Expression patterns of Tgfbeta1-3 associate with myocardialisation of the outflow tract and the development of the epicardium and the fibrous heart skeleton.

Transforming growth factor-beta (Tgfbeta) is essential for normal embryogenesis. The cardiac phenotypes obtained after knockout of each of the three mammalian isoforms suggest different roles during morphogenesis. We studied cardiovascular expression of Tgfbeta1-3 in parallel tissue sections of normal mouse embryos from 9.5 to 15.5 days post coitum (dpc) by using radioactive in situ hybridisation. The Tgfbeta isoforms are differentially expressed in unique and in overlapping patterns during cardiovascular development. In the vessels, Tgfbeta1 is found in the intima, whereas Tgfbeta2 and -beta3 are mainly present in the media and adventitia of the great arteries. Tgfbeta1 is present in the endocardium at all stages examined. The Tgfbeta2 signal in the endocardium of the atrioventricular canal and outflow tract (9.5 dpc) shifts during epithelial-mesenchymal transformation toward the mesenchymal cushions (10.5-11.5 dpc) after which it exhibits a marked spatiotemporal expression pattern as the cushion differentiation progresses (11.5-15.5 dpc). The myocardium underlying the endocardial cushions and the atrial muscular septum are intensely positive for Tgfbeta2 at early stages (9.5-11.5 dpc) and expression decreases at 12.5 days. In contrast to earlier reports, we find marked overlap of Tgfbeta2 and -beta3 expression. Tgfbeta3 expression shows a characteristic distribution in the mesenchymal cushions, suggesting a role in cushion differentiation, possibly additional to Tgfbeta2. From 14.5 dpc onward, a strong Tgfbeta3 signal is found in the fibrous septum primum of the atrium and in the fibrous skeleton of the heart. Special attention was paid to the proepicardial organ and its derivatives. The proepicardial organ strongly expresses Tgfbeta2 as early as 9.5 days, and all isoforms are present in the epicardium from 12.5 dpc onward. The spatiotemporal cardiovascular expression of Tgfbeta1-3 supports both specific and complementary functions during cardiovascular development that might explain functional redundancy between the Tgfbeta-isoforms. The information provided favors novel roles of Tgfbeta1-3 in epicardial development, of Tgfbeta2 in myocardialisation, and of Tgfbeta3 in differentiation of the fibrous structures of the heart.