Injectable osteogenic microtissues containing mesenchymal stromal cells conformally fill and repair critical-size defects

Abstract

<150 Words):Repair of complex fractures with bone loss requires a potent, space-filling intervention to promote regeneration of bone. We present a minimally-invasive strategy combining mesenchymal stromal cells (MSC) with a chitosan-collagen matrix to form modular microtissues designed for delivery through a needle to conformally fill cavital defects. Implantation of microtissues into a calvarial defect in the mouse showed that osteogenically pre-differentiated MSC resulted in complete bridging of the cavity, while undifferentiated MSC produced mineralized tissue only in apposition to native bone. Decreasing the implant volume reduced bone regeneration, while increasing the MSC concentration also attenuated bone formation, suggesting that the cell-matrix ratio is important in achieving a robust response. Conformal filling of the defect with microtissues in a carrier gel resulted in complete healing. Taken together, these results show that modular microtissues can be used to augment the differentiated function of MSC and provide an extracellular environment that potentiates bone repair. Introduction:Bone has a remarkable capacity to regenerate through carefully orchestrated, cell-mediated repair processes [1]. However, healing in large and complex fractures is often impaired, leading to incomplete or functionally inferior bone regeneration. In some wounds, loss of the native vasculature and infection of the wound bed can further impair bone regeneration, resulting in a variety of pathologies, including delayed-, mal-, and non-unions. In such cases, therapeutic intervention to stimulate and accelerate the healing response is required. Bone grafting is a standard approach to this problem but needs invasive surgery to harvest and deliver the graft.Autologous grafts and flaps are limited in supply and can cause donor-site morbidity, including infection, hematoma, and pain [2]. Moreover, they are not suitable in 10-30% of cases due to difficulty in conforming the graft to the shape of the defect [3]. Allogeneic decellularized grafts and synthetic ceramic substitutes can also be used, but are biologically inferior compared to viable bone grafts due to the lack of cellular components. Although processes such as irradiation and lyophilization can reduce the risk of disease transmission from an allogeneic graft, they eliminate cellular components resulting in reduced osteoinductivity [4] and revascularization, resulting in higher bone resorption [5].The ideal bone substitute would exhibit osteoconductive, osteoinductive, and osteogenic properties and would promote concomitant neovascularization of larger defects [4]. Materialsbased approaches have been developed to promote osteoconductivity [1], and the immobilization and release of growth factors can be used as an osteoinductive cue. However, only cells can produce bone, and osteogenesis, therefore, requires either recruitment of endogenous cells or delivery of exogenous cells capable of forming bone. In large and ischemic defects, endogenous cell recruitment is impaired,...

Coupling Osteogenesis and Vasculogenesis in Engineered Orthopedic Tissues

“…Preculture with these biochemical factors has been shown to promote the bone-forming capacity of MSCs in several in vivo bone defect models. [73][74][75] Figure 5 shows results from a study in which osteogenically induced MSCs were compared with controls in a subcutaneous implant in a heterotypic nude mouse model, demonstrating that predifferentiation can significantly potentiate bone formation. 75 The degree and length of osteogenic preculture is also an important variable, which again depends on the therapeutic approach and intended application.…”

Modeling and Mechanical Performance Analysis of Porous Gradient Bone Scaffolds

“…However, when bone defects exceed a speci c size (greater than 2 cm) or when the bone perimeter around the defect exceeds 50%, the self-healing capacity is signi cantly impaired. This may lead to complications such as non-union, abnormal healing, or pathological fractures [5,6], potentially resulting in lifelong disabilities for the patients. Effectively repairing bone defects, alleviating healing burdens on patients, and shortening postoperative recovery periods are currently the primary focuses of orthopedic research.…”