On gross examination, the synovial surface appears pale pink and shiny. Although some folds can be seen with the naked eye, the characteristic villi that increase the effective surface area of the synovial membrane are visible only on microscopic examination. One or two layers of cells, with their long axes generally lying parallel to the surface, line the synovial membrane; these lining cells are not joined by intracellular junctions. The deeper tissue consists predominantly of loose connective tissue, fibrous tissue, or fat. Thus, the associated synovial membrane is described as areolar, fibrous, or adipose. Fibrous synovial membrane is found in areas that need more strength but less flexibility.

Capillaries and venules lie immediately beneath the lining cells. Lymphatics, which are difficult to identify with standard light microscopy, are most abundant in areolar synovial membrane. Nonmyelinated nerve fibers extend from the capsule into the adventitia of the synovial blood vessels.

Ultrastructural and immunopathologic studies have added considerably to the understanding of the synovial membrane. The lining cell layer consists of some cells that are rich in rough endoplasmic reticulum. These cells, called type B, are probably related to fibroblasts. Type B cells are most important because of their ability to secrete prostaglandins, collagenase, hyaluronan, and many other components of joint fluid.

Phagocytic cells (type A), which have prominent lysosomes, are now known to originate from monocytes. They often lie superficially to the type B cells. Some cells, which appear to have features of both type A and type B cells, are less well understood. Mast cells in perivascular areas, easily identified on electron microscopy, are a source of important vasoactive substances. Collagen (types I and III), fibronectin, and proteoglycans are present in the matrix.

Electron microscopic examination reveals that the superficial capillaries and venules have a fenestrated endothelium through which fluid, together with small amounts of low-molecular-weight protein, transudes to form the joint fluid. The addition of hyaluronan by the lining cells gives the joint fluid its characteristic viscosity. The deeper vessels, which have thicker walls, are the vessels through which most inflammatory cells emigrate.