Spatiotemporal Expression Patterns Of Transforming Growth Factor TGFβs Receptors TβRⅠ And TβRⅡ During The Development Of Embryonic Mouse Heart

Background:Transforming growth factor-βs and their receptors TβRⅠand TβRⅡare known to play crucial roles during development of embryonic heart.In embryos of TGFβs,TβRⅠ and TβRⅡgene knockout mice,different types of heart malformation were produced.Systematic investigation on expression patterns of TGFβs in normal mouse embryos had been carried out in many Labs,but expression patterns of their receptors TβRⅠand TβRⅡduring the normal development of embryonic mouse heart are still controversial.In this study, immunohistochemical method was used in order to inquire further into the spatiotemporal expression patterns of TβRⅠand TβRⅡand their roles during normal development of embryonic mouse heart.Methods:Serial sections of mouse embryos from embryonic day 9(ED9)to embryonic day 14(ED14)were stained with polyclonal antibodies against TGFβ1,TβRⅠand TβRⅡand monoclonal antibodies againstα-smooth muscle actin(α-SMA)andα-sarcomeric actin(α-SCA).Results:At ED9,primary heart tube consists of myocardium and endocardium with cardiac jelly in between,no mesenchymal cells can be detected in cardiac jelly.The outflow tract, ventricle and atrium of primary heart tube show strong expression ofα-SMA andα-SCA, indicating that different segments of primary heart tube are composed of myocardium. Expression of TGFβ1,TβRⅠand TβRⅡin myocardium of the outflow tract can not be detected, whereas in ventricle and inflow end,obvious TGFβ1,TβRⅠand TβRⅡgranules are easily found, with the extension of tubular heart towards the inflow end,the intensities of TGFβ1,TβRⅠand TβRⅡstaining are gradually decreased.With the development of the embryo into ED10,the increase in the length of the outflow tract is obvious,the distal end of the outflow tract myocardium is at the reflection with splanchnic epithelium on the dorsal wall of the pericardial cavity and shows strong expression ofα-SMA andα-SCA.Expressions of TGFβ1,TβRⅠand TβRⅡare found in the myocardial wall of the outflow tract,especially in the right wall,but as the myocardial wall extends towards the distal end of the outflow tract,expression intensities of TGFβ1,TβRⅠand TβRⅡare tapered off. Expressions of TGFβ1,TβRⅠand TβRⅡare higher in ventricle and atrium than that in outflow tract.Myocardial wall of fight and left horns of sinus venosus can be recognized because of strong expressions in both horns ofα-SMA andα-SCA that extend along the dorsal wall of the sinus venosus to the junction with the dorsal mesocardium,however,the expressions of TGFβ1, TβRⅠand TβRⅡare mainly restricted to the left horn,leaving the right horn and dorsal mesocardium TGFβ1,TβRⅠand TβRⅡnegative.Between ED11 and ED12,expressions of TGFβ1,TβRⅠ,TβRⅡand a-SMA in different parts of the embryonic heart reach their highest level and extend to the myocardium in the sinoatrial node except that,in the myocardium of the distal end of the outflow tract,intensity ofα-SCA staining becomes decreased.The expression patterns of TGFβ1,TβRⅠand TβRⅡin the myocardium of the outflow tract are the same as that ofα-SMA,α-SMA positive cells in the endocardial cushion of the outflow tract also show TβRⅡstaining.In the epithelium and cushion tissue of atrio-ventricular canal,positive expressions of TGFβ1,TβRⅠand TβRⅡare easily detected.From ED11 onwards,the walls of aortic sac,arch arteries and ascending aorta and pulmonary trunk show strong expression ofα-SMA,suggesting development of smooth muscle, but expressions of TGFβ1,TβRⅠand TβRⅡare always confined to the myocardium of the outflow tract.From the night of ED12 onwards,expression intensities of TGFβ1,TβRⅠ,TβRⅡbegin to decline,and at ED14,their expressions are found only in the part of compact and trabecular myocardium of the right ventricle.Conclusion:The results demonstrate that the expressions of TGFβ1,TβRⅠ,TβRⅡin the developing mouse embryonic heart are spatiotemporal specific,the expression patterns of TβRⅠand TβRⅡare coincident with those of TGFβs.The lack of TGFβ1,TβRⅠand TβRⅡexpression in the myocardium of outflow tract and right horn of sinus venosus between ED9 and ED10 indicates that the committed differentiation from mesenchyme into myocardium is not affected by the TGFβsignals.Strong expressions of TGFβ1,TβRⅠand TβRⅡbetween ED9-ED14 suggest that TGFβ1,TβRⅠ,TβRⅡmay not only regulate proliferation and further differentiation of embryonic myocardium,but influence the establishment of relationships of the future heart chambers as well by regulating processes of heart looping and internal subdivision and neural crest cell migration into outflow tract.Elucidation of normal expression patterns of TGFβ1,TβRⅠand TβRⅡin normal embryonic heart may provide evidences for explaining heart malformation in embryos after each of these genes being knocked out.