Impaired intermediate formation in mouse embryos expressing reduced levels of Tbx6.

Intermediate mesoderm (IM) is the strip of tissue lying between the paraxial mesoderm (PAM) and the lateral plate mesoderm that gives rise to the kidneys and gonads. Chick fate mapping studies suggest that IM is specified shortly after cells leave the primitive streak and that these cells do not require external signals to express IM-specific genes. Surgical manipulations of the chick embryo, however, revealed that PAM-specific signals are required for IM differentiation into pronephros-the first kidney. Here, we use a genetic approach in mice to examine the dependency of IM on proper PAM formation. In Tbx6 null mutant embryos, which form 7-9 improperly patterned anterior somites, IM formation is severely compromised, while in Tbx6 hypomorphic embryos, where somites form but are improperly patterned along the axis, the impact to IM formation is lessened. These results suggest that IM and its derivatives, the kidneys and the gonads, are directly or indirectly dependent on proper PAM formation. This has implications for humans harboring Tbx6 mutations which are known to have somite-derived defects including congenital scoliosis.