A simple two dimensional culture method to study the hypertrophic differentiation of rat articular chondrocytes

Chondrocytes hypertrophy is a physiological process observed in endochondral ossification during development until adolescence in human. It can also be observed during pathophysiological conditions such as osteoarthritis. Hypertrophic chondrocytes synthesise collagen X and express matrix metalloproteinase 13 and alkaline phosphatase. The cellular models available to study this process are either not convenient, they might lead to a rapid dedifferentiation of chondrocytes, or they are far from the physiological conditions. The objective of this study was to design an user-friendly 2D-primary cell culture of young articular chondrocytes of rat able to follow the terminal differentiation process.

After confluence, chondrocytes were cultured according to 4 differentiation protocols. Protocol 1 contained DMEM/F12 supplemented with 10% foetal bovine serum (FBS) and 2 μg/ml insulin. Protocol 2 contained alpha-MEM supplemented with 5% FBS and 2 μg/ml insulin. Protocol 3 contained 2% FBS and 2 μg/ml insulin. Protocol 4 contained DMEM/F12 supplemented with 2% FBS in absence or in presence of 2 μg/ml insulin and 37.5 μg/ml ascorbate. The cell morphology was observed by phase-contrast microscopy and the expression of markers specific of mature and hypertrophic chondrocytes were assessed by RT-qPCR.

The effect of a decrease in nutrient quality of the culture medium after confluence was tested using protocols 1, 2 and 3. Protocol 1 did not allow the maintenance of chondrocyte phenotype more than one week, because cells became fibroblastic. A decrease in Sox9 mRNA expression, in collagen II/collagen I and in aggrecan/versican mRNA ratios was also found with protocol 1. Protocol 3 was the best when compared with protocols 1 and 2. It allowed chondrocytes to adopt a hypertrophic morphology. Cells also expressed the collagen X specific hypertrophic marker, and presented an increase in collagen II/I and aggrecan/versican ratios after 15 days of culture post-confluence. The effect of the insulin/ascorbate supplementation was studied using protocol 4. The insulin/ascorbate supplementation allowed an earlier chondrocytes conversion to terminal differentiation with a prolonged effect till 3 weeks post-confluence, compared to control without insulin/ascorbate. Finally, the profile of chondrocyte differentiation was checked during 5 successive sub-cultures. Only the first passage could be used to study hypertrophy.

A convenient protocol to study chondrocyte hypertrophy is proposed. Protocol 4 offers the possibility to study this differentiation phenotype which is crucial for the development of articular diseases such as osteoarthritis. Our model could also be used in tissue engineering for cartilage repair strategies in which hypertrophic differentiation of chondrocyte should be avoided.