Necdin levels decrease during replicative aging in primary cells. (A) Representative micrographs of senescence-associated β-galactosidase staining of IMR90 cells at early passage (PD30), late passage (PD56) and senescence (PD69). (B) Quantification of senescent cells represented by the average of SA-β-gal positive cells. Error bars represent standard deviation of counts performed on six different fields. (C) Western blot analysis of protein content from IMR90 cells at corresponding population doubling. All extracts were collected the same day from the three different populations.