Early Intraembryonic Vasculogenesis
Presomite stage (1.5-mm embryo) at approximately 20 days
Although not the first organ system to make its appearance in the embryo, the cardiovascular system reaches a functional state long before the other systems, and doing so while still in a relatively primitive state of development. The vascular system grows from a simple, bilaterally symmetric plexus into an asymmetric, complex system of arteries, veins, and capillaries a necessarily dynamic process involving the formation of new vessels and temporary detours, rerouting of the bloodstream, and the disappearance of previously dominant channels or even of entire vascular subsystems. The vascular system needs to enlarge as the embryo grows, adapting to marked changes in embryonic shape and developmental changes in other organ systems. While hard at work, the heart also must grow and differentiate from a simple tube into a complex, four-chambered organ with sets of valves. Finally, because the very young embryo is tiny compared to the mass of extraembryonic (placental) tissue, which the young heart also supplies with blood, this heart is relatively enormous compared with its relative size in the adult. Describing the development of the cardiovascular system first requires review of the intraembryonic coelom (“body cavity”) formed by the confluence of small, initially isolated spaces that appear in the lateral mesoderm and cardiogenic mesoderm. The spaces fuse together and form the single, horseshoe-shaped intraembryonic coelom that extends the length of the embryo in the lateral mesoderm on each side, communicating across the midline cranially in the cardiogenic mesoderm. Later in development, a communication develops on each side between the caudal ends of the intraembryonic coelom and the extraembryonic coelom. The formation of the coelom separates the lateral mesoderm into two layers: the parietal layer in contact with the ectoderm and the visceral layer in contact with the endoderm. The ectoderm with its parietal layer of lateral plate mesoderm is called somatopleure; endoderm with its visceral mesodermal layer is called splanchnopleure.
In the late presomite embryo, scattered masses of angiogenic cells differentiate in the cardiogenic mesoderm and the splanchnopleure mesoderm ventral to the entire extent of the horseshoe coelom. These cells also appear earlier in the wall of the umbilical vesicle (yolk sac). The angiogenic cell clusters, called blood islands, rapidly increase in number and size, acquire a lumen surrounded by a simple squamous endothelium, and unite to form a plexus of vessels. The angiogenic clusters can also form angioblastic cords that canalize to form vessels. In the cardiogenic mesoderm, paired endothelial heart tubes develop from paired angioblastic cords. The tubes are ventral to the central part of the U-shaped coelom that will form the pericardial cavity. The paired heart tubes begin to beat by day 21 or 22 and fuse into a single heart tube a few days later. Another pair of angioblastic cords appear bilaterally, parallel and rather close to the midline of the embryo. These cords also acquire a lumen and form a pair of longitudinal vessels, the dorsal aortae (aortas). These vessels connect with the dorsocranial aspect of the endothelial (endocardial) heart tubes to establish the arterial pole of the developing heart. The caudal ends of the endothelial heart tubes make contact with vessels arising in the yolk sac mesoderm (vitelline veins) and later with the developing umbilical veins and common cardinal veins. Thus the venous pole of the heart, still paired, is determined.
