#limb bud

Limb Development -l

Which experiment demonstrated that FGF10 is required for limb initiation?

Essential requirement for FGF10 in limb initiation was proven by generating FGF10-null mice which exhibit complete agenesis of forelimb and hindlimb buds, while flank lateral plate mesoderm remains present and expresses Tbx markers initially. Implantation of FGF10-soaked heparin beads into mutant flank rescues apical ridge induction evidenced by restored Wnt3a and FGF8 expression in overlying ectoderm and partial bud outgrowth. Overexpression of Wnt7a influences dorsoventral patterning, ridge removal tests maintenance, blocking RA synthesis expands fields, but only FGF10 ablation prevents initiation throughout development.

Ref: Nature Reviews Genetics: FGF10 knockout mice lacking limb buds and bead rescue experiments.

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Limb Development -l

Which experiment demonstrated that mesodermal signals induce limb bud formation?

Instructive capacity for limb formation resides in lateral plate mesoderm, demonstrated by classical transplantation in chick embryos. Excision of forelimb-destined lateral plate mesoderm and grafting to ectopic flank induced complete supernumerary limb, including an apical ridge newly induced from host flank ectoderm. Ectoderm grafts alone were non-inductive, Hox gene knockouts altered axial position without testing induction, and ridge removal truncates rather than initiates. Molecular mechanism centers on FGF10 secreted by lateral plate mesoderm activating Wnt3a-FGF8 signaling cascade in overlying ectoderm, initiating budding and pattern.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 19: Classical LPM transplantation inducing ectopic limb buds.

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Limb Development -lll

What happens when an extra AER is grafted onto a developing limb bud?

AER serves as distal outgrowth engine generating proximal-distal extension. Its FGF signals maintain progress zone proliferation, keep Meis genes repressed distally, and progressively specify more distal fates through Hox temporal activation and RA antagonism. Transplanting an additional AER onto dorsal or ventral surface of limb bud creates a second active signaling center, bifurcating underlying mesenchyme into an ectopic outgrowth with its own progress zone and autopod. Result is limb bifurcation and supernumerary skeletal elements branching from graft site. Removal instead causes truncation, early removal yields only humerus formation.

Ref: Saunders 1948, Gilbert Chapter 20: Extra AER graft induces supernumerary limb structures.

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Tetrapod

What happens when an extra AER is grafted onto a developing limb bud?

Extra AER grafted onto dorsal or ventral surface of limb bud creates ectopic signaling center secreting FGF8, FGF4 independent of endogenous ridge. Local subjacent mesenchyme remains proliferative and initiates secondary outgrowth axis with its own proximal-distal polarity. Depending on graft position, supernumerary limb elements including duplicated stylopod, zeugopod or autopod form branching from primary limb. This experiment demonstrated AER sufficiency for outgrowth induction and reprogramming of surrounding mesenchyme. Niswander and colleagues confirmed FGF bead implantation similarly induces duplicated structures mimicking AER transplantation.

Ref: Gilbert, Developmental Biology, 12th ed., Chapter 19: Extra AER graft and supernumerary limb induction.

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