Age-related changes in rat joint capsule.

[Purpose] We investigated morphological and histopathological changes in the joint capsule of rats with aging. [Materials and Methods] A total of 18 male Wistar rats were categorized into two groups: the control group (n=8), and the aged group (n=10). The aged group was reared until 75 weeks of age, while the control group was maintained until 11 weeks of age. At the end of the experiment period, the knee joints were sampled, joint capsules were subjected to histopathological analysis, and their thickness was measured. [Results] The joint capsule in the aging group exhibited significantly greater thickness compared to the control group. Histopathological examination revealed distinct differences between the two groups. The control group displayed gaps between the collagen fibers in the posterior joint capsule, along with loosely overlapping connective tissue and the presence of fat cells. Conversely, in the aged group's joint capsule, these gaps between the collagen fibers almost disappeared and fibers became densely packed and thickened. [Conclusion] These results were similar to our previous study in rats with immobilized hindlimb knee joints. Similar findings, including collagen fiber thickening, densification in the joint capsule, and reduced hindlimb knee joint range of motion, were consistent with those observed in the present investigation.