Vertebrates have characteristic and conserved left-right (L-R) visceral asymmetries, for example the left-sided heart. In humans, alterations of L-R development can have serious clinical implications, including cardiac defects. Although little is known about how the embryonic L-R axis is established, a recent study in the chick embryo revealed L-R asymmetric expression of several previously cloned genes, including Cnr-1 (for chicken nodal-related-1), and indicated how this L-R molecular asymmetry might be important for subsequent visceral morphogenesis. Here we show that nodal is asymmetrically expressed in mice at similar stages, as is Xnr-1 (for Xenopus nodal related-1) in frogs. We also examine nodal expression in two mouse mutations that perturb L-R development, namely situs inversus viscerum (iv), in which assignment of L-R asymmetry is apparently random and individuals develop either normally or are mirror-image-reversed (situs inversus), and inversion of embryonic turning (inv), in which all individuals develop with situs inversus. In both, nodal expression is strikingly affected, being reversed or converted to symmetry. These results further support a key role for nodal and nodal-related genes in interpreting and relaying L-R patterning information in vertebrates. To our knowledge, our results provide the first direct evidence that iv and inv normally function well before the appearance of morphological L-R asymmetry.

During gastrulation, the three germ layers of the embryo are formed and organized along the anterior-posterior body axis. In the mouse, gastrulation involves the delamination of ectodermal cells through the primitive streak and their differentiation into mesoderm. These processes do not occur in embryos homozygous for a retrovirally induced recessive prenatal lethal mutation, the strain 413-d insertional mutation. Instead of giving rise to mesoderm, embryonic ectoderm in 413-d mutants overproliferates and then rapidly degenerates, although extraembryonic lineages remain viable. Here we isolate a candidate for the mutated gene which encodes a new member of the transforming growth factor-beta (TGF-beta) superfamily. Expression is first detected in primitive streak-stage embryos at about the time of mesoderm formation. It then becomes highly localized in the node at the anterior of the primitive streak. This region is analogous to chick Hensen's node and Xenopus dorsal lip (Spemann's organizer), which can induce secondary body axes when grafted into host embryos (reviewed in refs 5 and 6). Our findings suggest that this gene, named nodal, encodes a signalling molecule essential for mesoderm formation and subsequent organization of axial structures in early mouse development.

The establishment of the main body axis and the determination of left-right asymmetry are fundamental aspects of vertebrate embryonic development. A link between these processes has been revealed by the frequent finding of midline defects in humans with left-right anomalies. This association is also seen in a number of mutations in mouse and zebrafish, and in experimentally manipulated Xenopus embryos. However, the severity of laterality defects accompanying abnormal midline development varies, and the molecular basis for this variation is unknown. Here we show that mouse embryos lacking the early-response gene SIL have axial midline defects, a block in midline Sonic hedgehog (Shh) signalling and randomized cardiac looping. Comparison with Shh mutant embryos, which have axial defects but normal cardiac looping, indicates that the consequences of abnormal midline development for left-right patterning depend on the time of onset, duration and severity of disruption of the normal asymmetric patterns of expression of nodal, lefty-2 and Pitx2.