| Background: Primary myocardium, aortico-pulmonary septum (AP-septum) and a -SMA positive cells in the endocardial ridges are involved in the septation of the outflow tract, considerable controversy continues to exist on the cellular mechanism by which the outflow tract is septated. We investigated the differentiation characteristics of the outflow tract primary myocardium and the spatio-temporal distribution pattern of the a -SMA positive cells during the septation of the outflow tract of the embryonic mouse heart to elucidate the relationship of outflow tract shortening and septation with the differentiation of the outflow tract primary myocardium and the spatio-temporal distribution pattern of a -SMA positive cells.Methods: Serial sections of mouse embryos from embryonic day 9 (ED9) to embryonic day 16 (ED 16) were stained with monoclonal antibodies against a -SCA, a -SMA and Desmin, while apoptosis was determined by the TUNEL assays.Results: In ED 9 embryos, distal boundary of the outflow tractmyocardium has reached to the bifurcation of the branchial arteries. At ED 10, the length of the outflow tract is greatly increased because of continuous differentiation of mesenchymal cells into the cardiomyocytes. During period of ED 11 to ED 12, the cardiomyocytes in the distal part of the outflow tract shed their a -SCA expression, especially on the right side, without going onto apoptosis, indicating the transdifferentiation of the myocytes into the mesenchymal cells. However, the transdifferentiated myocytes still show strong a -SMA expression. The disappearance of the myocardial phenotype in the distal part of the outflow tract is accompanied by the increase in the cell density of the endocardium that shows increased a -SMA expression when compared with the endocardium surrounded by the myocardial cuff, suggesting that the myocytes and the endocardium in the distal part of the outflow tract transdifferentiate into the free wall of the ascending aorta and pulmonary trunk near the semilunar valves. As a result of the transdifferentiation of the myocytes in the distal part of the outflow tract, the distal boundary of the outflow tract myocytes regresses proximally towards the ventricle and the outflow tract becomes shortened.The septation of the outflow tract starts at ED 10 with the formation of the a -SMA positive AP septum on the dorsal wall of the aortic sac and the migration of the a -SMA positive cells into the cardiac jelly of the outflow tract along the branchial arteries. With the development, the number of a -SMA positive cells is increased. At ED 11, in the proximal part of the outflow tract, endocardium lining the inside of the myocardial tube begin to accumulate locally to form two endocardial ridges being composed of a -SMA positive and a -SMA negative cells. From ED 12 to ED 13, theoutflow tract distal to the semilunar valves is septated into ascending aorta and pulmonary trunk by the AP septum. The AP septum is developed into the facing walls of both arteries. Few apoptotic cells are discovered in the AP septum. Proximal to the semilunar valves, after ED 13, the two endocardial ridges fuse to form the outlet septum between the subpulmonary and subaortic outlet of the ventricles. Fusion of the ridges is followed by accumulation of the a -SMA positive cells into a highly characteristic central whorl. From ED 13 onwards, the outlet septum is gradually myocardialized. The myocardialization of the outlet septum is accompanied by some of the mesenchymal cell apoptosis.Conclusion: The disappearance of the a -SCA expression in the distal part of the outflow tract is due to the transdifferentiation of the primary myocytes into the smooth muscle or fibroblast in the free walls of the aorta and pulmonary trunk close to the semilunar valves rather than apoptosis of myocytes. The transdifferentiation of the myocytes may be an important cellular mechanism for outflow tract shortening. During the myocardialization of the outlet septum, apoptotic mesenchymal cells are observed, a -SMA positiv... |
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