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, 9, 1876
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Trabecular Architecture Determines Impulse Propagation Through the Early Embryonic Mouse Heart

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Trabecular Architecture Determines Impulse Propagation Through the Early Embryonic Mouse Heart

Veronika Olejníčková et al. Front Physiol.

Abstract

Most embryonic ventricular cardiomyocytes are quite uniform, in contrast to the adult heart, where the specialized ventricular conduction system is molecularly and functionally distinct from the working myocardium. We thus hypothesized that the preferential conduction pathway within the embryonic ventricle could be dictated by trabecular geometry. Mouse embryonic hearts of the Nkx2.5:eGFP strain between ED9.5 and ED14.5 were cleared and imaged whole mount by confocal microscopy, and reconstructed in 3D at 3.4 μm isotropic voxel size. The local orientation of the trabeculae, responsible for the anisotropic spreading of the signal, was characterized using spatially homogenized tensors (3 × 3 matrices) calculated from the trabecular skeleton. Activation maps were simulated assuming constant speed of spreading along the trabeculae. The results were compared with experimentally obtained epicardial activation maps generated by optical mapping with a voltage-sensitive dye. Simulated impulse propagation starting from the top of interventricular septum revealed the first epicardial breakthrough at the interventricular grove, similar to experimentally obtained activation maps. Likewise, ectopic activation from the left ventricular base perpendicular to dominant trabecular orientation resulted in isotropic and slower impulse spreading on the ventricular surface in both simulated and experimental conditions. We conclude that in the embryonic pre-septation heart, the geometry of the A-V connections and trabecular network is sufficient to explain impulse propagation and ventricular activation patterns.

Keywords: cardiac conduction; mathematical modeling; mouse embryo; optical mapping; trabeculation.

Figures

Figure 1
Figure 1
Flow chart illustrating data acquisition and image processing (stack merging, segmentation, reconstruction, and creation of skeleton model).
Figure 2
Figure 2
Tensor images showing trabecular orientation in mouse embryonic hearts at ED10.5, ED11.5, and ED14.5. The cylinder size is proportional to the conductivity, the shape and color (blue to red) reflect the trabecular anisotropy. Posterior views of fully reconstructed hearts; note differences between the septal area and the ventricles as well as increased isotropy at ED14.5.
Figure 3
Figure 3
3D reconstructions of mouse embryonic Nkx2.5:eGFP hearts at ED9.5, ED10.5, ED11.5, and ED14.5. Expression of the eGFP in red, higher levels of expression in the atrioventricular canal (AVC), outflow tract (OT) and trabeculae in the right ventricle (RV) are highlighted in green; LA, left atrium; LV, left ventricle; RA, right atrium; scale bars 200 μm. All hearts are shown from the posterior aspect and 3D rendering is complemented by a single optical section through the A-V junction. His bundle is clearly present as a preferential dorsal atrioventricular conduction pathway terminating in the interventricular septum (IVS) at ED14.5, characterized by an increased expression of eGFP (and hence Nkx2.5). This pathway is, however, laid down much earlier (arrows and increased expression in the primary interventricular ring at ED10.5).
Figure 4
Figure 4
Examples of modeling of impulse propagation in mouse embryonic hearts at ED10.5, ED11.5, ED12.5, and ED14.5. Actual representative epicardial activation maps from wild type mouse embryonic hearts at the same stages are shown on the right side for comparison. The simulation and experimentally obtained map start to differ from ED12.5, suggesting a presence of an internal specialized conduction pathway. Scale bar 0.5 mm (all hearts were mapped at the same magnification).
Figure 5
Figure 5
Experimental evidence of left bundle branch (LBB) functionality in ED14.5 mouse embryonic heart. Left side of the interventricular septum (IVS) shows a biphasic action potential (AP), which is made clearer after calculation of the first derivative that shows two peaks 24 ms apart. This allowed construction of two activation maps: the one from the first peak only shows the passing of the action potential through the LBB, while the second one, starting 30 ms milliseconds later, shows the activation of the entire left ventricular (LV) myocardium, which has a typical monophasic action potential and the first derivative with a single peak. Posterior view, scale bar 0.5 mm. Temporal scale is 80 ms per division (upper two traces) and 15 ms per division (lower two traces). For visualization, see the Supplemental Movie 1.
Figure 6
Figure 6
Propagation of spontaneous activation vs. paced beat at ED14.5. Epicardial breakthroughs in the sinus rhythm are indicated by white asterisks. Note increased time needed to activate the entire left ventricular surface for the paced beat. Simulation data (right) correlate well with the experimentally obtained activation map.

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