Table 1.
Study group features and embryos acquired.
Fig 1.
Anterior segment of the straight heart tube is involved in the development of the apical and medial thirds of the interventricular septum.
(A, A’) Bright field and fluorescent microscopy images of the straight heart tube with the anterior segment delimited by the cephalic (blue arrow) and caudal (green arrow) labels. (B, B’–D, D’) C-, S-, and U-looped hearts. Note the anterior segment of the straight heart tube displaced to the supposed right ventricular primordium. The cephalic label (blue arrow) is located in the distal region of the ascending branch of the loop near the limit between the conus and the right ventricle primordium. The caudal label (green arrow) is located at the midpoint of the great curvature, at the level of the supposed interventricular septum precursor. (E, E’) Dorsal view of a tetracavitary heart (HH34) revealing the labels at both limits of the straight heart tube converging at the right ventricular apex. (F, F’) Histological sections of a tetracavitary heart showing the caudal label (green arrow) immersed in the myocardium at the middle third of IVS. Abbreviations: AV, atrioventricular; C, conus; LA, left atrium; LV, left ventricle; LVp, left ventricle primordium; PRA, primitive right atrium; PLA, primitive left atrium; RA, right atrium; RV, right ventricle.
Fig 2.
Posterior segment of the straight heart tube is involved in the development of the atrioventricular canal and left ventricle.
(A, A’) Straight heart tube with a label on the distal end of the posterior segments of the right (yellow arrow) and left (red arrow) branches. (B, B’) C-looped heart. Note the tubular shape of the proximal region of the posterior segment of the straight heart tube. The right and left caudal regions, which are supposed to be the primitive atria, remain bifurcated. (C, C’ and D, D’) S- and U-looped hearts. Note that the originally bifurcated posterior segment of the straight heart tube acquired a tubular shape. Labels at the right and left borders of the posterior segment of the straight heart tube converged at the currently named atrioventricular grooves, adjacent to the atrioventricular canal. (E, E’ and F, F’) Ventral and dorsal views of a mature heart. Observe labels at the level of the ventral (red arrow) and dorsal (yellow arrow) atrioventricular canal wall. Abbreviations: AV, atrioventricular; C, conus; LA, left atrium; LV, left ventricle; LVp, left ventricle primordium; PRA, primitive right atrium; PLA, primitive left atrium; RA, right atrium; RV, right ventricle. Dotted line represents the interventricular septum.
Table 2.
Displacement of labels from the cephalic and caudal limits of the anterior segment of the straight heart tube to the interventricular septum.
Table 3.
Displacement of Labels from the cephalic and caudal limits of the posterior segment of the straight heart tube to the LV and AV canal.
Fig 3.
Representative images of the in vitro explants assays showing that the atria did not arise from the first heart field-derived straight heart tube.
(A and B) Recently explanted straight heart semi-tubes (HH10). (A’ and B’) The same hearts after 15 h of in vitro culture. The C-looped heart tubes and a new vesicle formed at their caudal limit. (C) Panoramic view of a C loop showing the presence of cytoplasmic Troponin I (green fluorescence) and Sox-9 (red fluorescence) in cells along the cardiac tube. (C’) Newly formed cardiac vesicles at the caudal end of C loop comprising cells expressing nuclear Sox-9 alone.
Fig 4.
PCNA distribution patterns in the cardiac tube throughout torsion and looping.
(A, A′) Frontal view of a section of a straight heart tube (HH10). A small number of PCNA-positive fluorescent red cell nuclei are observed in the FHF delimited by two yellow dotted lines; (B, B’) Frontal view of a section of a C looped heart (HH12); (C, C’) left sagittal view of an S looped heart (HH14). Increased number of PCNA-positive fluorescent red cell nuclei are observed in cardiac segments derived from SHF; (D, D’). Right and left views of a section of a U looped heart (HH16). Notable increase in PCNA-positive fluorescent red cell nuclei in heart structures derived from both the first and second heart fields. (E) Graphical representation of the percentage of PCNA-labeled cells with respect to the total number of cells constituting each heart field at torsion and looping stages. Abbreviations: FHF, first heart field; SHF, second heart field.
Fig 5.
New proposal for the segmental pattern of the heart tube during torsion and looping.
(A) Straight heart tube. (B) “C” loop. (C) “S” loop. (D) “U” loop. (E) Middle stage of cardiac septation. (F) Mature heart. Anterior segment of the straight heart tube corresponds to the primordium of the interventricular septum (IVSp). Posterior segment is composed of precursors of the left ventricle (LV) and atrioventricular canal (AVC). The segment classically considered the conus of the C-looped heart, actually participates in the complete right ventricle (RV) development. The region previously considered the RV primordium actually participates in the development of the middle and apical thirds of the interventricular septum. The supposed left ventricular primordium actually contributes cell populations to the development of LV and AV canal. Abbreviations: AVCp, atrioventricular canal primordium; C, conus; LA, left atrium; LVp, left ventricle primordium; RA, right atrium; RVOF, right ventricle outflow; PLA, primitive left atrium; PRA, primitive right atrium; RVp, right ventricle primordium; Sig, sigmoid valves; T, truncus.