The number of nerve fibers in the limb of the mouse and its relation to regenerative capacity

In regeneration of the salamander limb the nerve must satisfy a quantitative threshold below which regeneration does not occur. The present work explores the reason for the inability of the mammalian limb to regenerate by analyzing the number of nerve fibers available in the wound of the amputated limb. The number expressed per unit area of amputation wound is only 16% of that of the newt, Triturus, a regenerating form. It is known that when the number of fibers in the newt's limb is reduced to about a third or less of the normal number, regrowth does not occur. The results are discussed against the background of published unsuccessful attempts to induce regrowth in the mammal. Finally, the results affirm findings published previously of an evolutionary trend in the vertebrate series of decline in quantity of peripheral innervation which attended elaboration of central nervous mechanisms.