Developmental changes in atrial conduction patterning.

<p><b>A</b>) Isochronal map (0.3 ms/div) depicting action potential propagation across the atria of a HH Stage 18 heart. The action potential initiates in the newly differentiated pacemaker cells (red) and propagates towards the left side of the atria (purple). <b>B</b>) Bright field image of the heart from (A), with the area of the atria and inflow region of the heart indicated by dashed lines. <b>C</b>) As in (A), for a HH Stage 24 atria. Note the emergence of finger-like projections in the isochronal map depicting regions of higher conduction velocity. <b>D</b>) Bright field image of heart from (C), with the area of the atria and inflow outlined by white dashed lines. <b>E</b>) As in (A), for a HH Stage 29 heart. Note the prominent finger-like extensions in the isochronal map demonstrating areas of rapid propagation. <b>F</b>) Bright field image of heart from (E), showing the location of the muscle bundles extending from Bachman's Bundle (BB) along the roof of the atria. <b>G</b>) Optical mapping data was superimposed on bright field images and the areas of the prominent muscle bundles were outlined (white dashed area) to demonstrate that rapid conduction followed the muscle bundles. Green signal = dV/dT max from a single frame of optical mapping recording. <b>H</b>) Quantification of average time for the right atria to activate at HH Stage 18, HH Stage 24, and HH Stage 29 (n = 8 for each). <b>I</b>) Quantification of right atrial conduction velocity from hearts in (H). P-values indicated significance relative to HH Stage 18 (***p<0.001, *p<0.05). v – ventral, l – left, d – dorsal, r – right, A - atria, LA – left atria, RA – right atria, BB – bundle of Bachman.