Fig 1.
SEM images of normal human articular cartilage after enzymatic depletion of the proteoglycan moiety and chondrocytes.
(a) SEM image of the collagen fiber meshwork from knee surface articular cartilage shows (i) the 67 nm D-band periodicity, (ii) the hierarchical organization of 5–7 threads of prototypic fibrils forming an individual collagen fiber; note that each prototypic fibril exhibits the 67 nm D-band periodicity; and (iii) a twisting of the prototypic fibril along the long axis of roughly about 400 nm (white arrow). (b) SEM image of hip surface articular cartilage with untwisted fibers (white arrows). (c) Collagen fibers were labeled by collagen II antibodies with 18-nm gold particles attached and directly inspected in cartilage by SEM. (d) imaging of 18-nm gold particles using the backscattering electron (BSE) mode in the SEM on extracted collagen type II fibers. (e) Graph shows the increase of collagen fiber diameter with the number of prototypic fibrils. (f) Comparison between fiber diameters in each zone in hip articular cartilage and knee articular cartilage. Scale bars, 100 nm (a to d).
Fig 2.
SEM images providing an overview of articular cartilage of patients undergoing knee or hip arthroplasties from osteoarthritic human articular cartilage after enzymatic depletion of the proteoglycan moiety and chondrocytes.
(a) SEM image of grade 3 osteoarthritic cartilage (knee), exhibiting breakdown of thicker collagen fibers with a diameter of 40–60 nm into thinner fibers down to bundles made of only one prototypic fibril of 18 ± 5 nm in diameter. (b) SEM image of grade 3 osteoarthritic cartilage (knee) shows the end-stage of fiber breakdown, that is a wool-like structure (white arrows) with filaments exhibiting a diameter of d = 13 ± 2 nm. (c) Degrading articular cartilage larger fibers split into smaller sized fibrils that are often arranged as a highly entangled fibrillar meshwork (white arrows). Scale bars, 500 nm (a and c); 100 nm (b).
Fig 3.
SEM images of grade 0 knee and hip articular cartilage show the breakdown of cartilage at the individual fiber level.
(a) SEM image of grade 0 articular cartilage (knee) shows a fiber that splits apart and forms two new perfect but smaller diameter fibers as indicated by the arrows. (b) SEM image of grade 0 articular cartilage hip cartilage exhibiting prototypic fibrils that are arranged in register. The thick fiber at the bottom of the image (white circle) has split into two fibers of smaller diameters. (c) Scheme suggesting that the splitting of fibers is induced by proteases. Scale bars, 500 nm (a); 100 nm (b).
Fig 4.
Schematic representation of fiber formation and fiber breakdown.
Three identical [α1(II)]3 chains assemble into a right-handed triple helix and form a collagen type II monomer that is approximately 300 nm long and 1.5 nm in diameter. In the next step of hierarchical organization collagen monomers assemble into highly ordered 67-nm staggered arrays of collagen monomers. The Hodge & Petruska model (Petruska and Hodge, 1964) explains this characteristic D-periodic banding formation. A collagen fiber in the knee surface zone is composed of threads of smaller fibrils that we identified as prototypic fibrils. Each individual collagen-containing fiber is assembled of multiple single prototypic fibrils. The prototypic fibrils are aligned in register. Schematics (upper right corner) explaining the splitting of fibers and formation of smaller fibers: Collagen fibers in the knee or hip surface zone after splitting into two perfect collagen fibers of smaller diameters. The prototypic fibrils are aligned in register, in the original fiber as well as in the daughter fibers.