Overlapping and differential localization of Bmp-2, Bmp-4, Msx-2 and apoptosis in the endocardial cushion and adjacent tissues of the developing mouse heart

The bone morphogenetic proteins BMP-2 and BMP-4 and the homeobox gene MSX-2 are required for normal development of many embryonic tissues. To elucidat e their possible roles during the remodeling of the tubular heart into a fu lly septated four-chambered heart, we have localized the mRNA of Bmp-2, Bmp -4, Msx-2 and apoptotic cells in the developing mouse heart from embryonic day (E) 11 to E17. mRNA was localized by in situ hybridization, and apoptot ic cells by TUNEL (TDT-mediated dUTP-biotin nick end-labeling) as well as b y transmission electron microscopy. By analyzing adjacent serial sections, we demonstrated that the expression of Msx-2 and Bmp-2 strikingly overlappe d in the atrioventricular canal myocardium, in the atrioventricular junctio nal myocardium, and in the maturing myocardium. of the atrioventricular val ves. Bmp-4 was expressed in the outflow tract myocardium. and in the endoca rdial cushion of the outflow tract ridges from E12 to E14. Msx-2 appeared i n the mesenchyme of the atrioventricular endocardial cushion from Ell to E1 4, while Bmp-2 and Bmp-4 were detected between Ell and E14. Apoptotic cells were also detected in the mesenchyme of the endocardial cushion between E1 2 and E14. Our results suggest that BMP-2 and MSX-2 are tightly linked to t he formation of the atrioventricular junction and valves and that BMP-4 is involved in the development of the outflow tract myocardium and of the endo cardial cushion. In addition, BMP-2, BMP-4 and MSX-2 and apoptosis seem to be associated with differentiation of the endocardial cushion.