Serotonin potentiates transforming growth factor-beta3 induced biomechanical remodeling in avian embryonic atrioventricular valves.

Embryonic heart valve primordia (cushions) maintain unidirectional blood flow during development despite an increasingly demanding mechanical environment. Recent studies demonstrate that atrioventricular (AV) cushions stiffen over gestation, but the molecular mechanisms of this process are unknown. Transforming growth factor-beta (TGFβ) and serotonin (5-HT) signaling modulate tissue biomechanics of postnatal valves, but less is known of their role in the biomechanical remodeling of embryonic valves.

Myocyte proliferation in the developing heart.

Regulation of organ growth is critical during embryogenesis. At the cellular level, mechanisms controlling the size of individual embryonic organs include cell proliferation, differentiation, migration, and attrition through cell death. All these mechanisms play a role in cardiac morphogenesis, but experimental studies have shown that the major determinant of cardiac size during prenatal development is myocyte proliferation.

Patterns of muscular strain in the embryonic heart wall.

The hypothesis that inner layers of contracting muscular tubes undergo greater strain than concentric outer layers was tested by numerical modeling and by confocal microscopy of strain within the wall of the early chick heart. We modeled the looped heart as a thin muscular shell surrounding an inner layer of sponge-like trabeculae by two methods: calculation within a two-dimensional three-variable lumped model and simulated expansion of a three-dimensional, four-layer mesh of finite elements. Analysis of both models, and correlative microscopy of chamber dimensions, sarcomere spacing, and membrane leaks, indicate a gradient of strain decreasing across the wall from highest strain along inner layers.

Fibulin-1 is required for morphogenesis of neural crest-derived structures.

Here we report that mouse embryos homozygous for a gene trap insertion in the fibulin-1 (Fbln1) gene are deficient in Fbln1 and exhibit cardiac ventricular wall thinning and ventricular septal defects with double outlet right ventricle or overriding aorta. Fbln1 nulls also display anomalies of aortic arch arteries, hypoplasia of the thymus and thyroid, underdeveloped skull bones, malformations of cranial nerves and hemorrhagic blood vessels in the head and neck. The spectrum of malformations is consistent with Fbln1 influencing neural crest cell (NCC)-dependent development of these tissues.

High-frequency ultrasonographic imaging of avian cardiovascular development.

The chick embryo has long been a favorite model system for morphologic and physiologic studies of the developing heart, largely because of its easy visualization and amenability to experimental manipulations. However, this advantage is diminished after 5 days of incubation, when rapidly growing chorioallantoic membranes reduce visibility of the embryo. Using high-frequency ultrasound, we show that chick embryonic cardiovascular structures can be readily visualized throughout the period of Stages 9-39.

Versican proteolysis mediates myocardial regression during outflow tract development.

An important phase of cardiac outflow tract (OFT) formation is the remodeling of the distal region of the common outlet in which the myocardial sleeve is replaced by with smooth muscle. Here we demonstrate that expression of the proteoglycan versican is reduced before the loss of myocardium from the distal cardiac outlet concomitant with an increase in production of the N-terminal cleavage fragment of versican. To test whether versican proteolysis plays a role in OFT remodeling, we determined the effects of adenoviral-mediated expression of a versican isoform devoid of known matrix metalloproteinase cleavage sites (V3) and an N-terminal fragment of versican (G1).

Changes in activation sequence of embryonic chick atria correlate with developing myocardial architecture.

To characterize developmental changes in impulse propagation within atrial musculature, we performed high-speed optical mapping of activation sequence of the developing chick atria using voltage-sensitive dye. The activation maps were correlated with detailed morphological studies using scanning electron microscopy, histology, and whole mount confocal imaging with three-dimensional reconstruction. A preferential pathway appeared during development within the roof of the atria, transmitting the impulse rapidly from the right-sided sinoatrial node to the left atrium.

Confocal imaging of the embryonic heart: how deep?

Confocal microscopy allows for optical sectioning of tissues, thus obviating the need for physical sectioning and subsequent registration to obtain a three-dimensional representation of tissue architecture. However, practicalities such as tissue opacity, light penetration, and detector sensitivity have usually limited the available depth of imaging to 200 microm. With the emergence of newer, more powerful systems, we attempted to push these limits to those dictated by the working distance of the objective.

Optical mapping of electrical activation in the developing heart.

Specialized conduction tissues mediate coordinated propagation of electrical activity through the adult vertebrate heart. Following activation of the atria, the activation wave is slowed down in the atrioventricular canal or node, after which it spreads rapidly into the left and right ventricles via the His-Purkinje system (HPS). This results in the ventricles being activated from the apex toward the base, which is a hallmark of HPS function.

Temporal and spatial expression pattern of beta1 sodium channel subunit during heart development.

Objectives: The aim of this study is to analyze Scn1b mRNA expression levels and protein distribution of Scn1b, a putative modulator of the pore-forming Na(+) channel subunit in the heart, during mouse cardiac development.

Methods: Scn1b mRNA levels were determined by real-time RT-PCR using embryonic hearts ranging from E9.5 to E18.

Targeted insertion of the Cre-recombinase gene at the phenylethanolamine n-methyltransferase locus: a new model for studying the developmental distribution of adrenergic cells.

To evaluate the developmental distribution of adrenergic cells in vivo, we inserted the Cre-recombinase gene into the locus encoding for the epinephrine biosynthetic enzyme phenylethanolamine n-methyltransferase (Pnmt) and crossed these Pnmt-Cre mice with ROSA26 reporter (R26R) mice to activate LacZ (encoding beta-galactosidase) expression in cells that were selectively derived from the adrenergic lineage. Our data show the following: (1) Insertion of Cre-recombinase into the Pnmt locus created a functional knockout of Pnmt expression with concomitant loss of epinephrine in homozygous Pnmt(Cre/Cre) mice; (2) Despite the reduction in Pnmt expression and epinephrine production in Pnmt(Cre/Cre) mice, these mice were viable and fertile, with no apparent developmental defects; (3) When crossed with R26R mice, Pnmt-Cre activation of LacZ expression faithfully recapitulated Pnmt expression in vivo; and (4) LacZ expression was activated in substantial numbers of pacemaking, conduction, and working cardiomyocytes.

Developmental transitions in electrical activation patterns in chick embryonic heart.

The specialized conduction tissue network mediates coordinated propagation of electrical activity through the adult vertebrate heart. Following activation of the atria, the activation wave is slowed down in the atrioventricular canal or node, then spreads rapidly into the left and right ventricles via the His-Purkinje system (HPS). This results in the ventricle being activated from the apex toward the base and is thought to represent HPS function.

Effects of deletion of the tissue inhibitor of matrix metalloproteinases-1 gene on the progression of murine thoracic aortic aneurysms.

Objective: The cause of thoracic aortic aneurysms (TAAs) is poorly understood. Previous work has suggested an association between development of aortic aneurysms and matrix metalloproteinase (MMP) activity. We hypothesized that removal of the primary endogenous aortic MMP inhibitor (TIMP) through TIMP-1 gene deletion will increase TAA progression.

A murine model of thoracic aortic aneurysms.

Background: The mechanisms of thoracic aortic aneurysm (TAA) formation are poorly understood, mainly due to the lack of a useful and reproducible model. Accordingly, the goal of this study was to test the hypothesis that abluminal calcium chloride (CaCl(2)) application could create TAAs in the mouse.

Materials And Methods: Adult 129/SvE mice (n = 8) were anesthetized and their thoracic aortas exposed via left thoracotomy.

The oldest, toughest cells in the heart.

We review here the evolution and development of the earliest components of the cardiac pacemaking and conduction system (PCS) and the turnover or persistence of such cells into old age in the adult vertebrate heart. Heart rate is paced by upstream foci of cardiac muscle near the future sinoatrial junction even before contraction begins. As the tubular heart loops, directional blood flow is maintained through coordinated sphincter function in the forming atrioventricular (AV) canal and outflow segments.

His-Purkinje lineages and development.

The heartbeat is initiated and coordinated by a multi-component set of specialized muscle tissues collectively referred to as the pacemaking and conduction system. Over the last few years, impetus has gathered into unravelling the cellular and molecular processes that regulate differentiation and integration of this essential cardiac network. One focus of our collective work has been the developmental history of cells comprising His-Purkinje tissues of the conduction system.

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

Patterns of DNA synthesis in the developing mouse heart between ED7.5-18.5 were studied by a combination of thymidine and bromodeoxyuridine labeling techniques.

Heart development in the spotted dolphin (Stenella attenuata).

Marine mammals show many deviations from typical mammalian characteristics due to their high degree of specialization to the aquatic environment. In Cetaceans, some of the features of limbs and dentition resemble very ancestral patterns. In some species, hearts with a clearly bifid apex (a feature normally present during mammalian embryogenesis prior to completion of ventricular septation) have been described.

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

The His-Purkinje system (HPS) is a network of conduction cells responsible for coordinating the contraction of the ventricles. Earlier studies using bipolar electrodes indicated that the functional maturation of the HPS in the chick embryo is marked by a topological shift in the sequence of activation of the ventricle. Namely, at around the completion of septation, an immature base-to-apex sequence of ventricular activation was reported to convert to the apex-to-base pattern characteristic of the mature heart.

Development of the cardiac pacemaking and conduction system.

The heartbeat is initiated and coordinated by a heterogeneous set of tissues, collectively referred to as the pacemaking and conduction system (PCS). While the structural and physiological properties of these specialized tissues has been studied for more than a century, distinct new insights have emerged in recent years. The tools of molecular biology and the lessons of modern embryology are beginning to uncover the mechanisms governing induction, patterning and developmental integration of the PCS.

Effect of increased pressure loading on heart growth in neonatal rats.

During embryonic and fetal development, the ventricular myocardium increases its mass principally by adding new cells (hyperplasia), while postnatally, it does so mainly through increase of cell size (hypertrophy). Switching between these two mechanisms of adaptation to increasing functional demand occurs in the early neonatal period. We investigated the response of the neonatal rat left ventricle to pressure overload induced by constriction of the abdominal aorta at postnatal day 2.

Functional and morphological evidence for a ventricular conduction system in zebrafish and Xenopus hearts.

Zebrafish and Xenopus have become popular model organisms for studying vertebrate development of many organ systems, including the heart. However, it is not clear whether the single ventricular hearts of these species possess any equivalent of the specialized ventricular conduction system found in higher vertebrates. Isolated hearts of adult zebrafish (Danio rerio) and African toads (Xenopus laevis) were stained with voltage-sensitive dye and optically mapped in spontaneous and paced rhythms followed by histological examination focusing on myocardial continuity between the atrium and the ventricle.

Cellular changes in experimental left heart hypoplasia.

Hypoplastic left heart syndrome (HLHS) is a rare but deadly congenital malformation, which can be created experimentally in the chick embryo by left atrial ligation (LAL). The goal of this study was to examine the cellular changes leading to the profound remodeling of ventricular myocardial architecture that occurs in this model. Hypoplasia of left heart structures was produced after 3H-thymidine prelabeling by partial LAL at stage 24, thereby reducing its volume, and redistributing blood preferentially to the developing right ventricle (RV).