What could be the reason for humans evolving with only two organs having a defence against scarring??

Question: What could be the reason for humans evolving with only two organs having a defence against scarring??
- Keywords:
  defence,
  evolution,
  evolve,
  evolving,
  human,
  organ,
  scarring
Asked by jammiedodger to John, Vicky on 23 Mar 2012.
- Vicky Young answered on 23 Mar 2012:
  
  Well all tissue has some defense… like your skin heals after you cut it! I think the ovary (and uterus) are better preserved though evolution because they are so important in reproducing that if they went wrong the human race could be wiped out. This is a system that needs to survive in evolutionary terms!

Question: What could be the reason for humans evolving with only two organs having a defence against scarring??