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That’s because as people age, those joints lose the cartilage and viscous fluid, known as synovial fluid, that keeps them supple.

Located in the outer synovial lining, CD55+ cells secrete synovial fluid, allowing for frictionless joint movement.

For example, liquid biopsies of cerebrospinal fluid could be used to study or diagnose the brain, synovial fluid could be used to study joints, and urine could be used to study the kidneys.

The synovium is a membrane-like structure that lines the knee joint and helps to keep the joint happy and healthy, mainly by producing and maintaining synovial fluid.

Instead cartilage gets help from what experts call dynamic loading—putting stress or weight on the joint, which causes nutrient-carrying synovial fluid to flow in and out.