Geometry of carriers controlling phenotypic expression in BMP-induced osteogenesis and chondrogenesis

The effect of the geometry of extracellular matrices on bone morphogenetic protein (BMP)-induced osteogenesis has not been systematically studied. Geometry is crucially important for the scaffold in bone and joint tissue engineering. The purpose of this study was to elucidate principles of geometry of matrices in designing new scaffolds and matrices for use in reconstruction of bone and joints.

More than ten biomaterials with different geometries, including a unique device of honeycomb-shaped hydroxyapatite, were combined with BMPs of recombinant (rhBMP-2) or natural bovine origin (S300 BMP cocktail) and implanted subcutaneously into 4-week-old Wistar-King rats. The implanted pellets were removed at 1-4 weeks and analyzed for bone and cartilage formation by histological and biochemical methods.

BMP-induced bone and cartilage induction was highly dependent on the geometric properties of the carrier. Some carriers such as porous particles or blocks of hydroxyapatite induced osteogenesis directly, without detectable chondrogenesis, whereas other carriers such as fibrous glass membrane induced cartilage exclusively. Still other carriers induced mostly cartilage followed by bone formation. Solid particles of hydroxyapatite and fibrous glass membrane with too tight a meshwork did not induce bone or cartilage. The optimal pore size for bone-forming efficacy in porous blocks of hydroxyapatite was a diameter of 300-400 microm. In straight tunnel structures with various diameters in honeycomb-shaped hydroxyapatite, tunnels with smaller diameters (90-120 microm) induced cartilage followed by bone formation, whereas those with larger diameters (350 microm) induced bone formation directly within the tunnels.

BMP carriers were classified into three types: bone-inducing, cartilage-inducing, and cartilage-bone-inducing. From the analysis of causative factors inducing osteogenesis and chondrogenesis in the BMP system, we concluded that the geometry of the carrier is crucially important and vasculature-inducing geometry should be considered in designing effective scaffolds for bone formation. We propose a classification of geometry of the artificial extracellular matrices that is useful for designing a scaffold for tissue engineering of bone and related tissues.

Conventional requisites of the BMP carriers for clinical use have mainly concerned the affinities of carriers with cells and biomolecules and their mechanical strength. The vasculature-inducing geometry of carriers adds a new criterion in designing systems for effective bone and joint reconstruction. The geometries of porous structures-their sizes, continuity, and straightness as verified by hydroxyapatite in this study-will be applicable for other biomaterials for clinical reconstruction therapy.