The associated paper to the article is:

https://journals.plos.org/plosbiology/article/file?id=10.137...

p.15 has a summary of what they propose, e.g., density of p21+ chondrocytes, as an inhibitor:

"Because we found that the extent of compensatory proliferation does not linearly correlate with cell density—but it does with the proportion of p21+ chondrocytes — we posit that density plays a permissive rather than an instructive role and that stress signals emanating from p21+ chondrocytes are needed as well."