Further analysis of Oct4 and Nanog indicates that although both are detected by immunohistochemistry in B-iPSC and F-iPSC (Fig. 1b), Oct4 mRNA is fully expressed from a demethylated promoter in both types of iPSC, whereas Nanog mRNA is sub-optimally expressed from a promoter that retains considerable methylation in F-iPSC (Supplementary Fig. 6). When assessed by blastocyst chimerism, B-iPSC contribute to all tissues, including the germ line, whereas F-iPSC contribute only poorly (Supplementary Fig. 7a), although they can be found in SSEA1+ germ cells of the gonadal ridge (Supplementary Fig. 7b). Thus, while both B-iPSC and F-iPSC generate robust multi-lineage teratomas, satisfying criteria for pluripotency typically applied to human cells3, broader functional assessments available in the mouse system confirm their differential degree of reprogramming. In this comparison of iPSC derived from accessible tissues of aged adult mice, bone marrow yields stem cells with superior features of pluripotency, but neither iPSC is equivalent to ntESC or fESC.
