Denervation effects on DNA replication and mitosis during the initiation of limb regeneration in adult newts

Abstract

Histological and cellular events during the first 2 weeks after amputation of adult newt forelimbs were compared under conditions of innervation and denervation in order to ascertain the first nerve-dependent step in the regeneration process. Formation of a wound epithelium, histolysis, and dedifferentiation occurred, despite the absence of nerves, in a manner similar to that of the regenerating limbs. Autoradiography showed that cells in the denervated limb-stumps incorporate [ 3H]thymidine into DNA beginning on day 4 and that during the first week after amputation these limb stumps resembled the contralateral controls in overall labeling indices, in the distribution of the labeled nuclei, and intensity of labeling. Microspectrophotometric determinations of nuclear DNA contents in the dedifferentiated cells of denervated and innervated limbs revealed that in both cases replication of the entire chromosomal complement occurred. Cell division, which first began 6 days after amputation, and early blastema formation were observed only in the innervated limb stumps. A likely explanation of these results, consistent with the preponderance of the literature on regeneration, is that limb stumps can initiate the regeneration process in the absence of nerves but that the cells are blocked in the G 2 phase of the first cell cycle and consequently do not proliferate to form a blastema.