We agree with Dr Epstein that longevity refers to living cells. In the study we published, the DNA from a bilayered skin substitute could not be detected 2 months after the initial grafting. The study did not provide any clues as to how long viable cells survive, but we can certainly conclude that there are no viable cells remaining at 8 weeks when the DNA is no longer detectable.
If our results had shown the persistence of DNA, it would not have been possible to tell whether the DNA came from a living cell or retained cellular fragments. The length of time living cells survive on the wound is of interest but living cells persisting in the wound might be present in very small numbers, or not grow in culture, and would be difficult to detect. However, this is a moot point because the DNA could not be detected at 8 weeks.