All structures labelled in each heart. In red, structures identified on the sections; in dark grey, structures that are yet to emerge and in light grey, structures that have become obsolete due to advanced development, such as the common atrium (Atrial_myocardium). The striped box represents a structure that was not on the sections that were included for annotation. N=1 for each time point, no statistical tests were performed.

Growth curves of the myocardium, cardiac jelly or cushions, and lumen of the embryonic hearts. All myocardial labels (grey), all cardiac jelly or cushion labels (yellow), and the labels for lumen (green) only within myocardial structures, as listed in Fig. 2, were pooled to generate total volumes. There is no indication that the total amount of cushion mesenchyme decreases (logistic regression curve fit R²=0.91, N=1 for each time point).

Growth of the cardiac chambers and myocardial structures. (A) Absolute growth of the inflow tract (IFT; including IFT_myocardium, R_sinus_horn_myocardium and L_sinus_horn_myocardium), common atrium (A; Atrial_myocardium), right atrium (RA; including RA_myocardium, RA_trabecular_myocardium, R_sinuatrial_valve, L_sinuatrial_valve, Septum_spurium_myocardium, Inferior_rim_oval_fossa_myocardium and Secondary_atrial_septum_sulcus), left atrium (LA; including LA_myocardium, LA_trabecular_myocardium and Pulmonary_vein_myocardium), common ventricle (V; including Ventricular_myocardium and Ventricular_trabecular_myocardium), left ventricle (LV; including LV_compact_myocardium and LV_trabecular_myocardium), and right ventricle (RV; including RV_compact_myocardium and RV_trabecular_myocardium). There is always significantly less left atrial than right atrial myocardium (P<0.0001) and less right ventricular than left ventricular myocardium (P=0.002). (B) Absolute growth of the septa (S; including Primary_atrial_septum_myocardium and Interventricular_septum_myocardium), atrioventricular canal (AVC; AV_canal_myocardium), and the outflow tract (OFT; OFT_myocardium). (C) Relative growth of chamber myocardium. Rest includes Primary_atrial_septum_myocardium, Interventricular_septum_myocardium and Myocardialised_AV_cushion. (D) Four chamber view cross-sections of hearts of CS12, 13 and 14 illustrating the transition from common atrium (dark blue) and common ventricle (light green) to recognisable right and left atria (intermediate and light blue) and ventricles (orange and yellow). N=1 for each time point.

Atrial and ventricular compact and trabecular myocardial growth. For the atria, distinction between outer myocardium and the trabecular pectinate muscles is made. For the ventricles the trabecular myocardium, including the papillary muscles, is segmented separated from the compact myocardium. Common atrium (includes Atrial_myocardium), right atrium (RA; compact includes RA_myocardium, Inferior_rim_oval_fossa_myocardium and Secondary_atrial_septum_sulcus; trabecular includes RA_trabecular_myocardium and Septum_spurium_myocardium), left atrium (LA; compact includes LA_myocardium and Pulmonary_vein_myocardium; trabecular includes LA_trabecular_myocardium), common ventricle (includes Ventricular_myocardium and Ventricular_trabecular_myocardium), left ventricle (LV; compact includes LV_compact_myocardium; trabecular includes LV_trabecular_myocardium), right ventricle (RV; compact includes RV_compact_myocardium; trabecular includes RV_trabecular_myocardium). N=1 for each time point, no statistical tests were performed.