The left–right (LR) axis in vertebrate embryos is specified by a hierarchy of gene products that interact in complex biochemical pathways. Many of these genes exhibit LR asymmetric expression patterns in the embryo before organ formation and are believed to specify morphogenetic differences between the left and right lateral plate mesoderm (LPM) that are ultimately manifested in the LR asymmetries of developing organs (reviewed in Capdevila et al., 2000; Schneider and Brueckner, 2000). The distal-most component of the LR pathway, the transcription factor Pitx2 (specifically the Pitx2a and Pitx2c isoforms; Schweickert et al., 2000; Yu et al., 2001), is thought to directly impart LR information during organogenesis. Similar to earlier genes in the LR specification pathway, the first asymmetric expression of Pitx2 is detected in the left but not right LPM. However, unlike the other LR genes, Pitx2 is also found in asymmetric patterns in the actual organs that become lateralized (Logan et al., 1998; Meno et al., 1998; Piedra et al., 1998; Ryan et al., 1998; St. Amand et al., 1998; Yoshioka et al., 1998; Campione et al., 1999; Essner et al., 2000; Schweickert et al., 2000). Unfortunately, it has been difficult to infer the normal morphogenetic role of Pitx2 from the complex phenotypes found in Pitx2-misexpressing or Pitx2-deficient animals, because the organ anomalies are not always immediately obvious as LR defects and seem to vary depending on the isoform and residual expression level of the Pitx2 allele involved (Gage et al., 1999; Kitamura et al., 1999; Lin et al., 1999; Rankin et al., 2000; Liu et al., 2001; Lowe et al., 2001). Thus, although LR gene expression is crucial for the normal development of asymmetric organs in vertebrates, its exact role in shaping or positioning individual organs is still unclear.
This ambiguity might be at least partially attributed to the fact that the underlying mechanisms of asymmetric organ morphogenesis have not been identified. There is some speculation that differential cell fate specification between the left and right sides of the embryo may be a mechanism of generating morphologic asymmetries (Capdevila et al., 2000; Patterson et al., 2000). For example, the anatomically left-sided spleen is reportedly derived exclusively from cells located on the left side in Xenopus (Patterson et al., 2000). In addition, differential cell proliferation in the left and right sides of individual organs is also a commonly invoked mechanism for creating LR asymmetric morphology (Kitamura et al., 1999; Capdevila et al., 2000). However, the slight differences in proliferation reported for the left and right sides of the heart tube are not likely to drive the substantial morphogenetic dynamics of cardiac looping, suggesting that such a mechanism may not play a primary role in asymmetric morphogenesis (Sissman, 1966; Manasek, 1981). Furthermore, unlike the case of the heart, in most developing organs, the locations of cells derived from the original left or right sides of the embryo have not been defined; thus, it has not been possible to assign differential proliferation rates or other morphogenetic roles to contralateral tissues within particular organs. Thus, a direct link between specific LR asymmetric gene expression patterns and specific morphogenetic events in developing structures has yet to be demonstrated.
Knowledge of the mechanisms that shape the LR asymmetry of individual organs is vital for a complete understanding of LR patterning and the etiology of situs deformities. Here, we investigate the development of LR asymmetry in the digestive system of the Xenopus laevis embryo. We have defined the organ-specific contributions and topologic distribution of left and right LPM tissues within the primitive gut tube during the complex looping process that positions the tadpole's digestive organs. Our analysis shows that contralateral tissues do not contribute to different digestive organs but do form opposing concave and convex topologies during gut looping. Interestingly, we find that key concave surfaces correlate with distinct early and late phase Pitx2 expression patterns, and we show that misexpression of Pitx2 mRNA induces ectopic concavities. Finally, we demonstrate that the left concave and right convex surfaces of the gut tube undergo differential elongation during looping, with no detectable difference in cell proliferation. Taken together, our results suggest that the LR signaling pathway may influence the asymmetric morphogenesis of the Xenopus digestive system by differentially regulating the rate of elongation of the left and right sides of the primitive gut tube.