Practice question

Hindlimb specification utilizes TBX4 T-box factor expressed in posterior lateral plate mesoderm under control of Pitx1 and Islet1/beta-catenin pathways. TBX4 activates Fgf10 in hindlimb mesenchyme analogous to TBX5 in forelimb, leading to Wnt3a-mediated AER induction and outgrowth. Conditional deletion of Tbx4 produces severely truncated or absent hindlimbs without affecting forelimbs. Unlike Pitx1, TBX4 misexpression alone does not fully convert forelimb to hindlimb morphology, indicating it executes outgrowth rather than definitive identity, yet remains essential downstream effector of hindlimb program initiating FGF loop.