A joint space has a membranous lining similar to the mesothelium found in the pleural and peritoneal cavities. The synovial lining cells, which make up this membrane, produce synovial fluid, which lubricates the joints.
There is a minimal volume of fluid present in a normal joint. With joint trauma, such as infection or inflammation, the volume will increase and synovial lining cells may be noted on the cytospin preparation in addition to the cell types normally present with infection, inflammation, or hemorrhage.
Synovial lining cells are indicated by the arrow in the image on the right. They resemble miniature mesothelial cells or small macrophages. They can be found singly or in clumps and can have "foamy"-looking cytoplasm.