Hypoxia-preconditioned mesenchymal stem cell transplantation reduces fibrotic histopathologic changes in the bleomycin-induced pulmonary fibrosis mouse model. Representative images at day 21 of (A-D) hematoxylin and eosin and (E-H) Masson’s trichrome-stained histological sections from each group. Immunohistochemistry staining was performed to observe the distribution of LacZ in lung tissues at (I-L) day 7 and (M-P) day 21 of mesenchymal stem cell (MSC)-treated mice. (A,E,I,M) Phosphate-buffered saline (PBS) control, (B,F,J,N) bleomycin (BLM) + PBS, (C,G,K,O) BLM + normoxia-preconditioned MSCs (NP-MSCs), (D,H,L,P) BLM + hypoxia-preconditioned MSCs (HP-MSCs). Scale bar = 100 μm. Solid arrows around the bronchi indicate LacZ-positive cells (brown spots). (Q) The fibrotic changes in the lung were quantified by using Ashcroft scores on day 21 after administration, ranging from 0 (normal lung) to 8 (complete fibrosis). Quantitative real-time RT-PCR was performed to detect the expression of LacZ in the lung tissues of mice that received NP-MSCs, HP-MSCs or PBS on (R) day 4 and (S) day 18 following BLM administration on day 3. Values are normalized to the GAPDH values and expressed relative to the PBS group. *P < 0.05, ***P < 0.001. Each dot represents an individual mouse with the mean shown for n ≥ 3 per group. Ctrl, control.