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Osteogenesis is a complex process that involves the synergistic contribution of multiple cell types and numerous growth factors (GFs). To develop effective bone tissue engineering strategies employing GFs, it is essential to delineate the complex and interconnected role of GFs in osteogenesis. The studies investigating the temporal involvement of GFs in osteogenesis are limited to in vitro studies with single cell types or complex in vivo studies. There is a need for platforms that embody the physiological characteristics and the multicellular environment of natural osteogenesis. Marrow tissue houses various cell types that are known to be involved in osteogenesis, and in vitro cultures of marrow inherently undergo osteogenesis process. Self-inductive ossification of marrow explants in vitro can be employed as a representative multicellular and three-dimensional model of osteogenesis. Therefore, the aims of this study were to employ the rat bone marrow explant ossification model to determine (1) the temporal production profiles of key GFs involved in osteogenesis, (2) the relation between GF production and ossification, and (3) the relations between the GF levels throughout ossification. Temporal production profiles of transforming GF beta-1 (TGF-beta1), bone morphogenetic protein-2 (BMP-2), vascular endothelial GF (VEGF), and insulin-like GF-1 (IGF-1) and the bone-related proteins alkaline phosphatase and osteocalcin were obtained by enzyme-linked immunosorbent assays conducted at days 2, 7, 12, 14, 19, and 21. The final amount of ossification (ossified volume [OV]) was measured by microcomputed tomography at day 21. TGF-beta1, BMP-2, VEGF, IGF-1, alkaline phosphatase, and osteocalcin were produced by the ossifying marrow explants differentially over time. The early production of IGF-1 (day 2) correlated positively (r = 0.868) with OV; however, latent production of IGF-1 correlated negatively (day 14: r = -0.813; day 19: r = -0.865) with OV. OV also correlated with VEGF levels at day 12 (r = 0.988) and at day 14 (r = 0.970). Production of GFs also correlated to each other across time points, which indicates the complex and interconnected contribution of various GFs in osteogenesis. Therefore, tissue engineering strategies toward bone regeneration should consider the richness of GFs involved in osteogenesis and their dynamically varying participation over time.

Microdamage occurs in trabecular bone under normal loading, which impairs the mechanical properties. Architectural degradation associated with osteoporosis increases damage susceptibility, resulting in a cumulative negative effect on the mechanical properties. Treatments for osteoporosis could be targeted toward increased bone mineral density, improved architecture, or repair and prevention of microdamage. Delineating the relative roles of damage and architectural degradation on trabecular bone strength will provide insight into the most beneficial targets. In this study, damage was induced in bovine trabecular bone samples by axial compression, and the effects on the mechanical properties in shear were assessed. The damaged shear modulus, shear yield stress, ultimate shear stress, and energy to failure all depended on induced damage and decreased as the architecture became more rod-like. The changes in ultimate shear strength and toughness were proportional to the decrease in shear modulus, consistent with an effective decrease in the cross-section of trabeculae based on cellular solid analysis. For typical ranges of bone volume fraction in human bone, the strength and toughness were much more sensitive to decreased volume fraction than to induced mechanical damage. While ultimately repairing or avoiding damage to the bone structure and increasing bone density both improve mechanical properties, increasing bone density is the more important contributor to bone strength.

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Background Collagen-based scaffolds reinforced with hydroxyapatite (HA) are an attractive choice for bone tissue engineering because their composition mimics that of bone. We previously reported the development of compression-molded collagen-HA scaffolds that exhibited high porosity, interconnected pores, and mechanical properties that were well-suited for surgical handling and fixation. The objective of this study was to investigate these novel collagen-HA scaffolds in combination with human adipose-derived stem cells (hASCs) as a template for bone formation in a subcutaneous athymic mouse model. Methods Collagen-HA scaffolds and collagen-only scaffolds were fabricated as previously described, and a clinically approved bone void filler was used as a control for the material. Constructs were seeded with hASCs and were pre-treated with either control or osteogenic media. A cell-free group was also included. Scaffolds were implanted subcutaneously in the backs of athymic nude mice for 8 weeks. Mineral deposition was quantified via micro-computed tomography. Histological and immunofluorescence images of the explants were used to analyze their vascular invasion, remodeling and cellularity. Results Cell-free collagen-HA scaffolds and those that were pre-seeded with osteogenically differentiated hASCs supported mineral deposition and vascular invasion at comparable rates, while cell-seeded constructs treated with the control medium showed lower mineralization after implantation. HA-reinforcement allowed collagen constructs to maintain their shape, provided improved cell-tissue-scaffold integration, and resulted in a more organized tissue when pre-treated in an osteogenic medium. Scaffold type and pre-treatment also determined osteoclast activity and therefore potential remodeling of the constructs. Conclusions The results of this study cumulatively indicate that treatment medium and scaffold composition direct mineralization and angiogenic tissue formation in an ectopic model. The data suggest that it may be necessary to match the scaffold with a particular cell type and cell-specific pre-treatment to achieve optimal bone formation.