Effect of gradient biomineral concentrations on osteogenic and chondrogenic differentiation of adipose derived stem cells

Bioinspired gradient scaffolds for osteochondral tissue engineering

Repairing articular osteochondral defects present considerable challenges in self-repair due to the complex tissue structure and low proliferation of chondrocytes. Conventional clinical therapies have not shown significant efficacy, including microfracture, autologous/allograft osteochondral transplantation, and cell-based techniques. Therefore, tissue engineering has been widely explored in repairing osteochondral defects by leveraging the natural regenerative potential of biomaterials to control cell functions. However, osteochondral tissue is a gradient structure with a smooth transition from the cartilage to subchondral bone, involving changes in chondrocyte morphologies and phenotypes, extracellular matrix components, collagen type and orientation, and cytokines. Bioinspired scaffolds have been developed by simulating gradient characteristics in heterogeneous tissues, such as the pores, components, and osteochondrogenesis-inducing factors, to satisfy the anisotropic features of osteochondral matrices. Bioinspired gradient scaffolds repair osteochondral defects by altering the microenvironments of cell growth to induce osteochondrogenesis and promote the formation of osteochondral interfaces compared with homogeneous scaffolds. This review outlines the meaningful strategies for repairing osteochondral defects by tissue engineering based on gradient scaffolds and predicts the pros and cons of prospective translation into clinical practice.

Low-Temperature Synthesis of Hollow β-Tricalcium Phosphate Particles for Bone Tissue Engineering Applications

β-Tricalcium phosphate (β-TCP) has been extensively used in bone tissue engineering in the form of scaffolds, granules, or as reinforcing phase in organic matrices. Solid-state reaction route at high temperatures (>1000 °C) is the most widely used method for the preparation of β-TCP. The high-temperature synthesis, however, results in the formation of hard agglomerates and fused particles which necessitates postprocessing steps such as milling and sieving operations. This, inadvertently, could lead to introducing unwanted trace elements, promoting particle shape irregularity as well as compromising the biodegradability and bioactivity of β-TCP because of the solid microstructure of particles. In this study, we introduce a one-pot wet-chemical method at low temperatures (between 160 and 170 °C) to synthesize hollow β-TCP (hβ-TCP) submicron particles of an average size of 300 nm with a uniform rhombohedral shape. We assessed the cytocompatibility of the hβ-TCP using primary human osteoblasts (HOB), adipose-derived stem cells (ADSC), and antigen-presenting cells (APCs). We demonstrate the bioactivity of the hβ-TCP when cultured with HOB, ADSC, and APCs at a range of particle concentrations (up to 1000 μg/mL) for up to 7 days. hβ-TCP significantly enhances osteogenic differentiation of ADSC without the addition of osteogenic supplements. These findings offer a new type of β-TCP particles prepared at low temperatures, which present various opportunities for developing β-TCP based biomaterials.

In Situ Generation of Hydroxyapatite on Biopolymer Particles for Fabrication of Bone Scaffolds Owning Bioactivity

Hydroxyapatite (HAP) can endow a biopolymer scaffold with good bioactivity and osteoconductive ability, while the interfacial bonding is fairly weak between HAP and biopolymers. In this study, HAP was in situ generated on poly(l-lactic acid) (PLLA) particles, and then they were used to fabricate a scaffold by selective laser sintering. Detailedly, PLLA particles were first functionalized by dopamine oxide polymerization, which introduced abundance active catechol groups on the particle surface, and subsequently, the catechol groups concentrated Ca²⁺ ions by chelation in a simulated body fluid solution, and then, Ca²⁺ ions absorbed PO₄^3- ions through electrostatic interactions for in situ nucleation of HAP. The results indicated that HAP was homogeneously generated on the PLLA particle surface, and HAP and PLLA exhibited good interfacial bonding in the HAP/PLLA scaffolds. Meanwhile, the scaffolds displayed excellent bioactivity by inducing apatite precipitation and provided a good environment for human bone mesenchymal stem cell attachment, proliferation, and osteogenic differentiation. More importantly, the ingrowth of blood vessel and the formation of new bone could be stimulated by the scaffolds in vivo, and the bone volume fraction and bone mineral density increased by 44.44 and 41.73% compared with the pure PLLA scaffolds, respectively. Serum biochemical indexes fell within the normal range, which indicated that there was no harmful effect on the normal functioning of the body after implanting the scaffold.

Mineral Distribution Spatially Patterns Bone Marrow Stromal Cell Behavior on Monolithic Bone Scaffolds

Interfaces between soft tissue and bone are characterized by transitional gradients in composition and structure that mediate substantial changes in mechanical properties. For interfacial tissue engineering, scaffolds with mineral gradients have shown promise in controlling osteogenic behavior of seeded bone marrow stromal cells (bMSCs). Previously, we have demonstrated a 'top-down' method for creating monolithic bone-derived scaffolds with patterned mineral distributions similar to native tissue. In the present work, we evaluated the ability of these scaffolds to pattern osteogenic behavior in bMSCs in basic, osteogenic, and chondrogenic biochemical environments. Immunohistochemical (IHC) and histological stains were used to characterize cellular behavior as a function of local mineral content. Alkaline phosphatase, an early marker of osteogenesis, and osteocalcin, a late marker of osteogenesis, were positively correlated with mineral content in basic, osteogenic, and chondrogenic media. The difference in bMSC behavior between the mineralized and demineralized regions was most pronounced in an basic biochemical environment. In the mineralized regions of the scaffold, osteogenic markers were clearly present as early as 4 days in culture. In osteogenic media, osteogenic behavior was observed across the entire scaffold, whereas in chondrogenic media, there was an overall reduction in osteogenic biomarkers. Overall, these results indicate local mineral content of the scaffold plays a key role in spatially patterning bMSC behavior. Our results can be utilized for the development of interfacial tissue engineered scaffolds and understanding the role of local environment in determining bMSC behavior. STATEMENT OF SIGNIFICANCE: Soft tissue-to-bone interfaces, such as tendon-bone, ligament-bone, and cartilage-bone, are ubiquitous in mammalian musculoskeletal systems. These interfacial tissues have distinct, hierarchically-structured gradients of cellular, biochemical, and materials components. Given the complexity of the biological structures, interfacial tissues present unique challenges for tissue engineering. Here, we demonstrate that material-derived cues can spatially pattern osteogenic behavior in bone marrow stromal cells (bMSCs). Specifically, we observed that when the bMSCs are cultured on bone-derived scaffolds with mineral gradients, cells in contact with higher mineral content display osteogenic behavior at earlier times than those on the unmineralized substrate. The ability to pattern the cellular complexity found in native interfaces while maintaining biologically relevant structures is a key step towards creating engineered tissue interfaces.