Synovial Membrane: Structure and Function

The synovial membrane is a tissue layer that forms a bordering line along the synovial joints. This membrane is made up of a soft tissue that lines the non-cartilaginous surfaces within joints, which have cavities (synovial joints).

The word 'synovium' comes from the Latin word that means 'with egg' because the synovial fluid that is present in the joints resembles an egg white. This is an important membrane that acts as a lubricating agent, thus aiding free movement of these joints.

Although the structure of the synovium can vary, it is generally made up of two layers. The outer layer is also known as subintima. It can be of almost any type, i.e., made up of fibrous, fatty, or loose areolar tissue. The inner layer is also known as intima. It consists of a sheet of cells, the thickness of which is lesser than that of a paper.

The subintima is loose and the intima sits on a pliable membrane. This membrane, together with the cells of the intima, acts as an inner tube, which seals off the synovial fluid from the surrounding tissue. This is a protective reflex, which helps to prevent the joint from getting squeezed when subject to an impact.

The intimal cells are classified into two types - fibroblasts and macrophages.

Fibroblasts are responsible for constructing long-chain sugar polymers known as hyaluronan. This gives the synovial fluid its ropy consistency. This, along with a molecule called lubricin helps to lubricate the joint surfaces.

The macrophages are responsible for ingesting foreign molecules that are harmful. The surface of the synovium may be flat or covered with finger-like projections known as villi. These protrusions help to allow the soft tissue to change shape as the joints move.

The blood supply is carried out by a dense net of blood vessels that are present just beneath the intima. These help to provide nutrients to the synovium and the avascular cartilage.

Contrary to common belief, the space in which the synovial fluid is lodged is not very large. Thus, the membrane has a variety of functions, the most important of which is to provide a plane of separation or disconnection between solid tissues so that the movement can occur smoothly without any friction.

Some of these disorders are: 

Rheumatoid arthritis : Excess of synovial cell proliferation, edema and basically synovial membrane inflammation

Scleroderma : Superficial fibrin deposits

Hemochromatosis : Excess deposition of iron in synovial lining cells

Tuberculous arthritis : Caseating granulomas along with giant cells

Alkaptonuria : Shards of pigmented cartilage

Pigmented villonodular synovitis : Hypertrophy of villi

Gout : Deposition of crystals made of monosodium urate

Neoplasms : Malignant cells in synovium

This structure plays a very important role in ensuring that there is a smooth functioning of all the synovial joints in the body.

Disclaimer: This is for informative purposes only, and should not be used as a replacement for expert medical advice.