CFA injection into the geriatric knee joint induces synovial inflammation, macrophage infiltration, and neovascularization. (A) Schematic of the frontal view of a cross-sectioned mouse knee joint, indicating the location of the major cellular and structural changes occurring in the arthritic knee joint. (B-G) Longitudinal cross sections (10 μm thick) of the knee joint stained with safranin-O/fast green display the histopathologic changes in the synovium and capsule 28 days after initial injection of complete Freund's adjuvant (CFA). In the vehicle-injected mice, the synovium is very thin and relatively intact (B), whereas in CFA-injected mice, an extensive inflammation in the synovial membrane and thickening of the joint capsule are found (C). The outlined boxes in B and C illustrate the region from which the subsequent confocal images were obtained. Representative confocal images of CD68+ macrophages (D, E, red/orange), DAPI-labeled nuclei (blue), and a vascular endothelial marker, PECAM+ (F, G, red) in vehicle-injected (D, F) and CFA-injected (E, G) mouse knee-joint sections (20 μm-thick). Injections of CFA induce a significant infiltration of CD68+ macrophages into the synovium (E), as compared with vehicle-treated mice (D). In vehicle-injected mice, a low-level vascularization by PECAM+ vessels is observed in the synovial space of the knee joint (F). In contrast, in CFA-treated knee joints, a significant number of PECAM+ vessels have developed and have an enlarged and disorganized morphology (G), as compared with vehicle-treated mice.