Neural crest and separation of the embryonic heart.

The endocardial cushions of the outflow tract (OFT), which separate the aorta from the pulmonary trunk, are structurally similar to the atrio-ventricular (AV) cushions, which divide the atria and ventricles, and yet they have strikingly divergent developmental fates. To determine whether OFT and AV cushions differ in their pattern of gene activity, the expression of a panel of genes was studied during endocardial cushion development. This revealed that the transcription factors Msx-1 and Msx-2 are expressed mainly in the AV cushions, whereas the growth factors BMP-2 and BMP-4, and the extracellular matrix molecule tenascin-C, are mostly OFT restricted. The most prominent developmental difference between the AV and OFT cushions, is the population of the OFT by neural crest cells (NCCs), which may account for their differential gene expression. To investigate this hypothesis, the extent of NCC population of the OFT was determined in an animal model with a known NCC defect: the mutant mouse splotch2H (Sp2H). α-smooth muscle actin (α-SMA) was used as a NCC marker, revealing a reduced and abnormally distributed population of α-SMA-positive cells within the OFT of Sp2H/Sp2H embryos compared to their normal littermates. To test the hypothesis that NCCs regulate the differential gene expression detected between the AV and OFT cushions, spatio-temporal expression of the panel of genes was determined in Sp2H embryos. This study revealed no significant differences in gene expression in the endocardial cushions of Sp2H / Sp2H compared with their normal littermates, perhaps indicating that NCCs are not responsible for the differential gene expression observed between OFT and AV cushions. Septation of the atrium is achieved by the formation of the primary atrial septum. The spatial-temporal localisation of BMP-2 and BMP-4, described in this thesis, implicates these genes as candidates for induction of the primary atrial septum.