Fig. 1

Epithelial remodelling of the LHT during chamber formation. a Scheme of two-chambered zebrafish heart at 54 hpf (left) next to the smoothened outlines of a ventricle depicting shapes of the cardiomyocytes (right). According to six defined anatomical landmarks (number 1–6 in blue) the ventricle is segmented into six regions (blue dashed line) connected with the ventricular centroid, counter-clockwise: between 1–2: inner curvature (IC), 2–3: outflow tract (OFT), 3–5: outer curvature (OC) (near OFT), 5–6: OC (middle), 6–4: OC (near AV) and 4–1: atrio-ventricular junction (AV). In insets, the scheme depicts the cell orientation in four different segments defined as the angle between the cell elongation axis (red) and a 90° angle (black) placed above a tangent (green) on the smoothened outline of a ventricle. Cell orientation of an OC (yellow) and IC (turquoise) cell is ~0°, cell orientation of an OFT (orange) and an AV cell (green) is ~90°. b In wild-type (wt) hearts, OC cardiomyocytes are large and elongated (n = 240, area = 107 μm², circularity = 0.53), while IC cardiomyocytes are small and rounded (n = 211, area = 83 μm², circularity = 0.6). Hearts analysed, n = 8. Means ± s.d. ****P < 0.0001, unpaired t-test with Welch correction. c Based on schematic in a, AV (cells analysed, n = 92) and OFT (n = 86) cardiomyocytes assume ~90° angle, IC (n = 57) and OC (n = 295) cells assume ~0° angle. 8 hearts analysed. Variance of angle distribution is labelled in red. d Whole embryo time-lapse imaging of a resolving transition state in a tnnt2a-deficient heart expressing membrane-associated EGFP under myl7 promoter. At 26 hpf, the LHT displays a transition state of five cardiomyocytes sharing a common boundary (arrow in inset). During the following 5 h the transition state resolves with newly forming cell junction (arrowhead in inset). Transition state is colour-coded in the insets, corresponding cells marked with coloured circles. Scale bars, 20 μm