Catabolic response of HAC to different oxygen percentages during differentiation Phase I and II. (A - B) Quantification of MMP-1 (A) and MMP-13 (B) released in the medium by human articular chondrocytes (HAC) cultured in pellets for four weeks (two weeks of Phase I and two weeks of Phase II). Levels are normalized to the amount of DNA measured in relative pellets. Values are mean ± SD of measurements obtained from two independent experiments. * = significantly different from the group cultured with the same oxygen percentage in Phase I but with different oxygen tension in Phase II; ° = significantly different from the group cultured entirely at 19% O2 (Phase I and Phase II). (C) Immunohistochemical detection of type II collagen fragments of pellets cultured under conditions described in (A - B). Bar = 100 μm.