Intact bovine knee joints (, ) were obtained from the local abattoir (Atria Oyj, Kuopio, Finland). After opening the joint, osteochondral samples () were removed from the upper lateral quandrant of the patellae. Four samples () within an area smaller than 25 mm in diameter were harvested from each patella. The control samples (, ) were immersed in a cell culture medium [Dulbecco’s modified Eagle’s medium (DMEM) low glucose , without phenol red, Sigma Aldrich Co., St. Louis, MO], with penicillin, streptomycin, 10 mM HEPES buffer solution, 1 mM L-glutamine (EuroClone S.p.A., Milano, Italy), and μg/ml vitamin C (Sigma Aldrich Co.). For the other samples, 100 mM L-threose (Sigma Aldrich Co.) was added in liquid to induce cross-link formation.5 The samples were incubated in 5% air atmosphere at 37°C. The incubation time for the control samples and the 12 samples with threose was 40 and 100 h for the six samples with threose treatment. Control samples () for 100 h threose treatment were also prepared but had to be rejected due to algae found in the medium after incubation for 100 h. For additional noncontact optical measurements (data not shown), the samples were removed after every 5 h from the incubator during the first 40 h and after every 15 h thereafter. After incubation, all the samples were changed into fresh control liquid (the same as the control incubation liquid), and the cartilage thickness was determined using a stereomicroscope at four locations around the edge of the sample.18 After the thickness measurements, the samples were frozen () until the fluorescence measurements.