The word "synovium" is related to the word "synovia" (synovial fluid), which is the clear, viscid, 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.
Synovium is very variable but often has two layers.
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 being squeezed dry when subject to impact, such as running).
- The fibroblasts manufacture a long chain sugar polymer called hyaluronan which makes the synovial fluid "ropy" like egg-white, together with a molecule called lubricin, 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.
The surface of synovium may be flat or may be covered with finger-like projections or villi, which probably help to allow the soft tissue to change shape as the joint surfaces move one on another.
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, although the film is very thin.
Although a biological joint can resemble a man-made joint in being a hinge or a ball and socket, the engineering problems that nature must solve are very different because the joint works within an almost completely solid structure, with no wheels or nuts and bolts.
In general the bearing surfaces of man made joints interlock, as in a hinge. This is rare for biological joints (although the badger's jaw interlocks).
More often the surfaces are held together by cord-like ligaments. Virtually all the space between muscles, ligaments, bones and cartilage is filled with pliable solid tissue. The fluid-filled gap is at most only a twentieth of a millimetre thick. This means that synovium has certain rather unexpected jobs to do. These may include:
- Providing a plane of separation, or disconnection, between solid tissues so that movement can occur with minimum bending of solid components. If this separation is lost, as in a 'frozen shoulder' the joint cannot move.
- Providing a packing that can change shape in whatever way is needed to allow the bearing surfaces to move on each other.
- Controlling the volume of fluid in the cavity so that it is just enough to allow the solid components to move over each other freely. This volume is normally so small that the joint is under slight suction.
Synovium can become irritated and thickened in conditions such as rheumatoid arthritis. When this happens, the synovium can become a danger to the bearing surface structure in a variety of ways. Excess synovial fluid weeping from inflamed synovium can provide a barrier to diffusion of nutrients to cartilage. The synovial cells may also use up nutrients so that the glucose level in the tissue is almost zero. These factors may lead to starvation and death of cartilage cells. Synovial cells may also produce enzymes which can digest the cartilage surface, although it is not clear that these will damage cartilage with healthy cells