Contributions Of BMP-2 To Cardiac Outflow Tract In The Development Of Mouse Embryo

Objective:Embryonic cardiac outflow tract is a tubular structure between arterial sac and right ventricle, that is formed by addition of cardiomyocytes from the second heart field to arterial pole of the primary heart tube, different cellular signals are involved in regulating the outflow tract development and reconstruction. Outflow tract endocardial cushion mesenchymal cells is mainly originated from endocardial Epithelial-Mesenchymal Transition and neural crest cells. Bone morphogenetic proteins-2 are key factors in the regulation of embryonic heart development, which is not only involved in cardiac differentiation and Epithelial-Mesenchymal Transition but also induced neural crest cell migration. Currently, research about it focuses on the atrioventricular canal. Cardiac precursor cell marker protein ISL-1 is considered to be involved in regulation of cardiac precursor cells developmental transcription factors, ISL-1knockout result in heart development to remain in the primitive heart tube stage. BMP-2/4 Knockout of SHF result in differentiation of abnormal cardiac outflow tract and upregulation of proliferation factor FGF10, TBX-1, that tips BMP-2plays an important role in the development of the outflow tract. The immunohistochemical method was used in this study to observe the temporal and spatial expression patterns of α-sarcomeric actin(α-SCA), islet-1(ISL-1),proliferating cell nuclear antigen(PCNA), bone morphogenetic protein-2(BMP-2) in embryonic hearts from embryonic day 9(E9) to E15. We explore BMP-2 possible role in the outflow tract development, which provide morphologic evidence for congenital outflow tract malformations.Methods:Serial paraffin sections of mouse embryonic hearts from embryonic day 9(E9) to E15 were stained with antibodies against α-sarcomeric actin(α-SCA), islet-1(ISL-1),proliferating cell nuclear antigen(PCNA) or BMP-2 by the immunohistochemical method. 3 to 7 mice were examined for each stage.Results:At E9, no cell was observed in the cardiac jelly of OFT. Scarce PCNA expressing cells were found in OFT myocardium. Weak BMP-2 expressed in the OFT myocardium, endocardium and dorsal pericardium. During E9-11, with the addition of cardiomyocytes derived from the second heart field to the arterial pole, the outflow tract lengthened. The proliferation rate of these newly added cells dropped gradually during recruitment into the outflow tract. During E10-11, mesenchymal cells gradually filled the outflow tract ridges, some of them expressing BMP-2,PCNA. BMP-2 expression in myocardium of OFT was up-regulated to the peak at the stages and tapered off toward the poles, reaching up to the reflection of pericardium at the arterial pole. At E12, the OFT got shortened and its BMP-2 expression level was down-regulated. During E13-15, the mesenchymal outlet septum were gradually myocardialized and weak BMP-2 expression was restricted to the OFT myocardium adjacent to the great vessels. During E10-13, myocardium of the distal OFT showed low proliferation activity while myocardium of the proximal OFT and right ventricle proliferated into trabeculae and resulted in the development of right ventricle.Conclusion:BMP-2 induced the precursors from the second heart field to differentiate into cardiomyocytes and add to the arterial pole of the heart. BMP-2 participated in the development of OFT ridges. BMP-2 inhibited cardiomyocytes of OFT proliferating and its down-regulation in the proximal portion reinitiated cardiomyocytes proliferating, which led to the development of right ventricle and shortening of the OFT. Low level of BMP-2 might induce mesenchymal cells in OFT septum to differentiate into cardiomyocytes.