Resistance to reprogramming is also very evident when donor nuclei from different cell types are compared. In nuclear transfer to Xenopus oocyte experiments, ten times more transcripts of Sox2 are made by transplanted nuclei of mouse embryonic fibroblasts (MEFs) than by those of the more differentiated C2C12 cells [5]. Conversely, the transcripts of Oct4 and Nanog are five to eight times more abundant in transplanted C2C12 nuclei compared to nuclei of mouse embryo fibroblasts [5]. The difference between these two cell types in resistance is therefore at least 50-fold in respect of these genes. Because the reprogramming factors of an oocyte are the same for both kinds of nucleus, the 50-fold difference in responsiveness reflects the differential resistance of these genes in the two donor cell types.
