Fig. 1

A subset of atrial cardiomyocytes gradually elongate along the long axis of the heart.a Schematic of 3D longitudinal imaging. Created with BioRender.com, released under a Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International license. b, b’ 3D image processing used throughout the study. 3D airyscan images of a 100 hpf larva showing both the atrium and ventricle (b) and only the atrium (b’) after 3D cropping in Imaris; CM membranes shown in cyan (myl7:EGFP-Hsa.HRAS) and CM nuclei in yellow (myl7:H2B-mScarlet); magenta square and arrow indicate cropped region; the solid line in b’ is the longest line through one atrial CM, and the dashed lines show how the angle (relative to the AVC) of the highlighted atrial CM (yellow dashed outline) was measured. c–e’ 3D airyscan longitudinal imaging of the atrium at 100, 124, and 146 hpf; CM membranes shown in cyan (myl7:EGFP-Hsa.HRAS) and CM nuclei in yellow (myl7:H2B-mScarlet); one elongating CM shown in 3D (magenta). f 3D quantification of atrial CM shape at 76, 100, and 124 hpf (n = 43 atrial CMs from 6 hearts at 76 hpf, and n = 44 atrial CMs from 6 hearts at 100 and 124 hpf; each data point represents one atrial CM; average of 8 CMs per heart; ordinary one-way ANOVA with Tukey’s multiple comparison test). g Percentage of elongating atrial CMs at 76, 100, and 124 hpf (n = 6 hearts for all time points; each data point represents one heart; average of 20 CMs per heart; Kruskal-Wallis test with Dunn’s multiple comparison test). h–h”, Atrial CM angle measured at 76, 100, and 124 hpf (n = 43 atrial CMs from 6 hearts at 76 hpf, and n = 44 atrial CMs from 6 hearts at 100 and 124 hpf; each data point represents one atrial CM; average of 8 CMs per heart; Kruskal-Wallis test with Dunn’s multiple comparison test; 76 vs 100 hpf p-value = 0.32*10−4, 76 vs 124 hpf p-value = 0.67*10−7). Error bars are mean ± SD.