At the pregastrula stage of amniotes, the embryos consist of two concentric epithelial layers, the epiblast on the dorsal side, and the hypoblast on the ventral side. Our body develops from the epiblast layer of the blastula embryo. Fate map analysis using fluorescent dye or explantation germ layer experiments in avian embryos, which are amniotes and consist of disk-like flat germ layers similar to the human embryonic disc, shows that prospective heart cells reside in the posterior lateral region of the epiblast layer (red and blue dots in Fig. 1A; Hatada and Stern 1994; Yatskievych et al. 1997; Ladd et al. 1998; Matsui et al. 2005). Before the onset of gastrulation, prospective heart cells move to the anterior region of the posterior half of the epiblast midline, where the primitive streak will soon develop (arrows in Fig. 1A,B).
Figure 1. Two sources of heart progenitor cells contribute to form the four-chambered heart. (A) At the blastula stage, prospective heart cells reside in the posterior–lateral region of the epiblast (blue and red dots). At the onset of gastrulation, heart progenitor cells migrate to the region where the anterior primitive streak develops (arrows). (B) In the early gastrula, prospective heart cells reside in the anterior primitive streak in an anterior–posterior sequence (blue and red dots). Later, the prospective heart cells migrate anterolaterally to form the heart mesoderm (arrows). (C) In the late gastrula stage (ventral view), the heart forming mesoderm is developed in the visceral mesoderm of the pericardial coelom (red and blue dots). Due to the formation of the foregut pocket (FP), the left and right heart forming regions move to the midline to fuse (arrows). The lateral-most cells in the heart forming region mainly give rise to the left ventricle (red dots), whereas the medial region forms the outflow tract (OFT) and right ventricle (blue dots). (D) The primitive heart tube is developed from the lateral-most part of the heart forming mesoderm (red dots). The visceral mesoderm behind the primitive heart tube (blue dots) contains future heart regions, including the OFT, the right ventricle, the atrioventricular canal and the atria. (E) During heart looping, the OFT is developed from the anterior heart field (AHF) and the secondary heart field (SHF) in the pharyngeal and splanchnic mesoderm (blue dots in AHF/SHF). Heart progenitors in the caudal splanchnic mesoderm of the pericardial coelom contribute to the atrioventricular canal and the atria. (F) Two sources of progenitors are known in the four-chambered heart: the OFT originates from the medial region of the heart forming mesoderm (blue dots in C), while the LV arises from its lateral-most region (red dots in C). In contrast, the RV, the atrioventricular canal and the atria are derived from the entire heart forming mesoderm (blue and red dots in C). A and B, dorsal view; C-D, ventral view; AHF, anterior heart field; FP, foregut pocket (anterior intestinal portal); LA, left atrium; LV, left ventricle; OFT, outflow tract; PS, primitive streak; RA, right atrium; RV, right ventricle; SHF, secondary heart field.
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During the early primitive streak stage, prospective heart cells, including those of the endocardium, the myocardium and the epicardium, reside in the rostral half of the primitive streak in an anterior–posterior (rostro–caudal) sequence; that is, the cells that will contribute to the outflow tract (OFT) and right ventricle (RV) are included in an anterior region of the anterior half of the primitive streak (blue dots in Fig. 1B), whereas the left ventricle (LV), the atria, and the sinus venosus forming cells are located in the posterior region of the anterior half of the primitive streak (red dots in Fig. 1B; Garcia-Martinez and Schoenwolf 1993). During gastrulation, prospective heart cells ingress into the primitive streak, and thereafter migrate anterolaterally to form the anterior lateral plate mesoderm (arrows in Fig. 1B; Yang et al. 2002; Chuai et al. 2006). Later, the anterior lateral plate mesoderm splits into the dorsal somatic and ventral splanchnic (visceral) mesoderm, resulting in the formation of the pericardial coelom (cavity). At this time, prospective heart cells are restricted to the splanchnic (visceral) mesoderm and constitute the heart forming mesoderm (Linask 1992). Cell tracing experiments have shown that the lateral-most cells in the heart forming mesoderm are incorporated into the atria and the LV (red dots in Fig. 1C), whereas the mediocaudally located progenitor cells are incorporated mainly into the OFT and RV (blue dots in Fig. 1C). In association with the formation of the foregut pocket (anterior intestinal portal; FP in Fig. 1C, D), the heart fields invert along the anterior–posterior (rostro–caudal) axis of the coelom (120–130° in chicks and 180° in humans; arrows in Fig. 1C), bend ventrally, and fuse with each other to form a “myocardial trough,” which is contiguous with the non-cardiac splanchnic mesoderm across the dorsal mesocardium (Moreno-Rodriguez et al. 2006; Abu-Issa and Kirby 2008). Later, the dorsal mesocardium is broken down to generate the primitive heart tube; therefore, it connects with the artery at the anterior pole (OFT) and with the vein at the venous pole (inflow tract). The primitive heart tube contains the future LV, the atrioventricular canal, the atria, and some RV lineage cells (red dots in Fig. 1D), but not the OFT. Surgical deletion of the midline region of the endoderm results in a bilateral heart (cardiac bifida) (Moreno-Rodriguez et al. 2006), and Gata4-mutants affect endoderm development, thereby disrupting heart tube formation, leading to cardia bifida (Narita et al. 1997; Reiter et al. 1999). Therefore, an endoderm effect, the formation of the foregut pocket, appears to be crucial to controlling the migration of the left and right heart fields to the ventral midline during the formation of the primitive heart tube.
In chicks, the segmental polarity of the heart tube is not committed at the primitive streak stage, but rather is established just before the formation of the primitive heart tube (Redkar et al. 2001). Altering the anterior–posterior organization of the precardiac region by grafting affects heart morphogenesis at the primitive heart tube stage but not at earlier stages (Patwardhan et al. 2000). In Meilhac et al. (2003, 2004), retrospective clonal analysis using a lacZ reporter gene targeted to α-cardiac actin revealed clusters of dispersed cells along the venous-arterial axis of the heart tube and the existence of two lineages that segregate from a common precursor, suggesting that the embryonic cardiac regions arise by progressive restriction of cell dispersion rather than pre-pattern formation by lineage specification. Therefore, it is likely that the lineage specification of heart progenitor cells occurs during their migration/movement and is completed immediately before the formation of the primitive heart tube.