Serum factors and gene expression in heterochronic parabiosis. A: Fasting insulin was analyzed in young, old, old isochronic, and old heterochronic mice. There was no significant difference
between the groups (using both t test and ANOVA). Although IGF-1 levels were significantly higher in old mice (*P < 0.05; t test), no difference was detected between old isochronic mice and old heterochronic mice (no significant difference was found
in ANOVA; N = 4). B: Expression of selected cell-cycle genes in islets of young, old, and parabiosed mice. All genes were normalized to the young
mouse group except p21 and p16, which were normalized to the old mouse group. Parabiosis to a young partner did not change
the expression of any gene in old mice (N = 3). Het, heterochronic; Iso, isochronic; NS, not significant.