The synovial surface of the articular cartilage

Abstract

<jats:p>The articular cartilage has been the subject of a huge amount of research carried out with a wide array of different techniques. Most of the existing morphological and ultrastructural data on the this tissue, however, were obtained either by light microscopy or by transmission electron microscopy. Both techniques rely on thin sections and neither allows a direct, face-on visualization of the free cartilage surface (synovial surface), which is the only portion subject to frictional as well as compressive forces. In the present research, high resolution visualization by scanning electron microscopy and by atomic force microscopy revealed that the collagen fibrils of the articular surface are exclusively represented by thin, uniform, parallel fibrils evocative of the heterotypic type IX-type II fibrils reported by other authors, immersed in an abundant matrix of glycoconjugates, in part regularly arranged in phase with the D-period of collagen. Electrophoresis of fluorophore-labeled saccharides confirmed that the superficial and the deeper layers are quite different in their glycoconjugate content as well, the deeper ones containing more sulfated, more acidic small proteoglycans bound to thicker, more heterogenous collagen fibrils. The differences found between the synovial surface and the deeper layers are consistent with the different mechanical stresses they must withstand.</jats:p>

Spectral CT imaging of human osteoarthritic cartilage via quantitative assessment of glycosaminoglycan content using multiple contrast agents

“… 57,58 This is of particular importance in the superficial layer for healthy cartilage where the low GAG concentration and/or low GAG sulfation (i.e., low fixed charge density) is not as sensitive to the inverse relationship demonstrated in this study as with cationic agents. 16 However, the ability to distinguish iodine, gadolinium, and calcium using photon-processing spectral CT means that ioxaglate and gadopentetate dimeglumine concentrations can be selected to maximize the sensitivity of the measurement, rather than being constrained by the need to differentiate the contrast agent from bone. Furthermore, using a higher concentration of contrast agent with photon-processing spectral CT will provide increased sensitivity of GAG quantification.…”

“…1 ), causing a gradient in the negative fixed charge density of the cartilage. 16 The integrity and physical properties of the articular cartilage are mainly regulated by specialized cartilage cells, the chondrocytes. 17,18 It is thought that the chondrocytes fail to maintain healthy cartilage in OA due to multifactorial influences including biomechanical, biochemical, and immunological events.…”

Slippery when wet: The free surface of the articular cartilage

“…The thickness of the cartilage, from the surface down to the subchondral bone, has been subdivided into three (Gulisano et al, 1993; Fujoka, Aoyama & Takakuwa, 2013; Oinas et al, 2016; Mansfield, Mandalia, Toms, Winlove & Brasselet, 2019) or four (He et al, 2014; Boyanich et al, 2019) superimposed layers on the basis of their fibrils layout, cell appearance and matrix content. In particular, a superficial layer up to 200 μm thick (Kobayashi, Yonekubo & Kurogouchi, 1996) stains differently from the underlying matrix (Hyllested, Veje & Ostergaard, 2002), either because of a different acidity of its glycoconjugates (Basso, Caravà, Protasoni, Reguzzoni & Raspanti, 2020), or of its content of hyaluronic acid (Asari et al , 1994) or of collagen IX (Hagg, Bruckner & Hedbom, 1998). The collagen fibrils of the surface are also different from the underlying matrix (Holmes & Kadler, 2006; Gottardi et al, 2016; Basso et al, 2020).…”

“…In particular, a superficial layer up to 200 μm thick (Kobayashi, Yonekubo & Kurogouchi, 1996) stains differently from the underlying matrix (Hyllested, Veje & Ostergaard, 2002), either because of a different acidity of its glycoconjugates (Basso, Caravà, Protasoni, Reguzzoni & Raspanti, 2020), or of its content of hyaluronic acid (Asari et al , 1994) or of collagen IX (Hagg, Bruckner & Hedbom, 1998). The collagen fibrils of the surface are also different from the underlying matrix (Holmes & Kadler, 2006; Gottardi et al, 2016; Basso et al, 2020). Differences among other layers are more subtle.…”

“…Other technical approaches (He et al, 2014; Rieppo, Saarakkala, Jurvelin & Rieppo, 2014; Tadimalla, Tourell, Knott & Momot, 2018; Mansfield et al, 2019) lack an adequate resolution. The studies carried out with appropriate, high resolution visualization techniques are comparatively few, scattered along several decades (Walker et al, 1969; Ghadially, Ghadially, Oryschak & Yong, 1977; Clark, 1990; Jurvelin et al, 1996; Forster & Fisher, 1999; Sawae, Murakami, Matsumoto & Horimoto, 2000; Stolz e al., 2009; Wang, Peng, Wang & Jiang, 2013; Gottardi et al, 2016; Boyanich et al, 2019; Basso et al, 2020), and in some cases more focused on pathological alterations rather than on ultrastructure (Gulisano et al, 1993; Pellegrini, Lane Smith & Ku, 1994; Kobayashi et al, 1996; Chizhik, Wierzcholski, Trushko, Zhytkova & Miszczak, 2010).…”