Functional callus organoids reveal distinct cartilage to bone transition mechanisms across donors and a role for biological sex
Engineering a cartilage tissue intermediate in vitro that closely mimics the characteristics of the native fracture callus in vivo, ensures a high potency to undergo cartilage-to-bone transition when implanted in vivo. However, there is a lack of critical quality attributes that can be defined in vitro to predict in vivo functionality across donors, which is a significant limitation for the development of autologous tissue engineered implants according to Quality by Design (QbD) principles. In addition, for clinical translation, regulatory bodies require in-process biological data acquisition, preferably noninvasive, to enable closed system manufacturing.  In this study, we produced cartilaginous callus organoids of 10 human donors, both male and female, assembled scaffold-free donor-specific cartilaginous implants. We found two distinct organoid morphologies and transcriptome signatures, with 6 donors (5 male, 1 female) generating hypertrophic cartilage and 4 female donors producing organoids with a fibrocartilage phenotype. This distinction was already detectable early in the differentiation process with differences in proliferation and organoid morphology, leading to differential regulation of gene panels specific for reported periosteal progenitor populations. Further, we compared the secretome of bone-forming and non-bone-forming organoids, identifying a robust panel of 83 proteins that could be measured noninvasively as potency monitoring biomarkers.  These findings provide new insights in donor variability and outlines a quality-driven pathway for the translation of organoid-based tissue engineered implants. The addition of noninvasive quality metrics paves the way towards data-driven closed-system manufacturing of patient-specific living implants. Overall design: To investigate the requirements for bone formation, periosteum derived cells from 10 donors were chondrogenically cultured as organoids for 21 days and implanted ectopically in nude mice. As negative control, 5 donors were cultured as organoids for 21 days in basal medium.