Increased Ventricular Preload Is Compensated by Myocyte Proliferation in Normal and Hypoplastic Fetal Chick Left Ventricle

Hemodynamics influence cardiac development, and alterations in blood flow may lead to impaired cardiac growth and malformations. The developing myocardium adapts to augmented workload by increasing cell number (hyperplasia). The aim of this study was to determine the influence of alterations in ventricular preload on fetal myocyte proliferation by manipulation of intracardiac shunting at the atrial level. We hypothesized that partial clipping of the right atrial appendage would increase the blood flow to the left ventricle and, in turn, lead to an increase in chamber volume and myocardial mass based on myocyte proliferation. Using an ex ovo culture setup, we performed partial right atrial clipping on embryonic day 8 chick embryos. Ultrasound imaging was performed before and after the surgery to assess the changes in left ventricular volume. Sampling after 24 hours was preceded by 2 hour of pulse-labeling with 5-bromodeoxyuridine. Ultrasound imaging showed that partial right atrial clipping led to a significant increase in left ventricular end-diastolic volume, demonstrating increased blood flow and preload. Anti–5-bromodeoxyuridine immunolabeling revealed a significant increase in myocyte proliferation in the left ventricle and atrium. No significant changes were found in the right heart structures. Increased left ventricular myocyte proliferation and myocardial mass after right atrial clipping was also observed in embryos with experimental left ventricular hypoplasia. These results demonstrate the ability of fetal myocardium to respond to increased preload by myocyte hyperplasia and support the rationale for prenatal surgical interventions in certain cases of congenital heart disease such as hypoplastic left heart syndrome.

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. Propagation of initially peristaltoid contraction along and between these segments appears to be influenced by physical conditioning and orientation of inner muscle layers as well as by their slow relaxation; all characteristic of definitive conduction tissue. As classical elements of the mature conduction system emerge, such inner 'contour fibres' maintain muscular and electrical continuity between atrial and ventricular compartments. Elements of such primordial architecture are visible also in histological and optical electrical study of fish and frog hearts. In the maturing chick heart, cells within core conducting tissues retain early thymidine labels from the tubular heart stage into adult life, dividing only slowly, if at all. Preliminary evidence from mammals suggest similar function and kinetics for these 'oldest, toughest' cells in the hearts of all vertebrates.

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. Previously, we have proposed that hemodynamics and/or mechanical conditioning may be key epigenetic factors in development of the HPS. We thus hypothesized that the timing of the topological shift marking maturation of the conduction system is sensitive to variation in hemodynamic load. Spatiotemporal patterns of ventricular activation (as revealed by high-speed imaging of fluorescent voltage-sensitive dye) were mapped in chick hearts over normal development, and following procedures previously characterized as causing increased (conotruncal banding, CTB) or reduced (left atrial ligation, LAL) hemodynamic loading of the embryonic heart. The results revealed that the timing of the shift to mature activation displays striking plasticity. CTB led to precocious emergence of mature HPS function relative to controls whereas LAL was associated with delayed conversion to apical initiation. The results from our study indicate a critical role for biophysical factors in differentiation of specialized cardiac tissues and provide the basis of a new model for studies of the molecular mechanisms involved in induction and patterning of the HPS in vivo.

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. Spread of the excitation wave through the atria was uniform with average activation times of 20 +/- 2 and 50 +/- 2 ms for zebrafish and Xenopus toads, respectively. After a delay of 47 +/- 8 and 414 +/- 16 ms, the ventricle became activated first in the apical region. Ectopic ventricular activation was propagated significantly more slowly (total ventricular activation times: 24 +/- 3 vs. 14 +/- 2 ms in zebrafish and 74 +/- 14 vs. 35 +/- 9 ms in Xenopus). Although we did not observe any histologically defined tracts of specialized conduction cells within the ventricle, there were trabecular bands with prominent polysialic acid-neural cell adhesion molecule staining forming direct myocardial continuity between the atrioventricular canal and the apex of the ventricle; i.e., the site of the epicardial breakthrough. We thus conclude that these hearts are able to achieve the apex-to-base ventricular activation pattern observed in higher vertebrates in the apparent absence of differentiated conduction fascicles, suggesting that the ventricular trabeculae serve as a functional equivalent of the His-Purkinje system.