Lack of marked chromatin changes in senescent mouse fibroblasts. (a) SA β-gal staining in MEFs transduced with either control or H-RasG12V, drug selected and stained 7 days after drug selection. (b) DAPI staining of nuclei of cells shown in (a) and human IMR90. (c) MEF nucleus with exceptionally condensed heterochromatin. (d) Percent cells with condensed chromatin in senescent MEFs or IMR90 cells. In IMR90, cells with characteristic SAHF as in (b) were scored as positive. In MEFs, only cells with exceptional compaction as in (c) were scored as positive. Pericentromeric heterochromatin of normal MEFs as in (b) was not scored as positive. (e) DAPI staining of wild type mouse skin fibroblasts and generation 5 TERC-/-, WRN-/- skin fibroblasts.