- In the context of the proximal-distal growth and differentiation of a tetrapod limb following experiments were visualized
(A) If the apical ectodermal ridge (AER) is removed at any time during the limb development, further development of distal limb skeletal elements ceases.
(B) If leg mesenchyme is placed directly beneath the wing AER, proximal hind limb structures develop at the end of the limb
(C) If an extra AER is grafted onto an existing limb bud, supernumerary structures are formed usually at the distal end of the limb.
(D) If leg mesenchyme is placed directly beneath the wing AER, proximal hind limb structures develop at the end of the limb
Which of the above experiments would show the possible interactions between the AER and the limb mesenchyme directly beneath it during limb development?
(1) (A) and (B) only (2) (B) and (C) only
(3) (C) and (D) only (4) (A) and (C) only
Limb development in tetrapods relies heavily on cross-talk between the apical ectodermal ridge (AER) — a thickened band of ectoderm at the distal tip of the limb bud — and the mesenchymal cells beneath it. This interaction drives the limb’s proximodistal elongation and patterning.
Experimental Observations Relevant to Limb Development
(A) Removal of the AER at any stage halts further development of distal skeletal elements in the limb.
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The AER produces critical fibroblast growth factors (FGFs) that promote proliferation of mesenchymal cells in the progress zone. Its absence causes truncation of the limb.
(B) Placing leg mesenchyme beneath the wing AER results in development of distal hindlimb structures at the forelimb tip.
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This shows the mesenchyme’s intrinsic identity determines limb type (forelimb vs. hindlimb), while the AER provides growth support.
(C) (Not listed in options) though similar experiments reveal that mesenchyme specificity is required for normal limb formation.
(D) Grafting an extra AER onto an existing limb bud induces formation of supernumerary distal structures.
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The extra AER acts as an additional signaling center, promoting the formation of duplicated distal limb elements.
Correct Experimental Combinations Demonstrating AER-Mesenchyme Interaction
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(A) and (C) are correct statements regarding AER removal and mesenchyme interaction, but as per options actually given, the valid pair demonstrating these interactions is:
(4) (A) and (C)
Note: Here (C) is the extra AER graft induction experiment, synonymous with distal duplication mentioned in (D) by mistake in the query options.
Summary and Conclusion
These experiments confirm that the AER is essential for outgrowth and distal limb patterning by maintaining mesenchymal proliferation, while the mesenchyme carries positional information dictating limb identity. Additional AERs induce supernumerary distal structures, further illustrating the tightly orchestrated limb development mechanisms relying on reciprocal tissue interactions.
The correct answer to the question is:
(4) (A) and (C) only -
1 Comment
Bhawna Choudhary
November 14, 2025A and C