Dense fibrillar collagen matrices sustain osteoblast phenotype in vitro and promote bone formation in rat calvaria defect.

Two pure collagen materials were prepared from acidic collagen solutions at 5 and 40 mg/mL. Benefits of collagen concentration on bone repair were evaluated in vitro with human calvaria cells and in vivo in a rat cranial defect. Both materials exhibited specific structures, 5 mg/mL was soft with an open porous network of fibrils; 40 mg/mL was stiffer with a plugged surface and bundles of collagen fibrils. Osteoblasts seeded on 5 mg/mL formed an epithelioid layer with ultrastructural characteristics of mature osteoblasts and induced mineralization. Numerous osteoblasts migrated inside 5 mg/mL, triggering reorganization of their actin cytoskeleton, whereas on 40 mg/mL osteoblasts remained in a resting state. In rat calvaria defects, both materials induced active bone formation. Dual-energy X-ray absorption bone area measures after 4 weeks averaged 84.0% with 5 mg/mL, 88.4% with 40 mg/mL, and 36.7% in the controls (p < 0.05). Tartrate-resistant acid phosphatase-positive giant cells releasing amounts of metalloproteinase-2 progressively degraded the implants at 76.5% with 5 mg/mL and 38.2% with 40 mg/mL (p < 0.05), whereas alkaline phosphatase-positive osteoprogenitors invaded collagen remnant. Hence, the dense structure of collagen materials allowed cell invasion and raise their mechanical behavior without addition of chemical cross-linkers. Collagen concentration can be tuned to form 3D matrices for in vitro investigations or to fit degradation rate to different bone repair purposes.