Endochondral ossification (ECO), the major ossification process during embryogenesis and bone repair, involves the formation of a cartilaginous template remodelled into a functional bone organ. Adipose-derived stromal cells (ASC), non-skeletal multipotent progenitors from the stromal vascular fraction (SVF) of human adipose tissue, were shown to recapitulate ECO and generate bone organs in vivo when primed into a hypertrophic cartilage tissue (HCT) in vitro. However, the reproducibility of ECO was limited and the major triggers remain unknown. We studied the effect of the expansion of cells and maturation of HCT on the induction of the ECO process. SVF cells or expanded ASC were seeded onto collagen sponges, cultured in chondrogenic medium for 3-6 weeks and implanted ectopically in nude mice to evaluate their bone-forming capacities. SVF cells from all tested donors formed mature HCT in 3 weeks whereas ASC needed 4-5 weeks. A longer induction increased the degree of maturation of the HCT, with a gradually denser cartilaginous matrix and increased mineralization. This degree of maturation was highly predictive of their bone-forming capacity in vivo, with ECO achieved only for an intermediate maturation degree. In parallel, expanding ASC also resulted in an enrichment of the stromal fraction characterized by a rapid change of their proteomic profile from a quiescent to a proliferative state. Inducing quiescence rescued their chondrogenic potential. Our findings emphasize the role of monolayer expansion and chondrogenic maturation degree of ASC on ECO and provides a simple, yet reproducible and effective approach for bone formation to be tested in specific clinical models.

Alginate-hydroxyapatite (AL) scaffolds modified with fibronectin (FN) or bioactive glass (BGMS10) have recently been characterized for their physicochemical properties and proposed as promising candidates for bone regeneration. Here, we present their first systematic biological evaluation, focusing on adhesion, proliferation, osteogenic differentiation, and in vivo host response. We compared FN-, BG-, and unmodified AL scaffolds using an immortalized mesenchymal stromal cell line (M-SOD) and primary human bone marrow-derived (BM-MSCs) and adipose-derived stromal cells (ASCs). FN scaffolds enhanced initial adhesion across all cell types and supported proliferation in M-SODs, but primary BM-MSCs and ASCs showed minimal expansion, regardless of scaffold type. BG scaffolds promoted expression of late-stage osteogenic markers in BM-MSCs, consistent with their ion release profile, but had limited impact on ASCs. In vivo subcutaneous implantation of acellular scaffolds in nude mice revealed robust host cell infiltration and extracellular matrix deposition across all scaffold types, confirming biocompatibility and integration. However, vascularization remained limited and did not differ substantially between formulations. Together, these findings highlight a critical discrepancy between immortalized and primary stromal cell responses to scaffold cues, underscoring the choice of cell source when evaluating the biocompatibility of a novel scaffold. At the same time, the effective in vivo integration observed across scaffold types emphasizes the importance of host tissue responses for translational evaluation of functional biomaterials.

Developmental tissue engineering (TE) strategies recapitulating endochondral ossification (ECO)—the major ossification pathway in bone development and repair– are a promising avenue for the treatment of critical size and congenital bone defects. In this work, we develop a customizable approach using adipose stromal cells (ASC)‐derived cartilage organoids as building blocks to generate clinically relevant grafts. Our hypothesis is that progenitor cells rather than mature chondrocytes would enable robust cartilage organoids fusion allowing graft tunability and scaling up. Using a cyclic approach alternating chondrogenic and proliferative cues we produce cartilage organoids surrounded by stromal chondrogenic progenitors. When re‐exposed to chondrogenic medium, this perichondrial layer forms new cartilage tissue and acts as a biological cement in between cartilage organoids. A key feature of our approach is the iterative aspect of the protocol, where scaled up cartilage tissues obtained can themselves be used as building blocks to create larger tissue. Finally, in vivo, these grafts remodel efficiently into functional and mechanically apt bone organs mirroring ECO during skeletal development over the course of 24 weeks. Collectively, our findings provide a strong proof of concept of the envisioned TE strategy paving the way for a clinical application in the near future.