Studies on the mechanism of implant‐induced supernumerary limb formation in urodeles. I. The histology of supernumerary limb formation in the adult newt, triturus viridescens

Supernumerary structures were formed in adult Triturus forelimbs following subcutaneous implantation of pieces of frog kidney. The gross and microscopic events in supernumerary limb formation were described in this paper. The first few days after implantation comprise a stage of wound healing and phagocytosis, the latter by both the epidermis and inflammatory cells of the host. Following this the tubular portions of the implant degenerate. During the height of this degeneration, the first dedifferentiated cells of the host are seen. These cells increase in number and gather around a central focus to form a blastema. From the blastema, first cartilage and then muscle differentiate, and elongation proceeds concomitantly with this differentiation. The final result of the kidney implant may be no growth at all, periosteal chondrification not preceded by a blastema or a true supernumerary structure ranging in complexity from a cartilaginous nodule to a fully developed supernumerary limb. The results of this experiment indicate that aside from the initial stimulus and the ultimate degree of morphogenetic complexity attained, the development of a supernumerary structure is very similar to the regeneration of an amputated limb.