Comparison of loss and recovery of club cells, AT1s and AT2s between aged and young mice following influenza pneumonia. A-C. Club cell coverage index (A), AT1 coverage index (B), and AT2 density (number per mm2 tissue area) (C) in aged (square) and young (triangular) mice following influenza infection. All error bars represent standard errors. *p < 0.05, **p < 0.01 by Student t-test. D-W. Representative immunofluorescence staining of lung sections for SCGB1A1 (green) and pro-SPC (red) in D-O (DAPI stain is shown in blue in E and H), and PDPN (red) and DAPI (blue) in P-W at the indicated time-points following influenza infection. The boxed areas in D, G, P and T are shown in higher magnification in E, H, Q and U, respectively. The arrows point to loss of SCGB1A1-expressing club cells; in D-O the dotted lines circle the regions with loss of pro-SPC-expressing AT2s; in P-W the dotted lines circle the regions with loss of PDPN-expressing AT1s . Scale bars are 500 μm for the low magnification images, and 50 μm for the high magnification images. Ten aged and 10 young mice were used for each time-point.