Scaffold-free bioprinted osteogenic and chondrogenic systems to model osteochondral physiology

Abstract

Three-dimensional bioprinted culture platformsmimic the nativemicroenvironment of tissuesmore accurately than two-dimensional cell cultures or animalmodels. Scaffold-free bioprinting eliminates many complications associatedwith traditional scaffold-dependent printing as well as provides better cell-to-cell interactions and long-termfunctionality. In this study, constructs were produced frombone marrowderivedmesenchymal stemcells (BM-MSCs) using a scaffold-free bioprinter. These constructs were cultured in either osteogenic, chondrogenic, a 50:50mixture of osteogenic and chondrogenic ('osteo-chondro'), orBM-MSCgrowthmedium. Osteogenic and chondrogenic differentiation capacity was determined over an 8-week culture period using histological and immunohistochemical staining andRT-qPCR(Phase I).After 6weeks in culture, individual osteogenic and chondrogenic differentiated constructs were adhered to create a bone-cartilage interactionmodel.Adhered differentiated constructs were cultured for an additional 8 weeks in either chondrogenic or osteo-chondromediumto evaluate sustainability of lineage specification and transdifferentiation potential (Phase II).Constructs cultured in their respective osteogenic and/or chondrogenicmediumdifferentiated directly into bone (model of intramembranous ossification) or cartilage. Positive histological and immunohistochemical staining for bone or cartilage identification was shownafter 4 and 8 weeks in culture. Expression of osteogenesis and chondrogenesis associated genes increased betweenweeks 2 and 6. Adhered individual osteogenic and chondrogenic differentiated constructs sustained their differentiated phenotypewhen cultured in chondrogenicmedium.However, adhered individual chondrogenic differentiated constructs cultured in osteo-chondromediumwere converted to bone (model ofmetaplastic transformation). These bioprintedmodels of bone-cartilage interaction, intramembranous ossification, andmetaplastic transformation of cartilage into bone offer a useful and promising approach for bone and cartilage tissue engineering research. Specifically, thesemodels can be potentially used as functional tissue systems for studying osteochondral defect repair, drug discovery and response, andmany other potential applications.