The expression of <t>IL-6</t> in cardiac mesenchymal fibroblasts is affected by aging. A ) Expression of IL-6 in quiescent fibroblasts derived from young and old MSCs were analyzed by quantitative PCR and by ELISA, 24 h after the cell cycle was synchronized. B ) Double immunofluorescence labeling of IL-6 (red) and DDR2 (green) in 3- and 30-mo-old mouse hearts. Double positive cells are visualized in yellow/orange color. Middle panel shows magnified image of the section marked by white squares. Bottom panel shows morphometric analysis of IL-6 + DDR2 + cells in heart sections. Nuclei were stained with DAPI. Scale bar, 20 μm. P