Solution viscosity regulates chondrocyte proliferation and phenotype during 3D culture

Cells are surrounded by an extracellular matrix (ECM), which controls cellular functions through biological or physicochemical cues. In particular, cartilage tissues have abundant ECM that is viscoelastic and provides the necessary signals for the maintenance of chondrocyte activity and metabolism. The influence of ECM stiffness on chondrocyte functions has been broadly investigated using elastic hydrogels. However, it is not clear how viscosity impacts chondrocyte functions. In this study, a biphasic gelatin solution/hydrogel system was established for the three-dimensional culture of bovine articular chondrocytes (BACs) to investigate the influence of gelatin solution viscosity on chondrocyte proliferation, ECM secretion and the maintenance of the chondrocyte phenotype. Gelatin solutions of different viscosities supported chondrocyte proliferation and ECM production. However, the cell morphology, proliferation rate, secreted ECM quantity and gene expression levels were different, and these were dependent on the viscosity of the gelatin solutions. Low-viscosity solutions were more beneficial for proliferation, while high-viscosity solutions were more beneficial for ECM production and the expression of collagen type II and aggrecan. Chondrocytes had a more spread morphology in a low-viscosity gelatin solution than in a high-viscosity gelatin solution. The results suggested that high-viscosity was more beneficial for the maintenance of the chondrocyte phenotype, while low viscosity was more beneficial for cell expansion. Viscosity was demonstrated as one of the key parameters affecting cell morphology, proliferation and phenotype.