Delayed Wound Healing in Aged Rats is Associated with Increased Collagen Gel Remodeling and Contraction by Skin Fibroblasts, Not with Differences in Apoptotic or Myofibroblast Cell Populations

Aging has been anecdotally reported to result in prolonged wound healing. Measurement of punch biopsy wound closure in young (4 month old) and old (36 month old) rats indicated there was a significant delay in wound closure by old rats during the early phase of repair, after which closure rates were equivalent. The delay in granulation tissue accumulation in older animals could involve premature programmed cell death (apoptosis); however, apoptotic fibroblasts in sponge granulation tissue and tissue culture were less abundant in samples from old rats relative to young rats. Myofibroblasts express alpha-smooth muscle actin, and they are believed to be important in wound contraction. There were no significant differences in overall abundance or distribution of alpha-smooth muscle actin containing myofibroblasts in granulation tissue and in cultured granulation tissue fibroblasts regardless of the age of the donor rat. The spatial distribution of myofibroblasts and apoptotic cells was distinct. Fibroblasts from granulation tissue and skin explants were placed in a collagen gel contraction assay prior to the 5th passage to determine their in vitro contractility. While granulation tissue fibroblasts from young and old rats showed similar collagen gel contractility, skin fibroblasts from old rats displayed greater collagen gel contractile behavior than young skin fibroblasts. Greater gel contractility of fibroblasts from old rats appeared to result, in large part, from the ability of those cells to cause generalized gel degradation. Gelatin zymography indicated a greater abundance of matrix metalloproteinase-2 in supernatants from gels containing skin fibroblasts from old rats. Taken together, these results suggest that the age-associated healing delay in the rat may not be related to the appearance or abundance of distinct myofibroblast or apoptotic cell populations. Proteolysis may have a significant role in delayed wound healing in aged animals.