The threshold quantity of nerve required to induce limb regeneration in the chick embryo.

Wingbuds of 4-day chick embryos were amputated at the future elbow joint and a segment of embryonic neural tube was implanted longitudinally in the stump. The cross-sectional area and number of nerve bundles were determined in limbs in which limb regeneration occurred and compared with similar measurements in control limbs without neural tube implant. The number of nerve bundles in regenerated limbs was not significantly greater than in control limbs which did not regenerate. However, the cross-sectional area of nerve bundles was significantly greater in the limbs that had undergone regeneration. The data support the view that the amount of axoplasm available at the amputation (surface) site is the essential factor in determining the success or failure of regeneration.