Articular cartilage in synovial joints is usually hyaline cartilage but occasionally can be fibrocartilaginous, such as that in the temporomandibular joint. The cartilage is avascular and has the capacity to accumulate tissue fluid high in water content due to the presence of proteoglycans in it called aggrecan. Due to this property, articular cartilage can withstand compression better than bone. In the absence of blood vessels, the tissue fluid nourishes the chondrocytes present within the cartilage. The cartilage also lacks nerve endings; hence, damage to the cartilage can remain painless until a substantial amount of cartilage is damaged, and underlying bone with neural connections is exposed, as seen in osteoarthritis. Articular cartilage is separated from subchondral bone by a thin zone of calcified cartilage. There exists a complex and intimate relationship between the structures at this junction. Biocomposition and structure of bone and cartilage are suitable for providing optimum weight-bearing surface. It has been now proposed that any change in the composition of any of the related structures may have adverse effect on the other [4].

Nonarticular surfaces of the joints are covered by a thin, two- to three-cell-layered synovial membrane which secretes a thin film of viscous fluid called synovial fluid. Synovial membrane is composed of two layers, surface or intimal layer which is cellular and subintimal layer which is neurovascular. The vascular layer has the potential for angiogenesis and can accrue inflammatory cell infiltrate during inflammation. The synovium is made up of two types of synoviocytes. Type A synoviocytes are macrophage cells which phagocytose the debris, act as antigen-presenting cells, and are part of the innate immune response [5]. Immune activation of these cells can occur through pattern recognition receptors, immune complexes, pro-inflammatory cytokines, or tissue injury fragments such as fragments of hyaluronan and damaged connective tissues. Type B synoviocytes are fibroblasts that secrete lubricant and hyaluronan which help in reducing friction between surfaces [6].

In health, synovial membrane expresses number of different adhesion molecules like CD44, vascular cell adhesion molecule (VCAM)-1, and intercellular adhesion molecules (ICAM)-1 [7]. These molecules have a role to play in the recruitment of inflammatory cells during development of arthritis [8]. Angiogenic molecules such as vascular endothelial growth factor (VEGF) and fibroblast growth factor (FGF), which are normally restricted to capillary endothelium in the vascular layer of synovial membrane, are found in areas outside endothelium during inflammation [8]. In synovitis, the synoviocytes proliferate and the membrane becomes multilayered often forming fingerlike projections called villi and may even extend in to nearby bursa.

Synovial fluid secreted by the synovial membrane is an ultrafiltrate of plasma and helps nourish the cartilage cells along with providing lubrication for articular surfaces. Normally joint fluid is minimal and measures about 2.5 ml. Formation of synovial fluid through ultrafiltration of plasma is dependent on differences between intracapillary and intra-articular hydrostatic pressure and osmotic pressure differences between capillary plasma and synovial tissue fluid [8]. Synovial fluid albumin concentration is about 45 % of that in plasma, and concentration of other small molecules and electrolytes is almost equivalent to that in plasma [9]. Hyaluronic acid and lubricin, which aid in lubrication, are found in much higher concentrations in synovial fluid than in plasma.