Guiding Chondrogenesis and Osteogenesis with Mineral-Coated Hydroxyapatite and BMP-2 Incorporated within High-Density hMSC Aggregates for Bone Regeneration

Since hydroxyapatite and bone morphogenetic protein-2 (BMP-2) can regulate chondrogenesis and osteogenesis, their individual and combined effects on endochondral ossification within human bone marrow-derived stem cell (hMSC) aggregates were investigated. Hydroxyapatite was presented in the form of mineral-coated hydroxyapatite microparticles (MCM) capable of controlled BMP-2 delivery. Aggregates were treated with varied BMP-2 concentrations supplemented in the media and loaded onto MCM to examine the influence of BMP-2 amount and spatial presentation on regulating chondrogenesis and osteogenesis. MCM alone induced GAG and type II collagen production by week 5 for two of three donors, and BMP-2 may have accelerated MCM-induced chondrogenesis. ALP activity and calcium content of cells-only aggregates suggest that the BMP-2-induced osteogenic response may be concentration-dependent. Treatment with MCM and BMP-2 resulted in chondrogenesis as early as week 2, which may have promoted additional mineralization by week 5, suggesting the induction of endochondral ossification. Released BMP-2 had similar if not higher levels of bioactivity compared to that of exogenous BMP-2 with regard to chondrogenesis and osteogenesis. In addition to providing localized and sustained BMP-2 delivery, MCM incorporation within aggregates yields a self-sustaining system that may be injected or implanted more rapidly to heal bone defects through endochondral ossification without extended in vitro culture.