Experimental HA in F8^em1−/− mice after a needle knee injury. (A) Protocol for HA. Intra-articular bleeding was induced in male F8^em1−/− mice and WT control mice via subpatella needle injury of the right knee joint (see the supplemental Methods for details). (B) Relative increase in right knee joint diameter (percentage) after needle injury in the F8^em1−/− compared with WT control mice (n = 9-14 mice per group). Red crosses indicate deaths of F8^em1−/− mice. Data are represented as mean with SEM from 3 separate experiments. (C) Representative images of bleeding in the knee joint of F8^em1−/− mouse before (day 0), and 2, 7, 14, and 28 days after needle injury. Histology of F8^em1−/− mouse knee joints and scoring on day 0 and 28 days after needle injury, and stained with Prussian blue for hemosiderin (arrow) and deposition scores (D); H&E for synovial hyperplasia and inflammation (indicated x----x) and synovitis scores (E); and safranin O for cartilage (red) and degradation scores (F). Data are presented as mean ± SEM from 4 to 9 mice per group. The scale bars represent 500 μm (Prussian blue and H&E) and 200 μm (safranin O). qPCR, quantitative polymerase chain reaction.