Scarless Fetal Wounds Are Associated with an Increased Matrix Metalloproteinase-to-tissue-derived Inhibitor of Metalloproteinase Ratio

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, stained with hematoxylin and eosin, and examined by light microscopy. RNA was isolated from the remaining wounds and skin, and a reduced-cycle, primer-specific, reverse-transcriptase polymerase chain reaction was performed to semiquantitatively determine relative gene expression of MMP-1, MMP-2, MMP-7, MMP-9, and MMP-14 and of TIMP-1, TIMP-2, and TIMP-3. Significance was determined by unpaired two-tailed t test (p < 0.05) and analysis of variance. In both E16 and E19 wounds, reepithelialization was complete by 72 hours. E16 wounds healed scarlessly, whereas E19 wounds healed with scar. During late gestation, skin expression of MMP-1 and MMP-14 (membrane type-1 MMP) doubled, whereas MMP-2 expression increased nearly 50-fold. Levels of MMP-7 and MMP-9 were unchanged in developing skin. As for the TIMPs, skin expression of TIMP-2 increased more than four-fold, whereas TIMP-1 and TIMP-3 expression was unchanged. In both scarless and scarring wounds, up-regulation of MMP-1 and MMP-9 occurred. However, the maximal increase in MMP-1 and MMP-9 expression occurred much more rapidly and was much greater in the scarless E16 wounds (28-fold versus 23-fold for MMP-1 and 18-fold versus nine-fold for MMP-9). Unchanged in scarless wounds, MMP-2 levels decreased more than three-fold in scarring wounds. MMP-14 (membrane type-1 MMP) expression increased three-fold in scarless wounds but was unchanged in scarring wounds. In contrast, TIMP-1 and TIMP-3 expression in E19 scarring wounds increased six-fold and four-fold, respectively. MMP-7 and TIMP-2 expression did not change in response to injury. E16 scarless wounds have greater MMP relative to TIMP expression than E19 scarring wounds. This favors extracellular matrix turnover, facilitates migration of fetal cells, and promotes scarless repair.