Cardiovascular Development and Angiogenesis in the Early Vertebrate Embryo

Embryonic cardiovascular physiology (as opposed to that of the more developed fetus) is being more closely examined by developmental physiologists to explore the onset of cardiovascular function and its regulation, as opposed to the later maturation of these processes as is typically examined in fetal mammal models. As our understanding of embryonic physiology grows, the dogma that the early embryonic heart serves the same convective bulk transport role that it does in the fetal and adult heart is being carefully evaluated. Experimental approaches have involved genetic, surgical and environmental manipulation, and have revealed that blood flow generated by the early embryonic heart is not required for bulk transport of respiratory gases, nutrients, and wastes. Rather, the very small size of the typical vertebrate embryo enables this critical transport function to be achieved by simple diffusion alone. Surprisingly, however, the heart begins to beat (and so expend valuable energy) well before convective blood circulation is actually required. This review postulates that angiogenesis may be a driving factor for the "early" beat of the heart. Recent experiments examining the effect of increased blood pressure and flow pulsatility on proximal blood vessel development offer initial support for the "synangiotropy" hypothesis, namely that the onset of heart beat occurs synchronously with the need for peripheral angiogenesis. Yet, the complexity of the patterns of angiogenesis (regional variations of opposite sign) suggests that we have much more to be learned about the relationship between angiogenesis and the circulation in vertebrate embryos.