- How will the regeneration of a newt limb be affected if the nerve supply is severed before
amputation?
(1) Partial regeneration will occur where only the proximal structures will be formed.
(2) Regeneration of most of tissues of the limb occurs normally but the regeneration of the nerve will fail.
(3) Regeneration will occur but the identity of the limb is lost and normal proximal-distal
patterning will not occur.
(4) blastema will form but not grow and regeneration will fail.
How Severing the Nerve Supply Affects Limb Regeneration
-
Nerves Provide Essential Growth Factors:
Nerves release trophic factors (such as neuregulin and fibroblast growth factors like FGF2 and FGF8) necessary for the proliferation and maintenance of blastema cells—the progenitors of the regrowing limb. -
Blastema Formation Occurs but Growth Fails:
Severing more than 90% of the nerve supply allows initial blastema formation, as wound healing and apical ectodermal cap (AEC) development still occur, but the blastema does not grow or proliferate sufficiently for regeneration to proceed. -
AEC Alone Cannot Sustain Regeneration:
Although the apical ectodermal cap secretes factors such as FGF8, promoting blastema growth, these alone are not enough without nerve-derived signals. The AEC can stimulate blastema initiation but cannot compensate for the absence of nerve trophic support required for continued outgrowth and patterning. -
Regeneration Failure:
The net effect is that complete limb regeneration fails, leading to truncated or absent limb structures beyond initial blastema formation.
Why Other Options Are Incorrect
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Outgrowth without nerves and loss of limb identity or patterning (Option 3) does not occur successfully; the regeneration halts at the blastema stage without substantial growth.
-
Limb regeneration proceeding normally without nerves (Option 2) contradicts well-established evidence that nerve presence is requisite.
-
Limb regeneration with normal nerve supply (Option 4) may happen, but the question specifies nerve severance before amputation, making this option irrelevant for the scenario.
Summary
Nerves are fundamental for successful newt limb regeneration. Severing most of the nerve supply before amputation results in formation but not growth of the blastema, rendering regeneration incomplete despite the presence of the apical ectodermal cap and its secreted factors. The nerve-borne mitogenic support is thus vital for sustained proliferation and morphogenesis during limb regeneration.
Final Answer:
(4) blastema will form but not grow and regeneration will fail. -
