208 Fig. 1 Histochemical and bio- chemical analyses of the lung Wbrosis in aging HPS double mutant mice. Panels a–d are rep- resentative photomicrographs of Masson trichrome stained lung sections from a HPS at 12 months of age, b HPS at 17 months, c WT at 17 months, and d CHS mice at 17 months. Panel e shows the lung hydroxy- proline levels (§SD) in HPS, WT, and CHS mice with aging; panel f shows the mean linear intercepts (Lm, §SD) of the lungs from HPS, WT, and CHS with aging. Samples of 3–6 mice were used for each time category and strain. Mouse strains: HPS = HPS double mutant mouse, C57BL/6-Hps1ep- Ap3b1pe; CHS = CHS mouse, C57BL/6-Lystbg-J-J; WT = wild type mouse, C57BL/6J. SigniW- cant diVerence P < 0.05: *HPS versus WT; #HPS versus CHS. No signiWcant diVerence between CHS and WT was found at any age tested Histochem Cell Biol (2010) 134:205–213 Changes of distribution and morphology in AT II cells in HPS double mutant mice with aging From the lung sections stained for prosurfactant C, ATII cells exhibited a homogeneous distribution in HPS double mutant mice at 2 months of age (Fig. 2a), where 1–3 ATII cells were found per alveolus. A change in this distribution was observed in samples from 5 month old mutants, where these cells appeared sparsely scattered and no longer pres- ent among all alveoli (Fig. 2b). At 9 months of age, ATII cell-rich regions were observed, and with some devoid of this cell type (Fig. 2c). Compared with these observations, 123 ATII cells in WT and CHS samples at all ages tested were evenly distributed among all alveoli (Fig. 2d, e). The quan- tiWcation of ATII cells showed that there was a signiWcant cell number decrease at 5 months of age and a recovery of the cell number at the age of 9 months in the HPS strain (Fig. 2f). As the alveolar sizes were stable from 2 to 9 months of age in HPS (Fig. 1f), the change in ATII cell number represented an absolute reduction in the numbers of this cell type in 5 month old HPS lungs. Compared with HPS, ATII cell numbers in WT mice were constant at all ages tested. CHS ATII cells showed non-signiWcant but a trend of decreasing in number with aging (Fig. 2g).