Broad mesodermal and endodermal deletion of Nodal at postgastrulation stages results solely in left/right axial defects.

Nodal signaling is a critical regulator of multiple aspects of early vertebrate development including asymmetry along the left/right (LR) axis. To study Nodal function occurring specifically in the postgastrulation embryo, we have used Cre/loxP based conditional mutagenesis. A floxed allele of Nodal was generated and shown to have wild-type function. This allele was then used in conjunction with the T-Cre line, which expresses Cre recombinase broadly in the mesodermal and definitive endodermal lineages posterior to the cranial region. T-Cre activity leads to complete deletion of Nodal before its normal transient expression in the early somite stage lateral plate mesoderm, thereby causing severe LR developmental defects. No other abnormalities were found, suggesting that Nodal signaling has no additional essential functions in developmental patterning within the extensive mesodermal and endodermal domains marked by T-Cre activity.