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In the first part of our review of cardiac development,1 we explained the changes occurring during the transformation of the solitary primary heart tube into the primordiums of the definitive heart, describing how this involved the processes of looping, and subsequent formation from the primary tube of the components of the atriums and ventricles. In the second part of our review,2 we then accounted for the steps involved in separation of the atrial and ventricular chambers, emphasising that the processes were more complicated than the simple formation of partitions within the respective atrial and ventricular primordiums.
The subject of this, our third review, is the transformation of the initially solitary outflow portion of the heart tube into the intrapericardial parts of the aorta and the pulmonary trunk, their arterial valves and sinuses, and the subarterial ventricular outflow tracts. In our first review, we summarised some of the problems that continue to plague the understanding of the development of these outflow structures. Thus, initially the entirety of the primary heart tube contained within the confines of the pericardial cavity possesses a myocardial phenotype. Yet, in the definitive heart, the walls of the intrapericardial arterial trunks, along with the sinuses of the arterial valves, and small parts of the subarterial ventricular outlets, have an arterial or fibrous phenotype. The steps involved in the changes of the walls from the myocardial to the arterial and fibrous phenotypes have yet to be clarified. And then, cushions, or ridges, of endocardial tissue initially fuse to divide the entirety of the solitary outflow segment into the presumptive systemic and pulmonary outlets. With subsequent development, these cushions lose their septal function, as the arterial valves and trunks, along with the subpulmonary muscular infundibulum, develop as free-standing structures with their own discrete walls within the pericardial …