Effects on adult newt limb regeneration of partial and complete skin flaps over the amputation surface

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Abstract
The influence of the wound epithelium on the cellular events preceding blastema formation was examined by comparing dedifferentiation, DNA labeling indices, and mitotic indices of the distal mesodermal tissues in control regenerating newt forelimbs and in amputated forelimbs covered with a flap of full thickness skin. Three kinds of results were seen following the skin-flap graft operations. Epidermal migration across the amputation surface was completely inhibited in 22% (8) of the cases and these limbs repaired the amputation wound but did not form regeneration blastemas. In 11% (4) of the experimental limbs, essentially normal wound epithelia displaced the skin flaps and the limb stumps formed blastemas and regenerated. The majority of the skin grafts (67%) exhibited epidermal migration restricted to the free edges of the flaps. These limbs formed eccentric blastemas on the ventral side of the limb next to the dermis-free epidermis and regenerated laterally in that direction. Despite these differences caused by the skin-flap grafts, the experimental limbs all resembled control regenerating limbs one week after the operations in the dedifferentiated appearance of the distal mesodermal tissues and in the DNA labeling and mitotic indices of the mesodermal cells. While the labeling and mitotic indices increased by two weeks after the operations in limbs with wound epithelia, these parameters decreased in limbs with complete skin grafts which lacked dermis-free epidermis. When labeling and mitotic indices were determined two weeks after the operations in the limbs with partial skin flaps, these values had increased over those a week earlier in the areas next to the lateral wound epithelia, but decreased in the opposite areas contacting the complete skin of the graft. These results suggest that the epidermal influence in promoting blastema formation may serve to keep the dedifferentiating cells in the cell cycle and prevent their early redifferentiation.