What is the Synovial Membrane: Functions, Synovial Fluid and Problems

Definition: The synovial membrane (MS) covers the internal surface of the fibrous joint capsule and only four deep cells.

It has a discontinuous surface and is not connected to a basement membrane; therefore, the synovial membrane is not an epithelium. It has many blood vessels, nerves, and lymphatic vessels.

Synovial fluid (LS) is produced by the synovial membrane, which lubricates the joint’s articular surface and provides nutrients to the articular cartilage. The synovial fluid comprises hyaluronic acid, glycoproteins, and transudate capillaries within the synovial membrane.

The synovial membrane is composed of fatty and fibrous tissue; they have a smooth non-adherent surface that allows easy movement between tissues. The cover is permeable to proteins, water, and small molecules.

The synovial membrane contains two layers:

  • The intima: (a thin cellular layer) contains macrophages and fibroblasts.
  • The subintima (a deeper vascular layer).


The synovial membrane provides joint lubrication, limiting friction, and cleaning the joint cavity, determining what can enter the joint cavity and which must remain outside.

Synovial fluid

This is a transparent, colorless, thick, and fibrous liquid. Many consider it to be similar to egg white.

Synovial fluid has two main functions:

  1. Help in the nourishment of the articular cartilage to act as a means of transporting nutrients such as glucose.
  2. It helps in the mechanical function of the joints by the lubrication of the surfaces of the joints.

The concentration of synovial fluid glucose is usually approximately equal to that of blood. A decreased amount of glucose in the synovial fluid can be associated with joint diseases, especially septic and immune-mediated arthritis.


Synovial membrane problems

Synovial membranes are subject to diseases and injuries and are the main target in rheumatoid arthritis. This is an autoimmune disease where the immune system goes to attack when there is no threat of pathogens and, in this case, attacks the joints.

It affects approximately 1 in 100 people. The synovial membrane swells and produces extra fluid that causes the ligaments to stretch around the joint.

The result is a rigid, swollen, and often painful joint that can also damage the tendons, ligaments, and old-fashioned bones. The excess liquid released is aqueous and of low viscosity; therefore, it is helpful as a lubricant.