Effect of Severe Nerve Severance on Limb Regeneration

• Nerves Are Essential for Blastema Growth:
  Peripheral nerves secrete trophic factors that are necessary for the proliferation and maintenance of blastema cells, which are the progenitor cells responsible for regenerating limb tissues.

• Apical Ectodermal Cap (AEC) Alone Cannot Sustain Regeneration:
  The apical ectodermal cap (AEC), analogous to the apical ectodermal ridge (AER) in limb development, secretes essential factors such as FGF8 that support blastema growth. However, without adequate nerve-derived signals, the blastema cells cannot proliferate or maintain their regenerative potential.

• Denervation Leads to Regeneration Failure:
  When more than 90% of the nerve supply is cut, newts typically fail to regenerate the amputated limb despite the presence of the AEC and its secreted FGFs. This is because the nerve-dependent signals are indispensable for initiating and sustaining blastema growth beyond early stages.

• Evidence from Molecular Studies:
  Proteins such as nAG (newt Anterior Gradient protein), secreted in part by nerves, directly stimulate blastemal cell proliferation. Denervation sharply reduces nAG expression and thereby regeneration.

Why Other Options Are Incorrect

• Limb regeneration does not proceed without nerve supply (ruling out option 2).

• Outgrowth without nerves does not occur effectively, nor is the patterning maintained in denervated limbs (ruling out option 3).

• Limb regeneration with intact nerve supply is expected only if nerves are not severed (ruling out option 4).

Summary

Despite the AEC’s secretion of FGF8, more than 90% nerve severance before amputation disables limb regeneration in newts because nerve-derived factors are critical mitogens required for blastema cell proliferation and regenerative progression. The AEC supports regeneration, but is insufficient in the absence of nerve input.

The correct answer is:
(1) The apical ectodermal cap stimulates growth of the blastema by secreting FGF8 but regeneration does not take place.