Fig 2: The calcified apoVs from PROCR+ fibroblast markedly aggregated and induced tendon calcification in the early stage of HO. (a) H&E staining of Achilles tendons of rats from the sham and HO groups after 1 and 3 weeks. Scale bar: 50 µm. High magnification of the black rectangle outline in the low magnification image showing apoptotic cells (arrows). Scale bar: 10 µm. (b) Representative images of TUNEL staining from the sham and HO groups after 1 and 3 weeks. Scale bar, 50 µm. (c) Representative immunofluorescence staining images of cleaved caspase‐3 from the sham and HO groups after 1 and 3 weeks. Scale bar, 50 µm. (d) Quantitative analysis of TUNEL+ cells in (b) (n = 3). (e) Quantitative analysis of the relative fluorescence intensity of cleaved caspase‐3 of Achilles tendons from sham and HO groups after 1 and 3 weeks (n = 3). RFI: relative fluorescence intensity. (f) Representative images of immunofluorescence staining of an Achilles tendon derived from the HO group after 1 week. PROCR, red; Cleaved caspase‐3, green; DAPI, blue. Scale bar, 10 µm. High magnification of the white rectangle outline in the low magnification image showing the apoptotic PROCR+ fibroblasts (arrows). Scale bar: 5 µm. (g) Representative TEM images of Achilles tendons of rats from the sham and HO groups after 1 and 3 weeks. Scale bar: 1 µm. High magnification of the red rectangle outline in the low magnification image showing the cells and calcified apoVs (arrows). Scale bar: 300 nm. (h) Representative SEM images of Achilles tendons of rats from sham and HO groups after 1 and 3 weeks. Scale bar: 1 µm. High magnification of the red rectangle outline in the low magnification image showing apoptotic cells (arrows). Scale bar: 500 nm. (i) SEM images and elemental mapping showing the aggregation of the calcified apoVs in the 1‐week HO group. Scale bar: 20 µm. Data represent the means ± standard deviations (n = 3). Statistical analyses were performed using one‐way ANOVA with a post‐hoc Tukey's test. **p < 0.01, ***p < 0.001.