RNA and protein localisations of TGFβ2 in the early mouse embryo suggest an involvement in cardiac development

Abstract

We have performed a detailed analysis of the localisations of RNAs for TGFβ2 and β3, and of TGFβ2 protein in mouse embryos from 6.5 to 9.5 days post coitum, using in situ hybridisation and immunohistochemistry on serial sections, and whole-mount in situ hybridisation to complete embryos. TGFβ3 RNA was not seen in any of the tissue sections, but very low levels of the RNA were seen by whole-mount in situ hybridisation around the outflow tract of the heart at 8.5 days post coitum. TGFβ2 RNA is expressed at high levels in all cells with the potential to differentiate into cardiomyocytes. Additionally, the foregut endoderm, juxtaposed to the heart, and the neuroepithelium at the rostral extremity of the foregut, express very high levels of TGFβ2 RNA, between 8.5 and 9.5 days post coitum. As cardiomyogenesis proceeds, TGFβ2 RNA levels diminishes within the myocytes, with a concomitant increase in staining for TGFβ2 protein. TGFβ2 protein staining of cardiomyocytes persists throughout development and in the adult, in the absence of detectable levels of the corresponding RNA. Superimposed upon this myocardial pattern of expression, there is an upregulation of TGFβ2 RNA in the myocardium of the outflow tract and atrioventricular canal between 8.5 and 9.5 days post coitum, which returns to low levels by 11.5 days post coitum. The results are discussed in terms of a potential role of TGFβ2 in controlling cardiomyogenesis and in induc-tive interactions leading to cardiac cushion tissue formation.