Cartilage tissue engineering aims to combine cells, biomaterials, and bioactive factors to create transplantable and mechanically stable cartilage-like tissue. Chondrocytes are surrounded by and interact with a supportive pericellular matrix (PCM), rich in basement membrane (BM) proteins. However, chondrocytes are deprived of their PCM during cell harvest and culture, impeding the cells’ functions and, in particular their ability to withstand mechanical stimulation. To test the importance of BM proteins on in vitro chondrogenesis and on the cellular response to mechanical stimulation, we have cultured human articular chondrocytes in photo-crosslinkable 1.5% w/v alginate hydrogel mixed with BM proteins (growth factor-reduced BD Matrigel) at 0.045%, 0.15% and 0.45% w/v with and without stimulation in a compression bioreactor (10% strain, 1 Hz) for 30 min.
Matrigel proteins predominantly formed a pericellular layer around cells as shown by flow cytometry and fluorescence microscopy. Cell viability, glycosaminoglycan content and mRNA levels of chondrogenic marker genes were higher in gels containing low amounts of BM at day 14 of culture. However, at 0.45% w/v, BM proteins induced cell spreading and hampered chondrogenesis leading to down-regulation of aggrecan as well as up-regulation of COL1A1. Compressive loading one day after embedding chondrocytes in alginate alone led to a massive decrease of cartilage matrix gene expression, without affecting cell viability. In contrast, mRNA levels for chondrogenic markers were not different from free-swelling controls in compressed gels containing 0.15% w/v Matrigel or after 7 days of pre-culture.
Exogenous BM proteins support re-differentiation of chondrocytes in hydrogels only at low concentrations and help to prevent the detrimental effects of pre-mature loading. Our results point to an important role of BM proteins in chondrocyte resistance to injurious loading and suggest the use of BM matrices in cartilage repair strategies that employ cells without functional PCM.