Ultrastructural comparisons of human synovial membrane in joints exposed to varying stresses.

The writers used the electron transmission microscope to study the ultrastructure of the synovial membrane in joints or synovial areas that varied in their exposure to weight bearing or other mechanical stresses. They used fragments of synovial membrane obtained surgically from the knee and from the elbow joints. In the joints or synovial areas little exposed to weight bearing or traction, the three strata of which the synovial membrane is composed were well differentiated from the fibrous capsule. The intima contained abundant cells with a trophic function (chiefly A cells); the subintima was intensely vascularised and had numerous fat cells but few fibres; the subsynovia was predominantly fibrous. On the other hand, in joints or synovial areas notably exposed to weight bearing or traction stresses, the synovial membrane was thinner, with few cellular elements (mostly resembling fibroblasts) and few blood vessels, while the fibrillar component was particularly marked. These differences in the ultrastructure confirm Franceschini's hypothesis that the synovial fluid is produced in those areas of the synovia that are less exposed to mechanical stress.