School of Civil and Mechanical Engineering, Curtin University, Perth, Western Australia, Australia.
School of Molecular and Life Sciences/Curtin Institute for Functional Molecules and Interfaces, Curtin University, Perth, Western Australia, Australia.
J Microsc. 2019 Sep;275(3):159-171. doi: 10.1111/jmi.12824. Epub 2019 Jul 29.
The surface of articular cartilage plays a crucial role in attenuating and transmitting mechanical loads in synovial joints to facilitate painless locomotion. Disruption to the surface of articular cartilage causes changes to its frictional properties instigating the deterioration of the tissue. In this study, we physically peeled the most superficial layer, a transparent membrane of 20.0 ± 4.7 µm thick, from the central loading region of femoral condyles of sheep. The ultrastructure of this layer without interference from the underlying cartilage was independently investigated using confocal, second harmonic generation and atomic force microscopy. We found that the most superficial layer contains chondrocytes, densely packed collagen, coarse elastic fibres and a fine elastic network. The elastic fibres are most prevalent at the surface of the layer, where collagen and chondrocyte densities are lowest. At the interface of this most superficial layer with the underlying bulk cartilage, a dense fibrillar network exists, formed mainly by collagen fibrils and elastin microfibrils. By contrast, the interface of the underlying cartilage with the most superficial layer contains collagen fibrils, fine microfibrils and microfibrils distinctively laced on one side. The findings of this study will play an important role in understanding the mechanical function and wear resistance of articular cartilage, and in developing more promising tissue engineering techniques to treat cartilage defects and osteoarthritis. LAY DESCRIPTION: The chronic pain and dysfuction in synovial joints caused by osteoarthritis can have a debilitating impact on daily activities for sufferers. Osteoarthritis is characterised by the deterioration of the articular cartilage. Despite intensive research, the wear mechanism of articular cartilage and the progression of osteoarthritis remain unclear in the literature. Articular cartilage is a resilient tissue that provides a low friction surface to facilitate painless locomotion. The surface of articular cartilage plays a crucial role in attenuating and transmitting mechanical loads. Disruption at the surface of articular cartilage causes changes to its frictional properties, instigating the deterioration of the tissue. Despite this, the definition of the most superficial layer of articular cartilage, as well as its composition and microstructure, have endured a long history of debate, clouding our understanding of the early progression of osteoarthritis. In order to investigate the surface of articular cartilage independently from the underlying cartilage, we physically peeled a transparent membrane of 20.0 ± 4.7 µm thickness, the most superficial layer, from the central loading region of the femoral condyles of sheep. Using confocal, second harmonic generation and atomic force microscopy, we found that the most superficial layer contains cartilage cells (chondrocytes), densely packed collagen, coarse elastic fibres and a fine elastic network. The coarse elastic fibres are most prevalent at the surface of the layer where collagen and chondrocyte densities are lowest. Furthermore, we investigated the surfaces at the interface of the most superficial layer with the underlying articular cartilage. At the interface of this most superficial layer with the underlying bulk cartilage, a dense fibrillar network exists, formed mainly by collagen fibrils and elastin microfibrils. In contrast, the interface of the underlying cartilage with the most superficial layer contains collagen fibrils, fine microfibrils and microfibrils distinctively laced on one side. The findings of this study have confirmed that there is a most superficial layer that is able to be removed using a tangential force. Through the application of advanced imaging technologies, we have shown that this most superficial layer is cellular and have detailed its composition and ultrastructure. Due to the close association between the form and function of tissues, the findings of this study will play an important role in understanding the mechanical function and wear mechanism of articular cartilage. This may lead to the development of more promising tissue engineering techniques to treat cartilage defects and osteoarthritis.
关节软骨的表面在减轻和传递滑膜关节中的机械负荷方面起着至关重要的作用，从而促进无痛的运动。关节软骨表面的破坏会导致其摩擦特性发生变化，从而引发组织恶化。在这项研究中，我们从绵羊股骨髁的中央加载区域物理地剥离了最表层，这是一层 20.0 ± 4.7 µm 厚的透明膜。使用共聚焦、二次谐波产生和原子力显微镜独立研究了这一层的超微结构，而不受下面软骨的干扰。我们发现最表层含有软骨细胞、紧密堆积的胶原、粗弹性纤维和细弹性网络。最表层的弹性纤维最常见于表面，而胶原和软骨细胞的密度最低。在最表层与下面的软骨之间的界面处，存在一个密集的纤维状网络，主要由胶原纤维和弹性微纤维组成。相比之下，下面的软骨与最表层之间的界面含有胶原纤维、细微纤维和一侧明显交织的微纤维。这项研究的结果将在理解关节软骨的机械功能和耐磨性以及开发更有前途的组织工程技术来治疗软骨缺陷和骨关节炎方面发挥重要作用。
