Serum and synovial fluid proteins in arthritis.

Synovial fluid is generally considered to be a dialysate of plasma to which mucin has been added during its passage through the synovial membrane (Ropes and Bauer, 1953). The proteins of the serum and synovial fluid have been shown to be qualitatively identical (Schmid and MacNair, 1956) and the concentrations of the various proteins in synovial fluid probably depend upon the following factors: (1) The molecular weights of the various protein fractions; the smaller albumin molecules achieving relatively higher concentrations in the normal synovial fluid than the large globulin molecules (Decker, McKenzie, McGuckin, and Slocumb, 1959). (2) The permeability of the synovial membrane; the inflamed membrane is more permeable and allows through more protein and especially more of the larger globulins (Decker and others, 1959). (3) The rate of absorption of synovial fluid from the joints; there is some evidence that the rate of egress from the synovial fluid is particularly slow for large globulin molecules (Bauer, Short, and Bennett, 1933); also, in rheumatoid subjects, the absorption of proteins is more rapid from arthritic than from non-involved joints (Ahlstr6m, Gedda, and Hedberg, 1956).