Ultrastructural identification of a membrane-like structure on the surface of normal articular cartilage

Cytochemical and immunocytochemical approaches have been applied to the study of the surface of articular cartilage in humans, bovine and rats. Specimens were fixed in situ or soon after bioptic sampling with chemicals able to preserve and visualize proteins (glutaraldehyde, tannic acid), lipids (osmium tetroxide, malachite green, uranyl acetate) and proteoglycans (toluidine blue O, cuprolinic blue, cetyl pyridinium chloride). Mixtures of reagents were also used. Oriented serial thin sections were observed as such or after treatment with chemicals (chloroform-methanol, Triton X 100) or enzymes (chondroitinases, hyaluronidases, trypsin). Hyaluronan was detected by the use of glial-hyaluronate-binding-protein and antibodies against it. High concentration of osmium tetroxide or fixatives containing markers for lipid or for proteoglycans revealed that the surface of the articular cartilage, in all animal species examined, was covered by mono-multilayered discontinuous three-laminar sheets, which could be partly removed by chloroform-methanol and Triton X 100, were sensitive to hyaluronidase, chondroitinase and trypsin, and were immunopositive for hyaluronan. Each three-laminar sheet was 12-14 nm thick, was always separated from the cartilage itself and could be easily displaced. It is proposed that the surface of normal articular cartilage is covered by a discontinuous mono/multilayered pseudo-membrane, that can be better preserved by fixatives injected into the joint cavity and seems to consist of phospholipids, glycosaminoglycans and proteins. This membrane-like structure might have a protecting role in preventing direct contacts between the articular cartilage and toxic agents present in the synovial fluid and/or exert a lubricating effect within the articular joint.