Type II collagen is the main collagen in joint cartilage. Collagen makes up about two-thirds of adult cartilage's dry mass, and type II is its principal molecule, woven with smaller amounts of other collagens into a fibre network. On most measurements adults replace that network very slowly, if at all, though one group found faster turnover in ankle cartilage. That slow renewal is why the enzymes that cut it matter.
Joint cartilage is held together by a framework, and that framework is collagen. In adult joint cartilage, collagen makes up the extracellular framework and two-thirds of the dry mass, and type II collagen is the principal molecule (Eyre 2002). It isn't alone. Collagens III, VI, IX, X, XI, XII and XIV all contribute, and in developing cartilage the core fibre network is a cross-linked mix of collagens II, IX and XI. Collagen X sits mainly in the thin calcified layer where cartilage meets bone (Eyre 2002).
Each type II collagen molecule is a triple helix, three protein strands wound together. Breaking one takes an enzyme able to cut that helix (Mitchell 1996).
Aggrecan, the large water-holding molecule packed into this network, is a separate material. Its loss is thought to be early and reversible, while loss of type II collagen is considered irreversible (Li 2011).
Aggrecan holds water so cartilage can give under pressure. The collagen does the other job.
The type 2 collagen is endowing cartilage with tensile strength and is actually the main determinant of the cartilage structure.
Enzymes don't treat all collagen types alike. MMP13 prefers type II collagen and is five or six times less efficient at cutting types I and III (Knäuper 1996). Turnover differs too. From the slow chemical ageing of collagen proteins, Verzijl and colleagues estimated the half-life of cartilage collagen at 117 years and that of skin collagen at 15 (Verzijl 2000).
In 2016 Heinemeier and colleagues used the carbon-14 bomb-pulse method, which reads the date a protein was made from its carbon-14 level, to date collagen in knee cartilage from 23 people born between 1935 and 1997. There was virtually no replacement after skeletal maturity, and joint damage didn't change that (Heinemeier 2016). Collagen in the centre of the joint surface had formed several years earlier than collagen at the edges (Heinemeier 2016). The authors called the collagen network of adult cartilage an essentially permanent structure.
Cells do keep some activity going around themselves. In healthy ankle cartilage, signs of new type II collagen being made were present in all zones and didn't fall with age, and synthesis and breakdown were balanced in the matrix right next to the cells (Aurich 2002).
Not every measurement agrees. Using a different molecular clock, Virginia Kraus's group at Duke found that protein turnover in human cartilage was highest in the ankle and lowest in the hip, and linked it to a limited capacity for repair (Hsueh 2019). Kraus has said the soluble type II collagen her group measured turns over in a matter of years, while the radiocarbon study dated the insoluble network. The two methods looked at different fractions of the collagen, which may explain part of the gap.
Because cartilage collagen stays in place for decades, it collects chemical damage. Sugar-derived compounds called advanced glycation end products build up with age in both skin and cartilage collagen, faster in cartilage. When the authors corrected for turnover, the rates matched, which pointed to slow turnover as the main reason cartilage collects more (Verzijl 2000).
The gene for type II collagen, COL2A1, is one of the genes switched on by SOX9, a control protein in cartilage cells. In human cartilage cells, high concentrations of the hormone leptin lowered SOX9 and with it COL2A1 and the aggrecan gene (Primrose 2023). Other provocations studied in Auckland, including inflammation, oxidative stress and calcium crystals, also lowered SOX9 while raising MMP13 (Rong 2019).
When type II collagen is cut, fragments turn up in blood and urine. One is CTX-II, a piece from the end of the molecule. Trials and animal studies use it as a marker of cartilage collagen breakdown, alongside other fragments (Abo-Zalam 2025; Kumar 2025; Vaidya 2025).
No. Type II collagen is the fibre network; aggrecan is the large water-holding molecule held within it. Researchers describe aggrecan loss as early and reversible, and type II collagen loss as irreversible.
A fragment from the end of the type II collagen molecule, released when the collagen is broken down. Studies measure it in blood or urine as a marker of cartilage collagen breakdown.
MMP13. In a 1996 study it broke down type II collagen at least ten times faster than MMP-1.
Its collagen barely does, on most measurements. Radiocarbon dating of knee cartilage from 23 people found virtually no replacement of the collagen network after the...
MMP13 cuts type II collagen, the rope-like fibre that gives cartilage its strength. It cleaves each three-stranded collagen molecule at one spot, leaving a three-quarter...
It depends which part is damaged. Reviews describe losing aggrecan, the water-holding molecule in cartilage, as an early and reversible step, and losing type II collagen...
Both are collagenases, enzymes that can cut intact collagen, and they cut type II collagen at the same bond. MMP13 is much faster at it: at least ten times faster than...