Biochemical composition of the superficial layer of articular cartilage

Abstract

To gain more information on the mechanism of lubrication in articular joints, the superficial layer of bovine articular cartilage was mechanically removed in a sheet of ice that formed on freezing the cartilage. Freeze-dried samples contained low concentrations of chondroitin sulphate and protein. Analysis of the protein by SDS PAGE showed that the composition of the sample was comparable to that of synovial fluid (SF). Attenuated total reflection infrared (ATR-IR) spectroscopy of the dried residue indicated that the sample contained mostly hyaluronan. Moreover, ATR-IR spectroscopy of the upper layer of the superficial layer, adsorbed onto silicon, showed the presence of phospholipids. A gel could be formed by mixing hyaluronan and phosphatidylcholine in water with mechanical properties similar to those of the superficial layer on cartilage. Much like the superficial layer of natural cartilage, the surface of this gel became hydrophobic on drying out. Thus, it is proposed that the superficial layer forms from hyaluronan and phospholipids, which associate by hydrophobic interactions between the alkyl chains of the phospholipids and the hydrophobic faces of the disaccharide units in hyaluronan. This layer is permeable to material from the SF and the cartilage, as shown by the presence of SF proteins and chondroitin sulphate. As the cartilage dries out after removal from the joint, the phospholipids migrate towards the surface of the superficial layer to reduce the surface tension. It is also proposed that the highly efficient lubrication in articular joints can, at least in part, be attributed to the ability of the superficial layer to adsorb and hold water on the cartilage surface, thus creating a highly viscous boundary protection.

Does Periacetabular Osteotomy Have Depth-related Effects on the Articular Cartilage of the Hip?

“…On closer inspection, the biochemical changes in cartilage after PAO were greater in the superficial than in the deeper cartilage zone, suggesting a higher response of the superficial zone compared with the deeper zone to changes in biomechanics or the possible nonbiomechical factors including the inflammatory process in the joint after surgery. This confirms the findings of several other authors, indicating depth-related properties and metabolic specialization of the resident chondrocytes leading to characteristic zonal variations in cartilage adaption to mechanical loading [9,36,43]. In addition, the unique depth-dependent features and high vulnerability of the superficial layer have been demonstrated in a study investigating the effect of inflammatory cytokines in different cartilage layers [13].…”

“…However, no synovial fluid layer could be detected on a single morphological True-FISP reformat, most likely as a result of the extreme thinness of this layer. Furthermore, it has been reported that the protein composition of the superficial layer is almost indistinguishable from that of synovial fluid [9]; therefore, we considered that our approach is reasonable for evaluating the superficial zone. Second, the lack of a control group (natural history group) is a limitation.…”

“…Loss of GAG is one of the earliest events in cartilage degeneration. Histologically, the articular cartilage has a zonal organization where both the GAG content and structure of the collagen fibrils vary through the depth of the tissue [9,16,20]. In normal articular cartilage there are three major zones based on the orientation of collagen fibers: superficial, mid-, and deep layer [29], which can be distinguished on MR images [27].…”

Boundary Lubrication in Natural Articular Joints

“…In our laboratories, the superficial layer was removed mechanically from the articular surface of natural bovine cartilage and analysed for phospholipid, protein, glycosaminoglycan and hyaluronan [29]. It was found that the uppermost hydrophobic layer that forms on removal of the cartilage from synovial fluid was made up mostly of phospholipid.…”

Laser-induced regeneration of cartilage

“…An articular cartilage surface is covered by a cell-free lamina splendens (LS) consisting mainly of the HA and phospholipids. 17 An important structural and metabolic unit of articular cartilage is a chondron. 11 It includes a chondrocyte and its pericellular matrix (PM) bordered with a pericellular capsule (PC).…”