Rat embryo fibroblasts labeled with 14C-thymidine. Fibroblasts were cultured on silicon chips, deprived of serum for 24 h and pulsed with serum and 19 nmol 14C-thymidine/ml (1 mCi/ml). (a,b,c) Simultaneous quantitative mass images of a fibroblast at (a) 12C15N- (grayscale); (b) 14C- (pseudo-color); (c) overlay of the 14C- and 12C15N- images. Field: 50 × 26 μm; acquisition time 14 h. (d,e) Simultaneous quantitative mass images of a control rat embryo fibroblast at (a) 12C15N-; (b) 14C-. Field: 50 × 41 μm; acquisition time 2 h.