Pore Size of Porous Hydroxyapatite as the Cell-Substratum Controls BMP-Induced Osteogenesis

Abstract

To elucidate the biochemical mechanism of osteogenesis, the effect of matrix geometry upon the osteogenesis induced by bone morphogenetic protein (BMP) was studied. A series of five porous hydroxyapatites with different pore sizes, 106-212, 212-300, 300-400, 400-500, and 500-600 microns, was prepared. A block (approximately 5 x 5 x 1 mm, 40.0 mg) of each hydroxyapatite ceramics was combined with 4 micrograms of recombinant human BMP-2 and implanted subcutaneously into the back skin of rat. Osteoinductive ability of each implant was estimated by quantifying osteocalcin content and alkaline phosphatase activity in the implant up to 4 wk after implantation. In the ceramics of 106-212 microns, the highest alkaline phosphatase activity was found 2 wk after implantation, and the highest osteocalcin content 4 wk after implantation, consistent with the results observed with particulate porous hydroxyapatite [Kuboki, Y. et al. (1995) Connect. Tissue Res. 32: 219-226]. Comparison of the alkaline phosphatase activities at 2 wk and the osteocalcin contents at 4 wk after implantation revealed that the highest amount of bone was produced in the ceramics implants with pore size of 300-400 microns. In the ceramics with smaller or larger pore sizes, the amount of bone formation decreased as the pore size deviated from 300-400 microns. The results indicated that the optimal pore size for attachment, differentiation and growth of osteoblasts and vascularization is approximately 300-400 microns. This study using chemically identical but geometrically different cell substrata is the first demonstration that a matrix with a certain geometrical size is most favorable for cell differentiation.

Synthetic biodegradable polymers as drug delivery systems for bone morphogenetic proteins

“…Osteoconductive delivery systems have included collagenous materials, such as type I collagen (as sponges, gels, or fibrils) [19,[26][27][28][29][30], and type IV collagen [31,32]; inorganic ceramic materials, such as hydroxyapatite (HA) (as a powder, granules, or blocks) [33,34], tricalcium phosphate (TCP) [35], glass ceramic, and other inorganic materials; cartilage-or bone-derived materials, such as coral, chitin, and bone mineral,; and composites of different types of these materials [20]. BMP have also been used in combination with titanium and other metal alloys [36].…”

The effect of pore size on cell adhesion in collagen-GAG scaffolds

“…The scaffold pore structure has been observed to significantly affect cell binding and migration in vitro and influence the rate and depth of cellular ingrowth into the scaffold in vitro and in vivo [7,8]. Additionally, cell adhesion and activity has been observed to vary considerably depending on the cell type, as well as the scaffolds composition and pore size [9][10][11]. In porous silicon nitride scaffolds, endothelial cells bind only to pores smaller than 80 µm while fibroblasts preferentially bind to larger pores (>90 µm).…”

Novel Freeze-Drying Methods to Produce a Range of Collagen–Glycosaminoglycan Scaffolds with Tailored Mean Pore Sizes

“…The pore architecture of these scaffolds has been shown to have a significant effect on both physical properties and cellular activity. [1][2][3][4][5] It has been hypothesized that the pore diameter must be large enough to allow infiltration of the cells toward the center of the scaffold, while being small enough to present sufficient ligand density for cellular attachment. 4,6 In addition, the pore size also determines permeability, which in turn influences the diffusion of nutrients and waste products within the scaffolds as well as the stimulus applied by flow perfusion bioreactors.…”

“…Endothelial cells, for example, show favorable attachment to pores in the range of 20-80 mm, whereas osteoblasts require pores larger than 100 mm for bone formation. 2,3,8,9 Therefore, for a particular scaffold to be suitable for multiple applications, the ability to alter the pore size over a wide range is essential.…”