Synovial membrane (also known as synovium or stratum synoviale) is the soft tissue found between the articular capsule (joint capsule) and the joint cavity of synovial joints.

The word "synovium" is related to the word "synovia" (synovial fluid), which is the clear, viscous, lubricating fluid secreted by synovial membranes. The word "synvovia" or "sinovia" was coined by Paracelsus, and may have been derived from the Greek word "syn" ("with") and the Latin word "ovum" ("egg") because the synovial fluid in joints that have a cavity between the bearing surfaces is similar to egg white.

Structure

Synovium is very variable but often has two layers

The outer layer, or subintima, can be of almost any type: fibrous, fatty or loosely "areolar".

The inner layer, or intima, consists of a sheet of cells thinner than a piece of paper.

Where the underlying subintima is loose, the intima sits on a pliable membrane, giving rise to the term synovial membrane.

This membrane, together with the cells of the intima, provides something like an inner tube, sealing the synovial fluid from the surrounding tissue (effectively stopping the joints from being squeezed dry when subject to impact, such as running).

The surface of synovium may be flat or may be covered with finger-like projections or villi, which, it is presumed, help to allow the soft tissue to change shape as the joint surfaces move one on another.

Just beneath the intima, most synovium has a dense net of small blood vessels that provide nutrients not only for synovium but also for the avascular cartilage.

In any one position, much of the cartilage is close enough to get nutrition direct from synovium.

Some areas of cartilage have to obtain nutrients indirectly and may do so either from diffusion through cartilage or possibly by 'stirring' of synovial fluid.

The intimal cells are of two types, fibroblasts and macrophages, both of which are different in certain respects from similar cells in other tissues.

The fibroblasts manufacture a long-chain sugar polymer called hyaluronan, which makes the synovial fluid "ropy"-like egg-white, together with a molecule calledlubricin, which lubricates the joint surfaces. The water of synovial fluid is not secreted as such but is effectively trapped in the joint space by the hyaluronan.

The macrophages are responsible for the removal of undesirable substances from the synovial fluid.