Nerve Pattern in Regenerated Urodele Limbs

Abstract

The factors determining the course, grouping, distribution, and size of peripheral nerves in development are still very much in the dark. One way of investigating these factors is direct experimentation on the embryo (Harrison, 1 Braus 2 ). Another way is to study the development of nerves in regenerating limbs of adult urodele Amphibians. Limb regeneration in these animals is known to be a sort of second development (P. Weiss), differing from the first development in the mass of tissue involved, the time relations, the conditions of muscle differentiation and of nerve supply. A comparison of nerve patterns established during regeneration with those formed during embryogenesis can be expected to yield some clue as to which factors may, and which may not be significant. If merely the nerves supplying a limb are severed, nerve regeneration into the limb takes place from the proximal stumps, the outgrowing fibers mainly following the old degenerated peripheral pathways which have persisted as such. If the whole limb is amputated, differentiation of a new limb occurs out of an undifferentiated blastema in which no old pathways are present to guide the outgrowing nerve fibers. The question arises as to what sort of pattern will be assumed by the nerves in the regenerating limb.

In metamorphosed individuals of the newt, Eurycea bislincata (Green), of varying age and size, the left fore limb and the right hind limb were amputated at their bases. The limb nerves were cut at about the level of the girdle plexus. Eight animals were kept for from 176 to 202 days, and were then preserved for dissection. Regeneration had taken place in all of the hind limbs and in 5 of the fore limbs. The regenerates varied from one-half to three-quarters the length of controls on the opposite side. They were slenderer in proportion but otherwise well formed. Some of the animals died before observations were made on the function of the limbs, but in all those observed there was normal coordination of movements. In one case the extension of the elbow was somewhat impaired.