Fibroblasts start their outgrowth from the tissue piece within one to two weeks and are very easy to cultivate with a minimum of serum and medium [51]. The low methylation status of the promoter regions of OCT4 and NANOG in fibroblasts as well as the resulting low endogenous expression of these factors may be involved in the good reprogramming ability associated with transcriptional and epigenetic states favorable to reprogramming [51]. Depending on the fibroblast subtype, they exhibit a very high proliferation rate compared to other cell types. This looks like an advantage at first but also has disadvantages. Not reprogrammed fibroblasts bear the risk to overgrow the newly reprogrammed cells, hinder their proliferation, and deplete growth factors in the medium. However, for an efficient reprogramming process, skin fibroblasts should only be used at a very low passage, not higher than passage five [51]. With higher passages, the reprogramming efficiency is reduced and the risk of accumulated genomic alterations is increased.
