Practice question

Timing of limb identity determination was assessed by heterotopic transplantation swapping presumptive forelimb and hindlimb lateral plate mesoderm between cervical and lumbar axial levels. Donor mesoderm retained donor fate, forming wing structures in leg region and leg structures in wing region, indicating Tbx5 versus Pitx1-Tbx4 programming precedes FGF10-mediated outgrowth. Transplanting zone of polarizing activity alters digit pattern without switching limb type, Tbx5 knockout ablates forelimb, retinoic acid overexpression posteriorizes field, but identity swap experiment proves early intrinsic encoding of forelimb versus hindlimb selector program.