Development of the human heart

In 2014, an extensive review discussing the major steps of cardiac development focusing on growth, formation of primary and chamber myocardium and the development of the cardiac electrical system, was published. Molecular genetic lineage analyses have since furthered our insight in the developmental origin of the various component parts of the heart, which currently can be unambiguously identified by their unique molecular phenotype. Moreover, genetic, molecular and cell biological analyses have driven insights into the mechanisms underlying the development of the different cardiac components. Here, we build on our previous review and provide an insight into the molecular mechanistic revelations that have forwarded the field of cardiac development. Despite the enormous advances in our knowledge over the last decade, the development of congenital cardiac malformations remains poorly understood. The challenge for the next decade will be to evaluate the different developmental processes using newly developed molecular genetic techniques to further unveil the gene regulatory networks operational during normal and abnormal cardiac development.

Genome-Wide Association Study-Driven Gene-Set Analyses, Genetic, and Functional Follow-Up Suggest GLIS1 as a Susceptibility Gene for Mitral Valve Prolapse

Background Mitral valve prolapse (MVP) is a common heart valve disease, the most frequent indication for valve repair or replacement. MVP is characterized by excess extracellular matrix secretion and cellular disorganization, which leads to bulky valves that are unable to coapt correctly during ventricular systole resulting in mitral regurgitation, and it is associated with sudden cardiac death. Here we aim to characterize globally the biological mechanisms underlying genetic susceptibility to MVP to better characterize its triggering mechanisms. Methods We applied i-GSEA4GWAS and DEPICT, two pathway enrichment tools to MVP genome-wide association studies. We followed-up the association with MVP in an independent dataset of cases and controls. This research was conducted using the UK Biobank Resource. Immunohistochemistry staining for Glis1 (GLIS family zinc finger 1) was conducted in developing heart of mice. Knockdown of Glis1 using morpholinos was performed in zebrafish animals 72 hours postfertilization. Results We show that genes at risk loci are involved in biological functions relevant to actin filament organization, cytoskeleton biology, and cardiac development. The enrichment for positive regulation of transcription, cell proliferation, and migration motivated the follow-up of GLIS1, a transcription factor from the Krüppel-like zinc finger family. In combination with previously available data, we now report a genome-wide significant association with MVP (odds ratio, 1.20; P=4.36×10-10), indicating that Glis1 is expressed during embryonic development predominantly in nuclei of endothelial and interstitial cells of mitral valves in mouse. We also show that Glis1 knockdown causes atrioventricular regurgitation in developing hearts in zebrafish. Conclusions Our findings define globally molecular and cellular mechanisms underlying common genetic susceptibility to MVP and implicate established and unprecedented mechanisms. Through the GLIS1 association and function, we point at regulatory functions during cardiac development as common mechanisms to mitral valve degeneration.

VGLL4 plays a critical role in heart valve development and homeostasis

Heart valve disease is a major clinical problem worldwide. Cardiac valve development and homeostasis need to be precisely controlled. Hippo signaling is essential for organ development and tissue homeostasis, while its role in valve formation and morphology maintenance remains unknown. VGLL4 is a transcription cofactor in vertebrates and we found it was mainly expressed in valve interstitial cells at the post-EMT stage and was maintained till the adult stage. Tissue specific knockout of VGLL4 in different cell lineages revealed that only loss of VGLL4 in endothelial cell lineage led to valve malformation with expanded expression of YAP targets. We further semi-knockout YAP in VGLL4 ablated hearts, and found hyper proliferation of arterial valve interstitial cells was significantly constrained. These findings suggest that VGLL4 is important for valve development and manipulation of Hippo components would be a potential therapy for preventing the progression of congenital valve disease.

VGLL4, a new member of the Hippo pathway, is intensively investigated in inhibition of tumor progression via competing with YAP to bind TEADs, but its role in cardiovascular field remains unclear. Here we generated VGLL4 knockout mouse line and VGLL4-eGFP reporter mouse line. VGLL4-eGFP reporter mouse line showed VGLL4 was mainly expressed in valve interstitial cells from post-EMT stage to adult stage. Genetic loss of function and lineage tracing data demonstrated only endothelial loss of VGLL4 led to valve malformation with up-regulation of YAP targets. Of note, semi-knockout YAP could rescue this phenotype of VGLL4 knockouts. This is the first study to show the Hippo pathway plays a critical role in valve remodeling, maturation and homeostasis. Our findings suggest that mutations in VGLL4 may underlie human congenital heart valve dysplasia.

The role of pygopus in the differentiation of intracardiac valves in Drosophila

Cardiac valves serve an important function; they support unidirectional blood flow and prevent blood regurgitation. Wnt signaling plays an important role in the formation of mouse cardiac valves and cardiac valve proliferation in Zebrafish, but identification of the specific signaling components involved has not been addressed systematically. Of the components involved in Wnt signal transduction, pygopus (pygo), first identified as a core component of Wnt signaling in Drosophila, has not yet to be investigated with respect to valve development and differentiation. Here, we take advantage of the Drosophila heart model to study the role of pygo in formation of valves between the cardiac chambers. We found that cardiac-specific pygo knockdown in the Drosophila heart causes dilation in the region of these cardiac valves, and their characteristic dense mesh of myofibrils does not form and resembles that of neighboring cardiomyocytes. In contrast, heart-specific knockdown of the transcription factors, arm/β-Cat, lgs/BCL9, or pan/TCF, which mediates canonical Wnt signal transduction, shows a much weaker valve differentiation defect. Double-heterozygous combinations of mutants for pygo and the Wnt-signaling components have no additional effect on heart function compared with pygo heterozygotes alone. These results are consistent with the idea that pygo functions independently of canonical Wnt signaling in the differentiation of the adult interchamber cardiac valves.

Computational simulation of hemodynamic-driven growth and remodeling of embryonic atrioventricular valves

Embryonic heart valves develop under continuous and demanding hemodynamic loading. The particular contributions of fluid pressure and shear tractions in valve morphogenesis are difficult to decouple experimentally. To better understand how fluid loads could direct valve formation, we developed a computational model of avian embryonic atrioventricular (AV) valve (cushion) growth and remodeling using experimentally derived parameters for the blood flow and the cushion stiffness. Through an iterative scheme, we first solved the fluid loads on the axisymmetric AV canal and cushion model geometry. We then applied the fluid loads to the cushion and integrated the evolution equations to determine the growth and remodeling. After a set time of growth, we updated the fluid domain to reflect the change in cushion geometry and resolved for the fluid forces. The rate of growth and remodeling was assumed to be a function of the difference between the current stress and an isotropic homeostatic stress state. The magnitude of the homeostatic stress modulated the rate of volume addition during the evolution. We found that the pressure distribution on the AV cushion was sufficient to generate leaflet-like elongation in the direction of flow, through inducing tissue resorption on the inflow side of cushion and expansion on the outflow side. Conversely, shear tractions minimally altered tissue volume, but regulated the remodeling of tissue near the cushion surface, particular at the leading edge. Significant shear and circumferential residual stresses developed as the cushion evolved. This model offers insight into how natural and perturbed mechanical environments may direct AV valvulogenesis and provides an initial framework on which to incorporate more mechano-biological details.

Human or animal homograft: could they have a future as a biological scaffold for engineered heart valves?

Tissue-engineered heart valves (TEHVs) promise to be the ideal heart valve replacement: they have the potential to grow and repair within the host, to minimise inflammatory and immunological responses and to limit thromboembolism. Viable cells included in TEHVs can theoretically adapt to a growing and changing environment exactly as a native biological structure. This could be extremely important in case of paediatric applications, where reoperations are frequently required to replace failed valve substitutes or to accommodate the growth of the patient. At present time the biological matrix from allogenic or xenogenic decellularized valves represents an appropriate valve scaffold in TEHVs, showing theoretically an ability to grow and repair within the host. Viable cells included in extracellular valve matrix can theoretically adapt to a growing and changing environment like the native biological structure. The aim of this paper is to present a review concerning the use of homograft and allograft valves as an ideal substrate for cardiac engineered tissue valves that represent an exciting possibility for in situ regeneration and repair of heart valves.

The role of organ level conditioning on the promotion of engineered heart valve tissue development in-vitro using mesenchymal stem cells

We have previously shown that combined flexure and flow (CFF) augment engineered heart valve tissue formation using bone marrow-derived mesenchymal stem cells (MSC) seeded on polyglycolic acid (PGA)/poly-L-lactic acid (PLLA) blend nonwoven fibrous scaffolds (Engelmayr, et al., Biomaterials 2006; vol. 27 pp. 6083-95). In the present study, we sought to determine if these phenomena were reproducible at the organ level in a functional tri-leaflet valve. Tissue engineered valve constructs (TEVC) were fabricated using PGA/PLLA nonwoven fibrous scaffolds then seeded with MSCs. Tissue formation rates using both standard and augmented (using basic fibroblast growth factor [bFGF] and ascorbic acid-2-phosphate [AA2P]) media to enhance the overall production of collagen were evaluated, along with their relation to the local fluid flow fields. The resulting TEVCs were statically cultured for 3 weeks, followed by a 3 week dynamic culture period using our organ level bioreactor (Hildebrand et al., ABME, Vol. 32, pp. 1039-49, 2004) under approximated pulmonary artery conditions. Results indicated that supplemented media accelerated collagen formation (approximately 185% increase in collagen mass/MSC compared to standard media), as well as increasing collagen mass production from 3.90 to 4.43 pg/cell/week from 3 to 6 weeks. Using augmented media, dynamic conditioning increased collagen mass production rate from 7.23 to 13.65 pg/cell/week (88.8%) during the dynamic culture period, along with greater preservation of net DNA. Moreover, when compared to our previous CFF study, organ level conditioning increased the collagen production rate from 4.76 to 6.42 pg/cell/week (35%). Newly conducted CFD studies of the CFF specimen flow patterns suggested that oscillatory surface shear stresses were surprisingly similar to a tri-leaflet valve. Overall, we found that the use of simulated pulmonary artery conditions resulted in substantially larger collagen mass production levels and rates found in our earlier CFF study. Moreover, given the fact that the scaffolds underwent modest strains (approximately 7% max) during either CFF or physiological conditioning, the oscillatory surface shear stresses estimated in both studies may play a substantial role in eliciting MSC collagen production in the highly dynamic engineered heart valve fluid mechanical environment.

Atrioventricular valve development during late embryonic and postnatal stages involves condensation and extracellular matrix remodeling

Although the signaling molecules regulating the early stages of valvular development have been well described, little is known on the late steps leading to mature fibrous leaflets. We hypothesized that atrioventricular (AV) valve development continues after birth to adjust to the postnatal maturation of the heart. By doing a systematic analysis of the AV valves of mice from embryonic day (E) 15.5 to 8 weeks old, we identified key developmental steps that map the maturation process of embryonic cushion-like leaflets into adult stress-resistant valves. Condensation of the mesenchymal cells occurred between E15.5 and E18.5 and was accompanied by increased cellular proliferation and adhesion. Cellular proliferation also contributed transiently to the concomitant elongation of the leaflets. Patterning of the extracellular matrix (ECM) proteins along the AV axis was achieved 1 week after birth, with the differentiation of two reciprocal structural regions, glycosaminoglycans and versican at the atrial side, and densely packed collagen fibers at the ventricular side. Formation and remodeling of the nodular thickenings at the closure points of the leaflets occurred between N4.5 and N11.5. In conclusion, AV valve development during late embryonic and postnatal stages includes condensation, elongation, formation of nodular thickenings, and remodeling of tension-resistant ECM proteins.

Neural crest cells retain multipotential characteristics in the developing valves and label the cardiac conduction system

Multipotent neural crest cells (NCCs) are a major extracardiac component of cardiovascular development. Although recognized as contributing cells to the arterial valves at early developmental stages, NCC persistence in the valves at later times or in the adult heart is controversial. We analyzed NCC persistence and contributions to both semilunar and atrioventricular (AV) valves in the mature heart. Two NCC-specific promoters driving Cre recombinase, Wnt1-Cre and P0-Cre, were mated with floxed reporter mice, R26R or CAG-CAT-EGFP, to map NCC fate. Hearts were analyzed before aorticopulmonary (AP) septation through adult stages. As previously demonstrated, strong NCC labeling was detected in ventral and dorsal outflow cushions before AP septation. In contrast to previous reports, we found that substantial numbers of labeled cells persisted in the semilunar valves in late fetal, neonatal, and adult hearts. Furthermore, NCCs were also found in the AV valves, almost exclusively in the septal leaflets. NCCs in the AV valves expressed melanocytic and neurogenic markers. However, cells labeled in the proximal cardiac conduction system exhibited neurogenic and gliagenic markers, whereas some NCCs expressed no differentiation specific markers. These results suggest that cardiac NCCs contribute to the mature valves and the cardiac conduction system and retain multipotent characteristics late in development.

Human semilunar cardiac valve remodeling by activated cells from fetus to adult: implications for postnatal adaptation, pathology, and tissue engineering

BACKGROUND

The evolution of cell phenotypes and matrix architecture in cardiac valves during fetal maturation and postnatal adaptation through senescence remains unexplored.

METHODS AND RESULTS

We hypothesized that valvular interstitial (VIC) and endothelial cell (VEC) phenotypes, critical for maintaining valve function, change throughout life in response to environmental stimuli. We performed quantitative histological assessment of 91 human semilunar valves obtained from fetuses at 14 to 19 and 20 to 39 weeks' gestation; neonates minutes to 30 days old; children aged 2 to 16 years; and adults. A trilaminar architecture appeared by 36 weeks of gestation but remained rudimentary compared with that of adult valves. VECs expressed an activated phenotype throughout fetal development. VIC density, proliferation, and apoptosis were significantly higher in fetal than adult valves. Pulmonary and aortic fetal VICs showed an activated myofibroblast-like phenotype (alpha-actin expression), abundant embryonic myosin, and matrix metalloproteinase-collagenases, which indicates an immature/activated phenotype engaged in matrix remodeling versus a quiescent fibroblast-like phenotype in adults. At birth, the abrupt change from fetal to neonatal circulation was associated with a greater number of alpha-actin-positive VICs in neonatal aortic versus pulmonary valves. Collagen content increased from early to late fetal stages but was subsequently unchanged, whereas elastin significantly increased postnatally. Collagen fiber color analysis revealed a progressive temporal decrease in thin fibers and a corresponding increase in thick fibers. Additionally, collagen fibers were more aligned in adult than fetal valves.

CONCLUSIONS

Fetal valves possess a dynamic/adaptive structure and contain cells with an activated/immature phenotype. During postnatal life, activated cells gradually become quiescent, whereas collagen matures, which suggests a progressive, environmentally mediated adaptation.

Bmp2 instructs cardiac progenitors to form the heart-valve-inducing field

A hallmark of heart-valve development is the swelling and deposition of extracellular matrix in the heart-valve region. Only myocardium overlying this region can signal to underlying endothelium and cause it to lose cell-cell contacts, delaminate, and invade the extracellular space abutting myocardium and endocardium to form endocardial cushions (EC) in a process known as epithelial to mesenchymal transformation (EMT). The heart-valve myocardium expresses bone morphogenetic protein-2 (Bmp2) coincident with development of valve mesenchyme. BMPs belong to the transforming growth factor beta superfamily (TGF-beta) and play a wide variety of roles during development. We show that conditional ablation of Bmp2 in cardiac progenitors results in cell fate changes in which the heart-valve region adopts the identity of differentiated chamber myocardium. Moreover, Bmp2-deficient hearts fail to induce production and deposition of matrix at the heart-valve-forming region, resulting in the inability of the endothelium to swell and impairing the development of ECs. Furthermore, in collagen invasion assays, Bmp2 mutant endothelium is incapable of undergoing EMT, and addition of BMP2 protein to mutant heart explants rescues this phenotype. Our results demonstrate that Bmp2 is both necessary and sufficient to specify a field of cardiac progenitor cells as the heart-valve-inducing region amid developing atria and ventricles.

Sex-dependence of the relative number of elastic fibres in human heart valves

The aim of the current study was to find out whether there are sex-dependent differences in the relative number of elastic fibres in human heart valves. Twenty-six aortic valves, 26 mitral valves, 33 pulmonary valves and 28 tricuspid valves of both sexes were obtained at autopsy from newborn to 89-year-old patients who died of noncardiac diseases. The quantitatively morphometric investigations were carried out on conventionally stained (Resorcinfuchsin) histological sections. The results were qualitatively examined with immuno-histochemically marked (anti-elastin antibodies) histological sections. Earlier examinations by Leutert [1976. Z. Gesamte Inn. Med. 31, 97-104] showed that the atrioventricular valves have the following layers: endothelium, atrial fibroelastic tissue (S1), fibrous tissue, ventricular fibroelastic tissue (S2) and endothelium. In our study, the ventricular side of the semilunar valves corresponds to side S1, whereas the vessel side corresponds to side S2. Three regions of interest were examined on each side of the valves: base, mid and tip. The number of elastic fibres per measuring area for all four human heart valves was significantly higher (p < 0,001) in fibroelastic tissue of side S1 than in fibroelastic tissue of side S2. Neither on side S1 nor on side S2 were there significant gender-related differences in the relative number of elastic fibres per measuring area. The results suggest a characteristic distribution of the elastic fibre system which is not sex-dependent but closely related to the function of the heart valves.

Is there a possibility for a glutaraldehyde-free porcine heart valve to grow?

OBJECTIVE

A challenging issue is to create a heart valve with growth and remodeling potential, which would be of great interest for congenital heart valve surgery. This study was performed to evaluate the growth and remodeling potentials of a decellularized heart valve.

METHODS

In 4 juvenile sheep (age 12 +/- 1 weeks) with a weight of 24.3 +/- 4.4 kg, a 17-mm diameter decellularized porcine valve was implanted as pulmonary valve replacement. Valve growth was evaluated by transthoracic echocardiography. At explantation, valves were evaluated by gross examination, light microscopy (hematoxylin and eosin, von Kossa, Sirius red, Weigert and Gomori staining), electron microscopy and immunohistochemistry. Atomic absorption spectrometry was performed to evaluate calcium content.

RESULTS

All animals showed fast recovery. The mean follow-up was 9.0 +/- 1.8 months. All sheep at least doubled their weight (54.3 +/- 9.2 kg). Echocardiography showed no regurgitation and a flow velocity of 0.7 +/- 0.1 m/s at the latest follow-up. The valve diameter increased from 17.6 +/- 0.5 to 27.5 +/- 2.1 mm (p < 0.018). Gross examination showed a similar wall thickness of the implanted valve and native pulmonary wall, with smooth and pliable leaflets. Histology showed a monolayer of endothelial cells, fibroblast ingrowth and production of new collagen. No calcification was seen at von Kossa staining, confirmed by low calcium content levels of the valve wall and leaflets at atomic absorption spectrometry.

CONCLUSIONS

This glutaraldehyde-free heart valve showed not only the absence of calcification, but also remodeling and growth potential.

Heart valve development: endothelial cell signaling and differentiation

During the past decade, single gene disruption in mice and large-scale mutagenesis screens in zebrafish have elucidated many fundamental genetic pathways that govern early heart patterning and differentiation. Specifically, a number of genes have been revealed serendipitously to play important and selective roles in cardiac valve development. These initially surprising results have now converged on a finite number of signaling pathways that regulate endothelial proliferation and differentiation in developing and postnatal heart valves. This review highlights the roles of the most well-established ligands and signaling pathways, including VEGF, NFATc1, Notch, Wnt/beta-catenin, BMP/TGF-beta, ErbB, and NF1/Ras. Based on the interactions among and relative timing of these pathways, a signaling network model for heart valve development is proposed.

Angiogenic modulators in valve development and disease: does valvular disease recapitulate developmental signaling pathways?

PURPOSE OF REVIEW

Neovascularization is a recognized feature of many valvular diseases and is established by numerous angiogenic modulators. Less known is that angiogenic modulators are multifunctional and have additional roles in valve development and disease. Recent advancements in this area are described.

RECENT FINDINGS

Initiation of epithelial to mesenchymal transformation, a developmental induction that specifies primordial interstitial cells (mesenchymal cells), requires vascular endothelial growth factor A, which stimulates matrix metalloproteinase 2 production and the invasive migration of mesenchymal cells. Epithelial to mesenchymal transformation also requires the matrix component hyaluronan to facilitate signaling through ErbB2/ErbB3 receptors and then is terminated by an increase in vascular endothelial growth factor A expression. Fibroblast growth factor 4 has been implicated in stimulating the following stage of proliferative expansion. Subsequently, in the remodeling phase, heparin-binding epidermal growth factor-like growth factor limits mesenchymal cell proliferation by signaling through the EGFR/ErbB1 receptor. Many adult valvular lesions appear similar to the embryonic proliferative expansion phase as they exhibit accumulations of extracellular matrix and myofibroblasts (a mesenchyme-like interstitial cell). The origins of such lesions may involve transforming growth factor beta 1. Similar to epithelial to mesenchymal transformation, tumor growth factor beta1 can induce cultured valvular endothelial cells to transdifferentiate to a myofibroblast-like phenotype. This scenario may occur in carcinoid valve disease because serotonin can induce interstitial cell expression of tumor growth factor beta1. Additionally, prolonged tumor growth factor beta1 activity may predispose to calcific degeneration. Calcific leaflets also exhibit tenascin-C, which may facilitate inflammatory cell migration through upregulation of pro-matrix metalloproteinase 2.

SUMMARY

Numerous angiogenic modulators control multiple stages of valvulogenesis and in the context of adult valvular disease may recapitulate their embryonic roles. Thus, lessons learned from valvulogenesis may provide insights into the molecular basis of adult valvular disease.

The use of animal models in developing the discipline of cardiovascular tissue engineering: a review

Cardiovascular disease remains one of the major causes of death and disability in the Western world. Tissue engineering offers the prospect of being able to meet the demand for replacement of heart valves, vessels for coronary and lower limb bypass surgery and the generation of cardiac tissue for addition to the diseased heart. In order to test prospective tissue-engineered devices, these constructs must first be proven in animal models before receiving CE marking or FDA approval for a clinical trial. The choice of animal depends on the nature of the tissue-engineered construct being tested. Factors that need to be considered include technical requirements of implanting the construct, availability of the animal, cost and ethical considerations. In this paper, we review the history of animal studies in cardiovascular tissue engineering and the uses of animal tissue as sources for tissue engineering.

Bioreactors for Cardiovascular Cell and Tissue Growth: A Review

Heart disease is a major cause of death in the Western world. In the past three decades there has been a number of improvements in artificial devices and surgical techniques for cardiovascular disease; however, there is still a need for novel devices, especially for those individuals who cannot receive conventional therapy. The major disadvantage of current artificial devices lies in the fact that they cannot grow, remodel, or repair in vivo. Tissue engineering offers the possibility of developing a biological substitute material in vitro with the inherent mechanical, chemical, biological, and morphological properties required in vivo, on an individual patient basis. In order to develop a true biological cardiovascular device a dynamic physiological environment needs to be created. One approach that employs the use of a simulated biological environment is a bioreactor in which the in vivo biomechanical and biochemical conditions are created in vitro for functional tissue development. A review of the current state of the art bioreactors for the generation of tissue engineered cardiovascular devices is presented in this study. The effect of the simulated physiological environment of the bioreactor on tissue development is examined with respect to the materials properties of vascular grafts, heart valves, and cardiac muscles developed in these bioreactors.

Control of endocardial cushion and cardiac valve maturation by BMP signaling pathways

Congenital heart defects, the leading cause of deaths from birth defects, are estimated to occur in close to 1% of live newborns. Among these, abnormal septation of the heart and valve anomalies are the most frequent forms. Despite progress defining several genes involved in normal heart development, we still have a limited understanding of the signaling pathways involved in morphogenesis of the outflow tract (OFT) and, to date, very few genes have been identified that are responsible for defects in humans. Bone Morphogenetic Protein (BMP) signaling pathways are emerging as vital regulators of multiple aspects of cardiogenesis, including the septation of the OFT and valve maturation. Genetic and other in vivo evidence is now supporting the role for BMPs as inducers of endocardial cushion epithelial-to-mesenchymal transformation that was suggested by in vitro explant studies as well as by their patterns of expression in the developing heart. Here, we review briefly the in vitro data, and detail the novel mouse models where perturbed BMP signaling pathways result in impaired OFT septation and semilunar valvulogenesis. We propose that growth of the OFT valve cushions is regulated by the level of BMP signaling, under the control of other signaling pathways.

Cardiovascular tissue engineering: constructing living tissue cardiac valves and blood vessels using bone marrow, umbilical cord blood, and peripheral blood cells

Although atherosclerosis and valvular heart disease are among the leading causes of morbidity and mortality in developed nations, the substitute blood vessels and heart valves currently available all have significant limitations. During the past 10 years, a new field called tissue engineering has emerged, and several research groups are focusing their efforts on constructing living tissue replacement blood vessels and heart valves. In 2001 several exciting developments occurred with the use of progenitor and stem cells. This article introduces the essential concepts of cardiovascular tissue engineering, reviews achievements in the field, discusses the basic developmental biology of heart valves and blood vessels, and summarizes the 2001 research on progenitor and stem cells.

Pathogenesis of streptococcal and staphylococcal endocarditis

Although streptococcal and S. aureus IE share the same primary site of infection, their pathogenesis and clinical evolution present several major differences. Streptococci adhere to cardiac valves with pre-existing endothelial lesions. In contrast, S. aureus can colonize either damaged endothelium or invade physically intact endothelial cells. These interactions are mediated by multiple surface adhesins, some of which have been only partially characterized. Streptococci produce surface glucans (gtf and ftf), ECM adhesins (e.g., fibronectin-binding proteins, FimA), and platelet aggregating factors (phase I and phase II antigens, pblA, pblB, and pblT), all of which have been.

Structure of the conus arteriosus of the sturgeon (Acipenser naccarii) heart. I: the conus valves and the subendocardium

Sturgeons are bony fish that retain structural traits typical of the more primitive Chondrostei. From an evolutionary viewpoint, sturgeons are considered relic fish. However, they show remarkable ecological plasticity and are well adapted to contemporary environmental conditions. Although development of the cardiovascular system is critical for all organs and systems, and is affected by evolutionary changes, the structure of the sturgeon heart has been mostly overlooked. This is also true for the conus arteriosus, which, as in Chondrostei, is endowed with several rows of valves and a layer of contractile myocardium. This work reports on the structure of the valves, the endocardium, and the subendocardium of the conus arteriosus of the sturgeon (Acipenser naccarii) heart. It is part of a broader study that aims to cover the entire structure of the sturgeon heart. The conus arteriosus of 15 A. naccarii hearts, ranging in age from juveniles to sexually-differentiated adults, has been studied by conventional light, transmission (TEM), and scanning electron microscopy (SEM). In addition, maceration of the soft tissues with NaOH, and actin localization by fluorescent phalloidin has been used. The conus is a tubular chamber that arises from the right ventricular side and presents two constrictions at the conus-ventricle and conus-aorta junctions. The conus is endowed with three rows of valves: one distal and two proximal. The segment of the conus located between the distal and the two proximal rows is devoid of valvular structures. The distal row has four leaflets, while the two proximal rows show the greatest variation in leaflet number, size, and shape. All leaflets have collagenous chordae tendineae arising from the free border and from the parietal side of the leaflets. The endocardium is a flat endothelium which shows a thick, irregular basement membrane. The leaflet body is formed by a loose connective tissue which blends with the subendocardium. The subendocardium is a connective tissue consisting of myofibroblasts, collagen, and elastin. It is divided into two distinct areas: one proximal, which shows little elastin and poorly organized collagen; and one distal, which is rich in elastin, with cells and extracellular fibers organized into layers that are oriented in alternative circumferential and longitudinal directions. The present report is the first systematic analysis of the structure of the sturgeon conus. Descriptions of the conus valves should recognize the existence of three valve rows only. The variability in valve morphology, and the loose structure of the leaflet tissue make it unlikely that the valves play an effective role in preventing blood backflow. In this regard, the ventricle-conus constriction may act as a sphincter. The subendocardium is an elastic coat capable of actively sustaining the tissue deformation that accompanies the heart contractile cycle. Further comparative studies are needed to provide deeper insight into the structural changes that accompany phyletic diversification.

Anatomy, histology, and development of the cardiac valvular system in elasmobranchs

We report here on the anatomy, histology, and development of the three sets of cardiac valves in embryonic and adult elasmobranch fishes. The sinus venosus is the first segment of the heart to receive blood, and a pair of sinoatrial (SA) valves prevent backward flow of blood into the sinus venosus. The SA valves derive from two dorsolateral infoldings of the cardiac wall and consist of a simple endocardium covering transverse sheets rich in collagen. The SA valves are simple flaps of tissue without papillary muscles or chordae tendineae. Blood from the atrium passes the atrioventricular (AV; semilunar) valves, which are attached to papillary muscles in the ventricle by way of the chordae tendineae. A series of rows of conal or pocket valves (CV) in the conus arteriosus, equipped with chordae tendineae but no papillary muscles, prevent blood from reentering the ventricle. Chordae tendineae form in a similar fashion in both chambers. Elevations from the chamber wall emerge as a sheet covered on both surfaces with endocardium and separated by a core of connective tissue. Endocardial cells extend basal projections toward the opposing epithelium through their basal laminae. Basal cell projections make contact to create perforations that enlarge to produce spaces between the nascent chordae. Fibroblasts in the core of the chordae enlarge and strengthen the chordae by producing linear arrays of collagen fibers.

Absorbable rings for pediatric valvuloplasty. Preliminary study

Secondary valvular stenosis is a significant risk of annular ring implantation during valvular repair in children. To avoid this problem, we have developed an absorbable prosthetic ring (APR), which induces the generation of a biologic fibroelastic ring with mitotic capability and thereby allows atrioventricular annulus growth with age. Twelve polydioxanone (PDS) biodegradable APRs were surgically implanted in immature goats. To avoid possible embolic complications from ring fragments during the absorption period, APRs were covered with an extensible sewing sheath of high-porosity polyester, allowing contact between the PDS, blood, and endocardium. Four APRs were implanted into the right atrial cavity, four in the tricuspid position, and four in the mitral position. Histological studies at 6 months showed a considerable amount of remaining PDS. At 1 year, only small residual fragments of PDS were present, surrounded by collagen and elastic fibers as well as fibroblasts with mitotic activity. This histological structure represents a new biologic fibroelastic ring, which has originated from PDS rings. Echocardiography at 12 months showed that the elasticity of APRs and fibrotic tissue allows normal systolic and diastolic valve motions. These findings, which demonstrate histological changes in native valve annuli as well as stable and elastic annuloplasty without secondary stenosis in growing animals, call for new studies involving hearts with valvular disease.

[Growth and aging of connective tissue]

The organs and organ systems of neonates usually show a differently advanced development of connective tissue. This must be considered when determining the change caused by ageing in specific connective tissue structures, especially if it is desired to establish relations between structure, function and time. In those organs which assume important functions during or immediately after birth the connective tissue components are more highly developed than in those which, after birth, are not subject to full functional strains. This particular phenomenon has been demonstrated by specific examples.