Anabolic response of HAC to different oxygen percentages during differentiation Phase I and II. (A - B) Safranin O and type II collagen stainings of representative tissues generated by human articular chondrocytes (HAC) cultured in pellets for two weeks (Phase I) at 19% (I and II) or 5% (III and IV) oxygen and further cultured for two additionally weeks (Phase II) at 19% (I and III) or 5% (II and IV) oxygen. Bar = 100 μm. (C) Quantification of glycosaminoglycans (GAG) accumulated in pellets cultured as described in (A - B) normalized to the amount of DNA. (D - E) Amounts of newly synthesized collagen (D) and proteoglycan (E) measured in pellets cultured for 18 days (two weeks of Phase I and four days of Phase II). The upper and lower parts of the columns represent the released and accumulated fractions respectively. 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; a = accumulated, r = released.