#Question id: 7238

Following are the statements regarding to establishment of positive feedback loops for limb bud formation.

A. In the presumptive forelimb field, Tbx5 induces Wnt2b, which then upregulates Fgf10, which in turn positively feeds back to maintain activation of both Wnt2b and Tbx5.

B. Mesodermal Fgf10 tells the surface ectoderm to continue to make Fgf8, and the surface ectoderm continues to tell the underlying mesoderm to make Fgf10.

C. Fgf10 provides the signal to initiate and propagate the limb-forming interactions between ectoderm and mesoderm, and these signaling interactions directly promote limb bud formation and growth.

D. The Wnt protein acts through the canonical β-catenin pathway to induce Fgf8 expression in the ectoderm. This relay of signaling represents a foundational stage in the formation of limb buds, because once Fgf8 is made in the surface ectoderm, the surface ectoderm elongates to physically become the AER.

Which of the following statements correctly explained positive feedback loops during limb development?