Abstract
Heart development depends on the spatio-temporally regulated contribution of progenitor cells from the primary, secondary and anterior heart fields. Primary heart field (PHF) cells are first recruited to form a linear heart tube; later, they contribute to the inflow myocardium of the four-chambered heart. Subsequently cells from the secondary (SHF) and anterior heart fields (AHF) are added to the heart tube and contribute to both the inflow and outflow myocardium. In amniotes, progenitors of the linear heart tube have been mapped to the anterior-middle region of the early primitive streak. After ingression, these cells are located within bilateral heart fields in the lateral plate mesoderm. On the other hand SHF/AHF field progenitors are situated anterior to the linear heart tube, however, the origin and location of these progenitors prior to the development of the heart tube remains elusive. Thus, an unresolved question in the process of cardiac development is where SHF/AHF progenitors originate from during gastrulation and whether they come from a region in the primitive streak distinct from that which generates the PHF. To determine the origin and location of SHF/AHF progenitors we used vital dye injection and tissue grafting experiments to map the location and ingression site of outflow myocardium progenitors in early primitive streak stage chicken embryos. Cells giving rise to the AHF ingressed from a rostral region of the primitive streak, termed region ‘A’. During development these cells were located in the cranial paraxial mesoderm and in the pharyngeal mesoderm. Furthermore we identified region ‘B’, located posterior to ‘A’, which gave rise to progenitors that contributed to the primary heart tube and the outflow tract. Our studies identify two regions in the early primitive streak, one which generates cells of the AHF and a second from which cardiac progenitors of the PHF and SHF emerge.
Highlights
The heart is the first organ that becomes functional in the developing vertebrate embryo and initially appears in the ventral midline as a linear endocardial tube, ensheathed by myocardial cells
Our results demonstrate that region A is the site of ingression of cells which contribute to the cranial paraxial mesoderm (CPM) by stage HH8 and to the first branchial arc at stage HH15
They show that region B is the site of ingression of cardiac progenitor cells (CPCs), which by HH8 are located in the splanchnic mesoderm, including the dorsomedial region, described to be part of the anterior heart fields (AHF) [25]
Summary
The heart is the first organ that becomes functional in the developing vertebrate embryo and initially appears in the ventral midline as a linear endocardial tube, ensheathed by myocardial cells. This primary linear heart tube develops from the ventral midline fusion of bilateral heart fields located in the lateral plate mesoderm. These bilateral heart fields, known as cardiogenic mesoderm, consist of cardiac progenitor cells (CPCs), which are among the first cells during gastrulation to ingress from the primitive streak in amniotes [1]. As the anterior intestinal portal continues to move caudally, the splanchnic mesoderm folds ventrally and medially and fuses at the midline by HH9, resulting in the formation of the primary heart tube by HH10
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