Wound contraction and scar synthesis during development of the amphibian Rana catesbeiana

Certain characteristics of skin wound healing in the North American bullfrog, Rana catesbeiana, were studied grossly and histologically at various stages of development. The contraction kinetics of excisional skin wounds were monitored for 50 days in animals from four larval developmental stages (tadpoles) and from adults (frogs). The percentage of original wound area which remained after contraction had stopped in the four larval groups decreasing steadily from 59.2%+/- 6.8% for the least developed larvae to 9.9%+/- 2.3% for the most developed ones. Wounds in adults contracted at a much lower rate than for larvae and the percentage of original wound area remaining after 50 days was less than 10%. The morphologic features characterizing the intact dermis and epidermis outside the wound bed in the larvae were also observed in qualitatively similar detail inside the wound bed. In the adults, the subepidermal connective tissue inside the wound bed was distinctly different from the physiologic dermis outside and was classified as amphibian scar. We conclude that, during larval development, wound contraction increasingly displaced skin regeneration as a mechanism for wound closure. Adults closed skin wounds by a combination of contraction and synthesis of scar tissue. It is hypothesized that the mechanism for synthesis of scar in this amphibian is established during metamorphosis from larva to adult.