The vascular system appears in the middle of the third week, when the embryo is no longer able to satisfy its nutritional requirements by diffusion alone. Cardiac progenitor cells lie in the epiblast, immediately lateral to the primitive streak. From there they migrate through the streak. Cells destined to form cranial segments of the heart, the outflow tract, migrate first, and cells forming more caudal portions, right ventricle, left ventricle, and sinus venosus, respectively, migrate in sequential order. The cells proceed toward the cranium and position themselves rostral to the buccopharyn-geal membrane and neural folds (Fig. 11.1). Here they reside in the splanchnic layer of the lateral plate mesoderm. At this time, late in the presomite stage of development, they are induced by the underlying pharyngeal endoderm to form cardiac myoblasts. Blood islands also appear in this mesoderm, where they will form blood cells and vessels by the process of vasculogenesis (see Chapter 5; p 103) (Fig. 11.1). With time, the islands unite and form a horseshoe-shaped endothelial-lined tube surrounded by myoblasts. This region is known as the cardiogenic field; the intraembryonic cavity over it later develops into the pericardial cavity (Fig. 11.1 D).
In addition to the cardiogenic region, other blood islands appear bilaterally, parallel and close to the midline of the embryonic shield. These islands form a pair of longitudinal vessels, the dorsal aortae.
Myoblasts and blood islands
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Cardiogenic field c
Cardiogenic field c
Figure 11.1 A. Dorsal view of a late presomite embryo (approximately 18 days) after removal of the amnion. Prospective myoblasts and hemangioblasts reside in the splanchnic mesoderm in front of the neural plate and on each side of the embryo. B. Transverse section through a similar-staged embryo to show the position of the blood islands in the splanchnic mesoderm layer. C. Cephalocaudal section through a similar-staged embryo showing the position of the pericardial cavity and cardiogenic field. D. Scanning electron micrograph of a mouse embryo equivalent to 19 days in the human, showing coalescence of the blood islands by vasculogenesis into a horseshoe-shaped heart tube (arrows) lying in the primitive pericardial cavity under the cranial neural folds (asterisks).
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