Q.91 Which one of the following defects do you expect to see if you were able to specifically block
apoptosis in the developing limb bud of a frog embryo?
(A) The digits will remain connected through a web-like extension
(B) The bones will not form, and the limb would look like a paddle
(C) The limb would look normal but would be larger in size
(D) The anterior-posterior polarity of the limb will be lost
Apoptosis plays a critical role in sculpting the limb bud during frog embryo development by removing interdigital mesenchyme cells, enabling digit separation. Blocking it specifically would prevent this separation, leading to webbed digits.
Option Analysis
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(A) The digits will remain connected through a web-like extension: Correct. In vertebrates like frogs, programmed cell death (apoptosis) in interdigital zones separates digits; inhibiting it causes syndactyly (webbing), as seen in amphibian limb models.
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(B) The bones will not form, and the limb would look like a paddle: Incorrect. Bone formation depends on chondrogenesis and differentiation signaling (e.g., BMP, FGF), not apoptosis; paddle-like limbs occur in earlier patterning defects.
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(C) The limb would look normal but would be larger in size: Incorrect. While excess cells might enlarge the limb slightly, the primary visible defect is webbing from failed interdigital cell death, not overall normalcy.
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(D) The anterior-posterior polarity of the limb will be lost: Incorrect. Polarity is set by ZPA/Shh signaling gradients early in limb bud outgrowth, independent of later apoptosis.
Introduction to Limb Bud Development
In frog embryo development, the limb bud forms through outgrowth, patterning, and sculpting, where block apoptosis developing limb bud frog embryo processes are pivotal for digit separation. Apoptosis removes specific mesenchymal cells between forming digits, preventing webbing. This GATE Life Sciences question tests understanding of programmed cell death’s role in morphogenesis.
Role of Apoptosis in Digit Separation
Apoptosis, triggered by BMP signaling in interdigital zones, eliminates cells post-patterning to shape free digits. Studies in amphibians show FGF modulates this; blocking apoptosis sustains these cells, yielding webbed structures like in ducks but pathologically in frogs. Without it, AER signaling continues outgrowth, but no separation occurs.
Why Option A is Correct
Blocking apoptosis halts interdigital cell death, so digits remain connected through web-like extension. This mirrors vertebrate syndactyly mutants, confirmed in chick/frog models where caspase inhibitors cause persistent webs.
| Option | Predicted Defect | Reason Incorrect (Except A) |
|---|---|---|
| A | Webbed digits | Matches interdigital apoptosis block |
| B | Paddle limb, no bones | Bone formation via Sox9/runx2, not apoptosis |
| C | Larger normal limb | Excess tissue webs, doesn’t normalize |
| D | Lost A-P polarity | Shh/ZPA sets polarity pre-apoptosis |
Implications for Developmental Biology
This defect highlights apoptosis as a “sculpting” mechanism, distinct from proliferation or axial patterning. In GATE prep, recall: frog limbs develop similarly to chicks, with AER/FGF for proximo-distal, ZPA/Shh for A-P, and apoptosis for separation.
