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Limb regeneration was induced in 4-day chick embryos by implanting stage 15 neural tube into the amputated stump of the limb. In control animals, amputation alone or amputation with implant of either notochord or somites resulted in development of proximal wing segments only. In experimental animals, more than one-fourth of the animals containing viable neural tube implants developed proximal, middle, and distal limb segments. These regenerated limb segments contained muscle groups, cartilage models, and nervous structures that approximated the normal situations. Since some new parts regenerated in more than one-fourth of the cases, it may be concluded that augmentation of nerve supply is an effective method of inducing regeneration in limbs of chick embryos. Volume measurements of the neural tube implant indicate that a "critical mass" of nerve tissue may be necessary for regeneration to occur.

We have previously shown that implanted fetal nerve tissue stimulates the regeneration of amputated chick limbs. The purpose of this study was to determine whether a similar phenomenon would occur in amputated rat limbs and if addition of applied direct current (DC) would affect this process. Thus, fetal nerve tissue was implanted into amputated stumps of 3-week-old rats; variable tissue regeneration was induced that was dependent on the age of the donor implant and the presence of applied DC. Twelve or 14 day fetal neural implants induced new accessory bones containing epiphyseal plates and marrow cavities and occasionally formed joint-like structures with the host humerus. Addition of DC to 12 day neural implants increased the number of new bones formed. Eighteen day neural tissue with applied DC did not induce new bone formation but stimulated the maximal elongation of the host humerus and outgrowth of nerve fibers to the cut surface. Implantation of fetal heart tissue or implantation of fetal neural tissue into unamputated limbs failed to induce new bone formation. Although true limb regeneration was not achieved, formation of new skeletal elements did occur and this effect was enhanced by applied DC.