The main limiting hurdles of reprogramming are the timeframe and the efficiency. The percentage of positively reprogrammed fibroblasts is stated at about 0.01–0.5%, which is quite low. It also takes a longer time period until stem cell colonies appear after infection compared to other cell types [7, 17]. The whole reprogramming process needs approximately three to five weeks [7]. In contrast, keratinocyte derived iPSC colonies reach the appropriate size for passaging already after 2-3 weeks. There are several hypotheses for the low efficiency rate and the rather long time period for reprogramming fibroblasts.
