Controlled Dual Growth Factor Delivery From Microparticles Incorporated Within Human Bone Marrow‐Derived Mesenchymal Stem Cell Aggregates for Enhanced Bone Tissue Engineering via Endochondral Ossification

A microparticle‐based growth factor delivery system was engineered to drive endochondral ossification within human bone marrow‐derived mesenchymal stem cell (hMSC) aggregates. Compared with cell‐only aggregates treated with exogenous growth factors, aggregates with incorporated transforming growth factor‐β1‐ and BMP‐2‐loaded microparticles exhibited enhanced chondrogenesis and alkaline phosphatase activity and a greater degree of mineralization. This microparticle‐incorporated system has potential as a readily implantable therapy for healing bone defects without the need for long‐term in vitro chondrogenic priming. Bone tissue engineering via endochondral ossification has been explored by chondrogenically priming cells using soluble mediators for at least 3 weeks to produce a hypertrophic cartilage template. Although recapitulation of endochondral ossification has been achieved, long‐term in vitro culture is required for priming cells through repeated supplementation of inductive factors in the media. To address this challenge, a microparticle‐based growth factor delivery system was engineered to drive endochondral ossification within human bone marrow‐derived mesenchymal stem cell (hMSC) aggregates. Sequential exogenous presentation of soluble transforming growth factor‐β1 (TGF‐β1) and bone morphogenetic protein‐2 (BMP‐2) at various defined time courses resulted in varying degrees of chondrogenesis and osteogenesis as demonstrated by glycosaminoglycan and calcium content. The time course that best induced endochondral ossification was used to guide the development of the microparticle‐based controlled delivery system for TGF‐β1 and BMP‐2. Gelatin microparticles capable of relatively rapid release of TGF‐β1 and mineral‐coated hydroxyapatite microparticles permitting more sustained release of BMP‐2 were then incorporated within hMSC aggregates and cultured for 5 weeks following the predetermined time course for sequential presentation of bioactive signals. Compared with cell‐only aggregates treated with exogenous growth factors, aggregates with incorporated TGF‐β1‐ and BMP‐2‐loaded microparticles exhibited enhanced chondrogenesis and alkaline phosphatase activity at week 2 and a greater degree of mineralization by week 5. Staining for types I and II collagen, osteopontin, and osteocalcin revealed the presence of cartilage and bone. This microparticle‐incorporated system has potential as a readily implantable therapy for healing bone defects without the need