Supramolecular Organization of Collagen Fibrils in Healthy and Osteoarthritic Human Knee and Hip Joint Cartilage

Abstract

Cartilage matrix is a composite of discrete, but interacting suprastructures, i.e. cartilage fibers with microfibrillar or network-like aggregates and penetrating extrafibrillar proteoglycan matrix. The biomechanical function of the proteoglycan matrix and the collagen fibers are to absorb compressive and tensional loads, respectively. Here, we are focusing on the suprastructural organization of collagen fibrils and the degradation process of their hierarchical organized fiber architecture studied at high resolution at the authentic location within cartilage. We present electron micrographs of the collagenous cores of such fibers obtained by an improved protocol for scanning electron microscopy (SEM). Articular cartilages are permeated by small prototypic fibrils with a homogeneous diameter of 18 ± 5 nm that can align in their D-periodic pattern and merge into larger fibers by lateral association. Interestingly, these fibers have tissue-specific organizations in cartilage. They are twisted ropes in superficial regions of knee joints or assemble into parallel aligned cable-like structures in deeper regions of knee joint- or throughout hip joints articular cartilage. These novel observations contribute to an improved understanding of collagen fiber biogenesis, function, and homeostasis in hyaline cartilage.

Surgical treatment of chondral knee defects using a collagen membrane – autologus matrix-induced chondrogenesis

“…Its oxygen and nutrient supply comes from the synovial fluid through the diffusion process, which is facilitated by cyclic compressive loads through a pumping mechanism during joint movements. 1 …”

Analysis of opticin binding to collagen fibrils identifies a single binding site in the gap region and a high specificity towards thin heterotypic fibrils containing collagens II, and XI or V/XI

“…The mean diameter of fibrils containing collagens II/IX/XI was slightly greater when formed in the presence of opticin compared to when opticin was absent, but this could simply represent opticin on the fibril surfaces rather than modification to the underlying collagen structure. These fibrils formed in vitro closely resemble the thin cartilage collagen fibrils found in vivo , which have the same molar ratio of collagens and a diameter of 15–18 nm [ 9 , 10 , 23 ] and vitreous collagen fibrils which have a diameter of 10–20 nm depending upon species and method of analysis [ 8 , 16 ]. The conclusion that opticin does not affect fibril morphology was further supported by comparing the diameter of vitreous collagen fibrils from opticin null and wild-type mice where no significant difference was observed.…”

“…The vitreous humour contains a network of fine collagen fibrils of uniform thickness (10-20 nm depending upon species) that impart gel-like properties to the tissue [8], whereas in human cartilage there are both thin~18 nm diameter fibrils and thicker fibrils [9,10]. The collagen fibrils from these two tissues are very similar in their collagen composition: They are heterotypic (mixed in composition) fibrils in which fibril-forming type II collagen predominate, the fibrils also contain collagen IX and XI or V/XI.…”

The synovial surface of the articular cartilage

“…The dimensional heterogeneity of the collagen content between the synovial surface and the deeper layers of articular cartilage is unambiguous and consistent with recent research. 28 A recent study 43 describes two discrete categories of fibrils with small and uniform fibrils, approximately 20 nm in diameter, distinct from other fibrils characterized by a broad distribution of diameters starting from 40nm, with no fibrils between 20 and 40 nm. SEM requires metal-coated specimens and is not really suited to take quantitative measurements of fibril diameter, but the dimensional homogeneity is unmistakable and consistent with other high-resolution studies.…”

“…SEM requires metal-coated specimens and is not really suited to take quantitative measurements of fibril diameter, but the dimensional homogeneity is unmistakable and consistent with other high-resolution studies. 28 …”