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Fig. 4 Atrial cardiomyocytes elongate through membrane protrusion formation.a Schematic of the membrane protrusion labeling of a few CMs, i.e., mosaic expression of the PH domain of Akt1 in CMs. Created with BioRender.com, released under a Creative Commons Attribution-NonCommercial-NoDerivs 4.0 International license. b–d’ 3D airyscan imaging at 76, 100, and 124 hpf; CM membranes shown in white (myl7:EGFP-Hsa.HRAS) and PH-Akt1 localization in magenta (myl7:PH-Akt1-tdTomato-PEST). e, f 3D confocal images of the atrium from control sibling and IRSp53DN overexpressing (myl7:gal4; UAS:IRSp53DN-RFP) larvae at 124 hpf; CM membranes shown in white (myl7:EGFP-Hsa.HRAS). g Percentage of elongating atrial CMs in 124 hpf control sibling and IRSp53DN overexpressing larvae (n = 9 control and n = 8 IRSp53DN; each data point represents one heart; two-tailed Mann-Whitney test). h–h”, Atrial CM angle measured in 124 hpf control siblings and IRSp53DN overexpressing larvae (n = 109 atrial CMs from 9 control hearts and n = 102 atrial CMs from 8 IRSp53DN overexpressing hearts; each data point represents one atrial CM; two-tailed Mann-Whitney test). i, j 3D confocal images of the atrium from DMSO-treated (i) and Rac1 inhibitor-treated (j) larvae at 124 hpf; CM membranes shown in cyan (myl7:EGFP-Hsa.HRAS) and CM nuclei in yellow (myl7:H2B-mScarlet). k Percentage of elongating atrial CMs in DMSO- or Rac1 inhibitor-treated larvae at 124 hpf (n = 7 DMSO and n = 6 Rac1 inhibitor; each data point represents one heart; two-tailed Mann-Whitney test). l–l”, Atrial CM angle measured in 124 hpf DMSO-treated and Rac1 inhibitor-treated larvae (n = 109 atrial CMs from 7 DMSO hearts and n = 105 atrial CMs from 6 Rac1 inhibitor hearts; each data point represents one heart; two-tailed Mann-Whitney test). Error bars are mean ± SD.