Increased angiogenesis and expression of vascular endothelial growth factor during scarless repair

Vascular endothelial growth factor (VEGF) is a dimeric heparin-binding glycoprotein that is a potent endothelial cell-specific mitogen with increased expression during adult cutaneous wound healing. VEGF activity is mediated by two receptors, VEGFR-1 (Flt-1) and VEGFR-2 (Flk-1/KDR), which are expressed primarily in vascular endothelial cells. Initiation of profibrotic cytokine expression likely coordinates the transition from scarless healing to scar formation in fetal wounds. Angiogenesis is an important component of the scarring repair process, but the function of VEGF and degree of angiogenesis during scarless repair has not been investigated. We hypothesize that VEGF and its receptors are differentially expressed in scarless compared with scarring fetal wounds because VEGF is implicated in angiogenesis during skin development and adult wound healing. Excisional wounds were created on fetal rats at gestational ages 16.5 days (E16) and 18.5 days (E18) (term = 21.5 days). Wounds were harvested at 24 and 72 hours (n = 12 wounds per time point). Nonwounded fetal skin (E17, E19, and E21) was used as control. Reduced-cycle, specific-primer, reverse-transcriptase polymerase chain reaction was performed to determine the expression of VEGF and its receptors, VEGFR-1 and VEGFR-2. Wounds at 72 hours and fetal skin controls were examined under high-power microscopy for blood vessel counts. Unpaired two-tailed t test was used (p < 0.05 was considered significant). VEGF expression increased 2.4-fold (p < 0.001) during normal skin development from E17 to E19. In scarless wounds (E16), VEGF expression increased 2.8-fold (p < 0.02) at 72 hours. No increased expression occurred in the scarring wounds (E18). VEGFR-1 and VEGFR-2 expression increased over 2-fold during normal skin development from E17 to E21. However, each was down-regulated 30 to 50 percent in scarless (E16) and scarring (E18) wounds. There is a 2-fold increase in mean vessel counts per high-power field in scarless (E16) wounds at 72 hours compared with age-matched control skin (p < 0.02) and a 1.7-fold increase in mean vessel count in scarring fetal wounds (E18) compared with age-matched control skin (p < 0.05). There is no difference in the total number of vessels found in scarless versus scarring wounds or between 19.5-day versus 21.5-day fetal skin. VEGF and its receptors, VEGFR-1 and VEGFR-2, increase expression during skin development and dermal differentiation. VEGF expression quickly elevates during scarless compared with scarring repair, which likely contributes to the more rapid scarless fetal repair rate. Similar numbers of new ves-sels are formed during scarless and scarring fetal repair.