PMC

Experimental setup of a heart in a pedestal bath with the apex facing up. LV, left ventricle; RV, right ventricle. The crossed wires are for alignment. The dotted circles show the flattened apex relative to the contour of the heart.

a Schematic of AAV9-shFosl1 virus injection to silence Fosl1 in the quiescent neonatal heart. b qPCR validation of Fosl1 knockdown in the neonatal heart after AAV9-shFosl1 injection (n = 5 hearts). c, d Representative images (c) and quantification (d) of EdU⁺ cardiomyocytes in ventricle apex (n = 8 hearts). Lower panels of c are the magnified confocal images of EdU⁺ cardiomyocytes. e Representative images and quantification of pH3⁺ cardiomyocytes in the ventricle apex (n = 8 hearts). Lower panels are the magnified images of pH3⁺ cardiomyocytes. f Schematic of AAV9-shFosl1 virus injection to silence Fosl1 in the injured heart at 4 dpr. g qPCR validation of Fosl1 knockdown in the injured heart (n = 3 hearts). h Quantification of heart weight (HW) to body weight (BW) ratio (n = 3 hearts). i, j Representative images (i) and quantification (j) of EdU⁺ cardiomyocytes in the ventricle apex (n = 8 and 5 hearts). Right panels of i are the magnified images of EdU⁺ cardiomyocytes. k, l Representative images (k) and quantification (l) of Ki67⁺ cardiomyocytes in the ventricle apex (n = 8 and 5 hearts). Right panels of k are the magnified images of Ki67⁺ cardiomyocytes. m, n Representative images (m) and quantification (n) of pH3⁺ cardiomyocytes in the ventricle apex (n = 8 and 5 hearts). Right panels of m are the magnified images of pH3⁺ cardiomyocytes. All data are presented as mean ± SEM. *p < 0.05, **p < 0.01, ***p < 0.001 versus shNC (Student’s t test).

Experimental setup for measuring the apex under the SQUID microscope. Inset: Apex stimulation setup: (a) SQUID sensor pickup coil, (b) SQUID microscope Dewar tail, (c) heart, (d) apex, (e) cathodal electrode, (f) Teflon post, (g) anodal electrode, and (h) perfusate bath. The cathodal stimulation occurred ∼50 μm below the surface of the apex.

Apical resection of the X. tropicalis heart and rapid blood coagulation after resection. a The heart was exposed for apical resection. The dash line shows the resection plane, corresponding to approximately 10% of the ventricle. b The heart immediate after surgical amputation. c Bleeding after ventricle amputation. d Rapid blood coagulation in the amputated heart after approximately 5 s of pressure with sterile cotton. e Outer surface of the amputated apex. The asterisk indicates the end of the apex on the outer surface. f Inner surface of the amputated apex. The circle with the dotted line indicates the bottom of the ventricle. g Lateral view of the amputated apex. The arrow indicates the bottom of the apex on the outer surface