## 2 Anatomy of Articular Cartilage
### Proteoglycans  
Proteoglycans are highly glycosylated protein monomers that consist of a protein core with attached glycosaminoglycans [116; 117]. They are the second largest macromolecular group in the ECM after collagen, accounting for 10-15% of the wet weight. There are several types of proteoglycans in articular cartilage, such as aggrecan, decorin, biglycan and fibromyalgia [2]. Aggrecan is the largest and most abundant proteoglycan, characterised by its ability to interact with hyaluron. This interaction allows the formation of large aggregates of proteoglycans through linker proteins [118]. Aggrecans, together with collagen, form a porous, strong, fibre-reinforced composite material. No covalent bonds between  
\begin{table}
\begin{tabular}{c c c c} \hline \hline
**Classification** & **Type** & **Molecules** & **Concentrations** \\  & II & 3 \(\times\)\(\alpha\)1(II)—triple helix & 90–95 \\ Fibrillar & XI & \(\alpha\)1(XI), \(\alpha\)2(XI), \(\alpha\)3(XI) & 3–5 \\  & V & \(\alpha\)1(V), \(\alpha\)2(V), \(\alpha\)3(V) & \(<\)1 \\ Fibrillar-associative (FACIT) & IX & 2 \(\times\)\(\alpha\)1(DX), \(\alpha\)2(IX)—collagen side-bonding & 1–2 \\ Microfibrillar & VI & 3 \(\times\)\(\alpha\)1(VI)—chondrocyte envelope & 1–2 \\ Cross-linking & X & Homo-trimers \(\alpha\)1(X)—calcified zone & \(<\)1 \\ \hline \hline \end{tabular}
\end{table}
Table 1: Types and molecular structure of collagen in articular cartilage.  
collagen and proteoglycans were found; however, the size of the proteoglycan aggregates is an extremely important factor affecting their retention in cartilage [117; 119]. The location of the aggrecan is in the interfibrillar space of the ECM cartilage. Its presence influences the osmotic properties of the cartilage, making it capable of resisting compressive loads. Decorin, biglycan and fibromyalgia are similar molecules due to their protein structure; however, they differ in their composition and function as glucosaminoglycans. Biglycan is located in the immediate vicinity of chondrocytes, allowing interaction with collagen VI. Fibromodulin and decorin interact in the matrix with fibrils of type II collagen.  
Proteoglycans consist of repeating carboxyl and sulphate groups along their chains [117]. When placed in an aqueous solution, these groups become negatively charged, causing a high solid charge density in the cartilage through their tight packing [120].  
With a higher concentration density of negatively charged proteoglycans, swelling pressure increases, which in turn leads to tensile stress on the surrounding collagen network [117; 121]. The swelling pressure also promotes the maintenance of a high degree of hydration in the articular cartilage. The degree of hydration of cartilage is determined by the balance between the expanding total swelling pressure, which is exerted by proteoglycans, and the restraining tensile force that develops in the surrounding collagen network [117; 121; 122; 123]. If the collagen network of the articular surface is damaged, thereby disturbing this balance, it will contribute to increased tissue hydration, as a result of which the ability of the articular cartilage to bear loads will change [117; 124; 125].