell and the amount actually required by the cell. The degree of hypoxic injury depends not only on the intensity and duration of the hypoxic stimulus, but also on the level of cardiac tolerance to oxygen deprivation. This variable changes significantly during phylogenetic and ontogenetic development. The heart of an adult poikilotherm is significantly more resistant as compared with that of the homeotherms. Similarly, the immature homeothermic heart is more resistant than the adult, possibly as a consequence of its greater capability for anaerobic glycolysis. Tolerance of the adult myocardium to oxygen deprivation may be increased by pharmacological intervention, adaptation to chronic hypoxia, or preconditioning. Because the immature heart is significantly more dependent on transsarcolemmal calcium entry to support contraction, the pharmacological protection achieved with drugs that interfere with calcium handling is markedly altered. Developing hearts demonstrated a greater sensitivity to calcium channel antagonists; a dose that induces only a small negative inotropic effect in adult rats stops the neonatal heart completely. Adaptation to chronic hypoxia results in similarly enhanced cardiac resistance in animals exposed to hypoxia either immediately after birth or in adulthood. Moreover, decreasing tolerance to ischemia during early postnatal life is counteracted by the development of endogenous protection; preconditioning failed to improve ischemic tolerance just after birth, but it developed during the early postnatal period. Basic knowledge of the possible improvements of immature heart tolerance to oxygen deprivation may contribute to the design of therapeutic strategies for both pediatric cardiology and cardiac surgery.

Coronary angiogenesis. From morphometry to molecular biology and back

We have reviewed briefly the current state of knowledge relating to the regulation of angiogenesis, including the role of angiogenic growth factors, and we have described the major structural components of the vascular wall and their changes during formation of new channels. Finally, we have described quantitative evaluations of vascular growth in cardiac muscle. Examples of substantial angiogenesis during the early postnatal stages of normal development were used together with examples depicting a more moderate stimulation of capillary growth in adult rat hearts treated with nifedipine or thyroxine. Adequate capillary supply is a precondition for tissue survival and proper function. This is probably why vascular growth is so tightly regulated by several systems which may alternately stimulate or inhibit the formation of new vessels. Recent advances in molecular biology have enabled us to study the mechanisms of angiogenesis under both in vitro and in vivo conditions, but the final assessment of vascular growth should be accomplished through morphometric analysis.

Myocardial capillaries: increase in number by splitting of existing vessels

To study myocardial vascular development, stereological parameters were estimated in 24 Wistar rat hearts of six different age groups, from newborn to adult. The vascular surface density showed a sharp increase in the first 2 weeks, a peak around the age of 2 weeks, and then a steady decrease until it flattened in adulthood. In contrast, the vascular volume percentage, when plotted against age, decreased continuously with the greatest change in the first week, after which the curve flattened. These findings are compatible with an increase in the number of capillaries with a concomitant decrease of their diameters. Qualitative scrutiny of the histology did indeed support the idea that vessels become thinner. Reconstructions of the histological sections showed the same change three dimensionally. The reconstructions also demonstrated very small holes that seemed to go through the capillaries in the younger stages. Corrosion casts of the blood vessels were made using a casting resin. This was injected into the umbilical artery of rat embryos from 15 days gestation to birth. In postnatal rats of six age groups methacrylate was injected directly into the left ventricle. These casts supported the stereological data by showing an increase in number and decrease in diameter of capillaries, while during pre- and postnatal development, the intervascular spaces lengthened from small, irregular spaces to long, rectangular ones. Small holes, the probable precursors of such spaces, were clearly visible in the wider vessels of the youngest stages. All data point to an interesting mode of capillary growth, i.e. growth by division of existing vessels.

Maturation of myocardial capillaries in the fetal and neonatal rat: an ultrastructural study with a morphometric analysis of the vesicle populations

The ultrastructural morphology of the cellular and extracellular components of the developing myocardial capillary wall--from the 16-day-gestation fetus of the rat to the 21-day neonate--was examined. A morphometric analysis of plasmalemmal vesicles and of coated vesicles and pits of capillary endothelial cells was performed during the same developmental period. As the lateral extensions of the capillary endothelial cells change from irregular to regular in their thickness during development, there is an increase in the number of plasmalemmal vesicles and a progression from clusters of plasmalemmal vesicles to a uniform distribution in the endothelial cell. The ratio of vesicles which are open to the luminal front, which are "free" in the cytoplasm, or which are open to the abluminal front of the endothelial cell was consistent throughout development. The numerical density of plasmalemmal vesicles demonstrates a gradual and significant increase. In contrast, the numbers of coated vesicles and pits are variable within a very narrow range, and no pattern of increase or decrease is discernible during development. Similarly, there is no change in interendothelial cell junctions, which consist of occluding and primitive adhesive junctional types, during development. The lamina densa of the basal lamina gradually develops from discontinuous, patchy densities along the abluminal surface of the endothelial cells to a continuous and distinct layer by 21 days gestation. The presence of the proteoglycan species in the developing basal lamina was assessed with the cationic dye ruthenium red (RR), and the appearance of RR-marked proteoglycans was found to parallel the appearance of lamina densa material. found to parallel the appearance of lamina densa material. The RR sites appear discontinuously in patches; and later, the RR sites appear in a continuous and regular planar lattice in the lamina rara interna and externa at 21 days gestation. A complete array of RR-stainable anionic sites outside a continuous lamina densa near birth indicates that the basal laminae of developing capillaries in the heart are morphologically, and in part biochemically, mature by the end of the first neonatal week. Our results show that the endothelial cells and the subtending basal lamina of myocardial capillaries gradually mature morphologically during the final days of gestation and the first neonatal week.(ABSTRACT TRUNCATED AT 400 WORDS)

Selective retrograde coronary venous perfusion

The theoretical concept of delivering oxygenated blood to an ischemic myocardium by way of the coronary venous system antedated by many decades the present widespread utilization of coronary artery bypass grafting. Diffuse arterial atherosclerosis has limited the effectiveness of coronary artery bypass grafting in about 15% of patients seen with significant angina pectoris. Consequently, there has been renewed interest in selectively reversing the flow in certain coronary veins through coronary venous bypass grafts. This collective review details the physiology and anatomy of the coronary venous system. It then discusses the early attempts to globally retroperfuse the entire coronary venous system through the coronary sinus. Finally, the current experimental and clinical attempts to selectively retroperfuse just one region of the coronary venous system are presented and reviewed.