Growing the coronary tree: the quail saga.

Coronary vessel development has become an active area of research thanks to the rediscovery of the precursors of the epicardium and coronary vessels and the deployment of new techniques. A long-time leader in the field, Dr. Robert Tomanek provides us with a paper in this issue that calls our attention to several important aspects of coronary vessel development in the quail, a valuable animal model for these investigations. His findings also bring up many ‘‘why’’ questions that underscore how much more work is required to understand fully how normal and abnormal coronary vessels develop and function or malfunction. Insights from these data would be relevant for therapy in adult diseases where new vessel growth is required, such as after heart attacks, or needs to be suppressed, as in cancer. Once the size and metabolic activity of an animal exceed the limits of diffusion, a circulatory system becomes a necessity. The same rule applies to individual organs, and the heart is an interesting example of an organ that solved this problem in unique ways, both phylogenetically and ontogenetically. The primitive pulsatile vessels or even sophisticated hearts of more complex invertebrates such as large marine crustaceans or mollusks are devoid of coronary vascularization. Instead, if an increase in myocardial mass is desired, the heart muscle is organized into a spongy meshwork of trabeculae. The same arrangement of myocardium can be found even in some lower vertebrates, typically cold-blooded, relatively sedentary animals with low blood pressure, notably some fish (Ostadal and Schiebler, 1971; Tota et al., 1983) and amphibian species such as Xenopus, that has a coronaryless ventricle, although with rudimentary coronary vessels supplying the atrioventricular canal and the arterial pole (Sedmera et al., 2003). Unlike most organs in the vertebrate body, the heart has to function continuously from very early stages of its development to support the embryo’s growth. Consequently, the coronaryless stages are recapitulated in the ontogenesis of the higher vertebrates, and the lacunar system of the trabeculated heart was recognized more than a century ago (Minot, 1901; Rychter and Ostadal, 1971) as a means to increase surface:volume ratio to facilitate exchange of nutrients and oxygen. The coronary blood supply becomes necessary at the point when increasing circulatory demands cannot be met by the trabeculated heart. Obviously, it would be too late to start recruiting parts and constructing coronary vessels when they are acutely needed; indeed, their development starts at the early trabeculated stages as soon as the heart becomes covered by the epicardium (reviewed in Tomanek, 2005). Interestingly, this is the time when hypoxia can be detected in the OFT myocardium by hypoxia indicator EF5 (Sugishita et al., 2004). The development of coronary arteries is typically divided into three steps. In the first step, delamination of migratory cells from the epicardial mesothelium occurs, and these multipotent precursors invade the heart. Proliferation and coalescence of these precursors produce the primitive vascular network (vasculogenesis), and some of these pluripotential cells give rise also to blood islands. This was demonstrated by Tomanek et al. (2006) using retrovirus lineage tracing that proved the blood cells and endothelial cells share common precursors. In the second step, further growth occurs by sprouting (angiogenesis) from this primitive network. The critical event culminating in this period is the connection of this network to the circulation, similar to plumbing systems, with the venous end connected first, then the arterial end later (Vrancken Peeters et al., 1997). The last step, involving differentiation and patterning of the arterial and venous parts of the coronary system, occurs after the connection to circulation and is probably in part controlled by the hemodynamic signals from the lumen. Differential hypoxia of the myocardium and subsequent activation of hypoxia-inducible factors such as HIF-1 may play a role in organizing the large vessels of the coronaries at the atrioventricular and interventricular sulci (Wikenheiser et al., 2006). Much of our knowledge about the coronary development comes from studies in the avian systems, in particular the Japanese quail (Coturnix coturnix japonica). Apart from the advantages inherent in in ovo systems (easy access for experimental manipulations, low cost), avian systems offer the opportunity of simple lineage tracking experiments using quail-chick chimera with detection of transplanted cells using quail-specific antibodies (QCPN, quail nuclear marker; QH1, quail endothelial and hemangioblast marker). Studies employing these techniques have shown that epicardial-derived cells give rise to all cell