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In contrast to adult cutaneous wounds, early fetal wounds heal scarlessly. Fetal rat skin transitions from scarless repair to healing, with scar formation between days 16.5 (E16) and 18.5 (E18) of gestation. Term gestation is 21.5 days. The composition of the extracellular matrix in fetal skin and wounds differs from that of the adult. Matrix metalloproteinases (MMPs) and their tissue-derived inhibitors (TIMPs) determine the architecture of the extracellular matrix. The authors hypothesized that differential expression of MMPs and TIMPs occurs during the ontogenetic transition to scar-forming repair in fetal skin and wounds. Full-thickness, excisional wounds (2 mm) were created on the dorsum of E16 (n= 42 fetuses) and E19 fetal rats (n= 42 fetuses). Wounds were harvested at 24, 48, and 72 hours. Nonwounded skin from littermates was also harvested as controls. Six E16 and E19 wounds were fixed 72 hours after injury …