Ex vivo model of mechanical injury to adult human articular cartilage explants. (a) Adjacent explants from human adult articular cartilage were dissected and placed in culture in separate bacteriological Petri dishes. After 6 days, 1 explant was injured. At different time points the cultures were terminated for gene expression analysis, histochemistry and immunohistochemistry. (b) Safranin O staining of: a, freshly dissected normal articular cartilage; b, an adjacent explant after 7 days in culture; c, a further adjacent explant after 6 days in culture before injury plus 1 additional day after injury; and d, a typical freshly dissected explant from a preserved area from a patient who had undergone joint arthroplasty for osteoarthritis. (c,d) Time course of metalloproteinase (MMP)-3 and MMP-13 mRNA differential expression in injured versus uninjured explants. Values are normalized for the housekeeping gene β actin and expressed as fold change of gene expression in the injured explants from paired uninjured controls. Diamonds indicate samples from preserved areas from joints affected by osteoarthritis; open squares indicate sample pairs from healthy cartilage. *p < 0.05; **p < 0.01. D, day(s); h, hours.