## 2 Anatomy of Articular Cartilage
### Zonation and Regional Organisation  
The structure of articular cartilage consists of a dense extracellular matrix (ECM) with a sparse distribution of highly specialised cells called chondrocytes. It is composed of water, proteoglycans, collagen, along with other less common non-collagenous proteins and glycoproteins [2; 4; 41; 42; 43]. The combination of such ingredients influences water retention in the ECM, which in turn contributes to maintaining its mechanical properties. Chondrocytes contribute to the zonal structure of articular cartilage. It is divided into four distinct zones: I superficial zone, II middle zone, III deep zone, and IV calcified zone. A schematic cross-sectional diagram of healthy articular cartilage is shown in Figure 1 [44]. Each of the above-mentioned zones exhibits different biomechanical and biochemical properties. Depending on the tissue layer, the proteoglycan concentration and water content are variable. The amount of proteoglycans is higher from the superficial to the deep zone, while the water concentration decreases from the superficial to the deep zone [4; 43; 45]. The superficial zone has a protective function for the deeper layers against shear stresses. It accounts for approximately 10-20% of the thickness of the articular cartilage [2; 4]. In this layer, there are numerous flattened and smaller chondrocytes with a higher density as opposed to cells deeper in the matrix. The collagen fibres are arranged very tightly, with their position aligned parallel to the articular surface. The zone is mostly water and a small amount of proteoglycans. The superficial zone, otherwise known as the tangential zone, directly borders the synovial fluid. It is responsible for the tensile properties of the articular cartilage, which is what allows it to resist the shearing, compressive and tensile forces exerted by the articulation. A characteristic feature of the superficial zone is its deformability and lower stiffness compared to the other zones. The middle zone, otherwise known as the transitional zone, accounts for 40-60% of the articular cartilage volume. It consists of proteoglycans and thicker collagen fibres organised obliquely. Chondrocytes are spherical and have a low density. This zone is responsible for resisting compressive forces. The third zone is the deep zone, which accounts for approximately 30% of the articular cartilage volume. Due to the perpendicular alignment of the largest diameter collagen fibres radially to the articular surface, it provides the greatest resistance to compressive forces. It also features the lowest water concentration and the highest proteoglycan content. The position of the chondrocytes is in a columnar orientation, perpendicular to the joint line and parallel to the collagen fibres. There is a tidal mark between the deep and calcified zones, separating the lime-poor cartilage from the lime-rich cartilage. It consists of strands of fibrils that are attached to collagen fibres anchored in a lime-poor layer, which prevents the cartilage from detaching from the bone [46]. The deepest zone is the calcified zone. It is characterised by numerous recesses, protrusions and interlacing, which contribute to the high resistance to shear forces and prevent the cartilage from detaching from the underlying bone. The calcified zone secures the cartilage to the subchondral bone through the collagen fibres of the deep zone, which are anchored to the subchondral bone. Chondrocytes in this zone are hypertrophied, and the cell population is sparse. The matrix is also made up of regions that differ in their proximity to chondrocytes, as well as in the composition, diameter and organisation of collagen fibrils. These regions include pericellular, territorial and interterritorial regions. The pericellular matrix is mainly composed of proteoglycans, glycoproteins and other non-collagenous proteins. It is a thin layer that adheres to the cell membrane and surrounds the chondrocyte.