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#forelimb specification

3 public questions tagged with this topic.

Limb Development -l

Which transcription factor is responsible for forelimb specification?

Forelimb versus hindlimb identity is encoded by T-box transcription factors expressed in lateral plate mesoderm before any morphological bud. Tbx5 is restricted to presumptive forelimb field, where it upregulates Fgf10 and Wnt2b and launches forelimb-specific enhancer program leading to arm or wing formation. Tbx4 together with Pitx1 and Islet1 marks hindlimb field and specifies leg development. Genetic ablation of Tbx5 abolishes forelimb initiation entirely, while ectopic Tbx5 expression in flank can induce supernumerary wing-like buds, establishing its master selector status for forelimb.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 19: Limb identity – Tbx5 forelimb specification and Tbx4/Pitx1.

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Limb Development -ll

Which gene is critical for forelimb specification in limb development?

Forelimb identity is encoded by TBX5 restricted to rostral lateral plate mesoderm downstream of Hoxb4 Hoxb5 and retinoic acid signaling. TBX5 activates a forelimb-specific enhancer of FGF10 and Wnt2b, triggering epithelial-mesenchymal signaling cascade leading to bud outgrowth and AER induction via Wnt3a. Loss of Tbx5 abolishes forelimb formation entirely, while ectopic Tbx5 expression in hindlimb field can induce forelimb-like features including wing-type morphology. TBX4 performs analogous hindlimb function, Pitx1 reinforces hindlimb character, Islet1 contributes early to both fields. Thus TBX5 is master forelimb specifier.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 20: TBX5 as critical forelimb specification gene.

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Limb Development -lll

Which of the following factors is essential for forelimb specification?

Forelimb field is defined by T-box transcription factor TBX5 expressed exclusively in rostral forelimb-forming lateral plate mesoderm downstream of axial Hox4-5 and Wnt2b signals. TBX5 activates FGF10 and Wnt2b enhancers, initiating forelimb bud and AER induction via Wnt3a beta-catenin. TBX4 and Pitx1 perform analogous hindlimb functions caudally with Islet1. Tbx5 knockout mice lack forelimbs entirely while hindlimbs form normally; human TBX5 haploinsufficiency causes Holt-Oram syndrome with thumb anomalies. Lmx1b patterns dorsal compartment, Pitx1 reinforces hindlimb. Therefore forelimb specification depends essentially on TBX5 upstream cascade.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 20: TBX5 specifies forelimb identity.

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