Biology of fetal repair: the presence of bacteria in fetal wounds induces an adult-like healing response.

The minimal acute inflammatory response to tissue injury is one of the most dramatic differences between fetal and adult wound healing. Considering the prominent role of inflammation in adult tissue repair, this study tested the hypothesis that the minimal fetal inflammatory response to tissue injury plays a central role in the "scarless" fetal repair process. Sponge implants were treated with lethally irradiated or live bacteria and placed subcutaneously in fetal rabbits to test the ability of the fetus to mount an acute inflammatory response to bacterial antigens present at the wound site and to analyze the effects of this inflammatory response on fetal fibroplasia and neovascularization. After harvest, these implants were examined histologically for inflammation, fibroblast infiltration, collagen deposition, and neovascularization, and collagen deposition was measured using hydroxyproline quantitation by high-performance liquid chromatography. Bacteria-treated implants showed dose-dependent acute inflammatory responses and significant increases in collagen deposition compared with control sponges. Implants containing live bacteria demonstrated maximal fibroplasia and neovascularization. These findings suggest that, despite neutropenia and immaturity of the fetal immune system, the fetus is capable of mounting an acute inflammatory response to avirulent bacteria present at the wound site. Fetal inflammatory cells which respond to this bacterial stimulus appear capable of initiating an adult-like healing response. Thus, by failing to provide a bacterial stimulus for leukocyte recruitment at the site of tissue injury, the sterile fetal environment appears to play a role in effecting "scarless" fetal wound healing.