Human Heart Morphogenesis: A New Vision Based on In Vivo Labeling and Cell Tracking

Abstract

Despite the extensive information available on the different genetic, epigenetic, and molecular features of cardiogenesis, the origin of congenital heart defects remains unknown. Most genetic and molecular studies have been conducted outside the context of the progressive anatomical and histological changes in the embryonic heart, which is one of the reasons for the limited knowledge of the origins of congenital heart diseases. We integrated the findings of descriptive studies on human embryos and experimental studies on chick, rat, and mouse embryos. This research is based on the new dynamic concept of heart development and the existence of two heart fields. The first field corresponds to the straight heart tube, into which splanchnic mesodermal cells from the second heart field are gradually recruited. The overall aim was to create a new vision for the analysis, diagnosis, and regionalized classification of congenital defects of the heart and great arteries. In addition to highlighting the importance of genetic factors in the development of congenital heart disease, this study provides new insights into the composition of the straight heart tube, the processes of twisting and folding, and the fate of the conus in the development of the right ventricle and its outflow tract. The new vision, based on in vivo labeling and cell tracking and enhanced by models such as gastruloids and organoids, has contributed to a better understanding of important errors in cardiac morphogenesis, which may lead to several congenital heart diseases.

Keywords: cardiogenesis; cell tracking; embryo; heart morphogenesis.

Figure 1

Chronology of the development of the human heart and the stages of the process. Abbreviations: A-V: atrioventricular, V-A: ventriculoatrial.

Figure 2

Diagrams and images of embryos at the gastrula stage. (A) Development of the trilaminar embryonic disk of chicken due to the migration (arrows) of epiblast cells through the primitive streak (PS). (B) A 16 ± 1-day embryo exhibiting the right (RCA) and left (LCA) cardiogenic areas spanning one-third of the primitive streak, the primitive node (PN), and the notochord (N) to the oropharyngeal membrane (OPM). (C) An 18 ± 1-day embryo showing the angiogenic plexuses (APs) arranged in the cardiac crescent, facing the neural plate (NPla). (D) Sagittal section of a 19–21-day human embryo with the first pair of somites showing the beginning of the development of the foregut (FG) and the neural plate. The somatopleure (SP), pericardial cavity (PC), and cardiogenic plate (CP) on the endoderm (En) of the yolk sac are also visible.

Figure 3

Formation of the heart in a straight tube (A–C). Cross-sectional diagrams of embryos depicting the tubulation process in embryos, which results in the fusion of the cardiac primordia to form the straight heart tube during late gastrulation (day 18 ± 1). (A) Note the two cardiac primordia composed of one endocardial tube and myocardial lineage cells (B), (C) Fusion of the two endocardial tubes (EnT) and formation of the straight heart tube (SHT), composed of one endocardial tube and a dorsally opened myocardial canal. (D) Frontal or ventral view of the straight heart tube derived from the cell populations of the first heart field. Abbreviations: FG: foregut, NT: neural tube.

Figure 4

New proposal for the segmental pattern of the heart tube during twisting and looping. (A) Straight heart tube (SHT) and the cephalic region of SHT (green), which was previously considered the primordium of the right ventricle, participate in the development of the middle region and apical third of the IVS. The caudal region of the SHT (purple), the supposed primordium of the LV, also contributes to the development of the LV and AVC and is the last precursor of the inflow tracts of both ventricles and the outflow tract of the LV. At this stage, both primordial segments, pIVS and pLV, are connected in series: one cephalad and the other caudal. (B) Subsequently, the cardiac tube presents different conformational changes during the segmental incorporation of the primordia from the second cardiac field, C-loop. The proximal segment of the conus (brown), which actually participates in the development of the RV, is found in the C-loop. The right and left atrial primordia are caudally integrated. At this stage, the straight heart tube is turned to the right, and the ventral wall becomes part of the greater curvature of the loop. (C) S-loop. The elongation of the heart tube continues, and the lesser curvature of the loop (*) becomes more pronounced, causing both ends of the tube (inlet and outlet) to be closer to each other. (D) Advanced loop. The distal segment of the conus is present (blue) and forms the RVOFT. The anatomical position reached by each of the primordial cardiac segments stands out: the pLV is caudal to the PA and to the right of the RV. The DC and pRV descend and assume their correct anatomic positions. (E) Late loop (start of septation). The segment known as truncus (T) appears (pale pink) and forms the sigmoid arteries and their insertion annulus. (F) Mature heart. The heart is made up of four cavities that are properly connected and segmented: the right (RA) and left (LA) atriums, as well as the RV and LV. (G) Diagram summarizing the integration of the cardiac primordia during the torsion and folding stages of the cardiac tube. Abbreviations: PLA: primitive left atrium, PRA: primitive right atrium, Sig: sigmoid valves.

Figure 5

Model for Nodal signalling in the left-lateral plate mesoderm and activation Pitx2 in the heart morphogenesis in normal embryos. In the initial event, that translates the motion of primitive nodal cilia (Hensen) toward the left side of the chicken embryo, which regulates the expression of Foxh1 and Pitx2c through the receptor of the activin signaling pathway. Foxh1 determines the rightward direction of the heart loop and the ventricular topology. Pitx2c expression determines the atrial identity. (A) Real embryo. (B) Representation of the possible flow of nodal from the primitive node and the interacting genes. (The PS is exemplified in the central dotted line). Based on Chen CM, Norris D, Bhattacharya S. Transcriptional control of left-right patterning in cardiac development [57].

Figure 6

Onset of cardiac septation in a rat embryo of an age equivalent to 28–32 days in humans. (A,B) Sagittal view of the left cavities. Note the mask-shaped primitive cardiac septum composed of the septum primum (SP), the primitive interventricular septum (PIVS), and the ventral (VCAVC) and dorsal (DCAVC) cushions of the atrioventricular canal. Note the foramen primum (1) and the primitive interventricular foramen (2). The structures are in the same plane without demarcation of limits. (C,D) Sagittal view of the right cavities showing the connection between the primitive right atrium (PRA) and its ventricle (black dotted line). Note that although the four-chambered heart has begun to establish, a single conus (C) emerges from the developing right ventricle (PTRRV). Abbreviation: T: truncus.