Model of contact coculture between cells and subchondral bone explants. BMSC, mixed cultures (BMSC + chondrocytes 1:1) and chondrocytes were embedded in fibrin gel and these cell-gel constructs were applied onto the surface of subchondral bone explants (co- and tricultures). As controls, cells were embedded in fibrin gels and cultured without subchondral bone (monocultures). All experimental setups were kept for up to 28 days in chondrogenic medium. Samples were harvested at days 7 and 28. BMSC, bone marrow-derived mesenchymal stem cells; F, fibrin gel-embedded monocultures (without subchondral bone explants); FB, fibrin gel-embedded co- and tricultures together with subchondral bone explants.