Synovial membrane histology

The synovial membrane, also called simply the synovium, is a thin layer that lines the inner surface of the joint capsule, the fibrous sac that wraps around a joint. It works like a case lining: it covers the wall from the inside and keeps its contents moist. Its cells make synovial fluid.

Two details explain how it works. First, the articular cartilage, the cartilage that covers the surface of the bones, is continuous with the membrane: they are not two separate pieces. Second, some blood vessels pass through the fibrous capsule and form a rich network deep within the membrane, a network with a name of its own, the circulus vasculosus.

The fluid the membrane produces does an essential job: it nourishes the articular cartilage, which is avascular hyaline cartilage, meaning it has no vessels of its own, and for that reason depends on the fluid for its nourishment. When the membrane becomes inflamed, that is called synovitis; and if synovitis persists in several joints at once, it may point to rheumatoid arthritis.