Human osteoclast formation and resorptive function on biomineralized collagen

Biomineralized collagen composite materials pose an intriguing alternative to current synthetic bone graft substitutes by offering a biomimetic composition more similar to native bone. We hypothesized that by creating a composite of fibrillary collagen type-I and hydroxyapatite with organized submicron architecture similar to native bone tissue, this composite could undergo cellular resorption and remodeling similar to that which occurs naturally in bone. To evaluate this, we investigated the formation and activity of human osteoclasts cultured on biomineralized collagen and pure collagen membranes in comparison to cortical bone slices. Human monocytes/macrophages from peripheral blood differentiated into multinucleated, TRAP-positive osteoclast-like cells on all substrates. These cells formed clear actin rings on cortical bone, but were unable to form similarly organized actin structures on biomineralized collagen or pure collagen membranes. Resorption was observed by SEM, and quantified by measuring resorbed area and volume by optical profilometry, as well as by quantification of Ca2+ concentration in cell culture supernatant. Osteoclasts readily formed resorption pits in cortical bone, resulting in higher Ca2+ concentration in the cell culture medium; however, osteoclast resorption of biomineralized collagen and collagen substrates did not measurably occur. Activity of key osteoclast enzymes – TRAP, CA-II, and CTS-K – was similar on all substrates, despite phenotypic differences in actin ring formation and functional resorption. Key material property differences may have prevented formation of stable actin rings and resorption, specifically the mesh-like fibrillary structure, relatively low stiffness, and lack of RGD-containing binding domains of the collagen-based membranes versus cortical bone. This insight can help guide further research toward the optimized design of biomineralized collagen composites as more biomimetic bone-graft substitutes – particularly if they can be readily resorbed by osteoclasts and remodeled into new bone.