Endochondral ossification for enhancing bone regeneration: converging native extracellular matrix biomaterials and developmental engineering in vivo

Abstract

Autologous bone grafting (ABG) remains entrenched as the gold standard of treatment in bone regenerative surgery. Consequently, many marginally successful bone tissue engineering strategies have focused on mimicking portions of ABG's "ideal" osteoconductive, osteoinductive, and osteogenic composition resembling the late reparative stage extracellular matrix (ECM) in bone fracture repair, also known as the "hard" or "bony" callus. An alternative, less common approach that has emerged in the last decade harnesses endochondral (EC) ossification through developmental engineering principles, which acknowledges that the molecular and cellular mechanisms involved in developmental skeletogenesis, specifically EC ossification, are closely paralleled during native bone healing. EC ossification naturally occurs during the majority of bone fractures and, thus, can potentially be utilized to enhance bone regeneration for nearly any orthopedic indication, especially in avascular critical-sized defects where hypoxic conditions favor initial chondrogenesis instead of direct intramembranous ossification. The body's native EC ossification response, however, is not capable of regenerating critical-sized defects without intervention. We propose that an underexplored potential exists to regenerate bone through the native EC ossification response by utilizing strategies which mimic the initial inflammatory or fibrocartilaginous ECM (i.e., "pro-" or "soft" callus) observed in the early reparative stage of bone fracture repair. To date, the majority of strategies utilizing this approach rely on clinically burdensome in vitro cell expansion protocols. This review will focus on the confluence of two evolving areas, (1) native ECM biomaterials and (2) developmental engineering, which will attempt to overcome the technical, business, and regulatory challenges that persist in the area of bone regeneration. Significant attention will be given to native "raw" materials and ECM-based designs that provide necessary osteo- and chondro-conductive and inductive features for enhancing EC ossification. In addition, critical perspectives on existing stem cell-based therapeutic strategies will be discussed with a focus on their use as an extension of the acellular ECM-based designs for specific clinical indications. Within this framework, a novel realm of unexplored design strategies for bone tissue engineering will be introduced into the collective consciousness of the regenerative medicine field.

FIG. 1.

Both IM and EC ossification occurs during the bone-healing process in three overlapping regeneration phases. Critical-sized bone defects favor EC ossification over direct IM ossification, primarily due to the large, avascular nature of these defects. Four ECM-based biomaterial strategies (boxes) are highlighted with regard to bone regeneration. The intermediate fibrocartilaginous ECM remains underexplored (**) as a strategy to potentially enhance quality and extent of EC ossification and overall bone regeneration. ABG, autologous bone grafting; EC, endochondral; ECM, extracellular matrix; IM, intramembranous. Color images available online at www.liebertpub.com/teb

FIG. 2.

Current landscape of EC ossification strategies (blue boxes) along with underexplored design space (dashed outlines) highlighted in this review. Some combined cell and material strategies are not shown explicitly (dashed arrows). This includes several in vitro priming studies that utilized cell and material approaches (**) and are further explored elsewhere (Tables 2–4). All strategies will be critically evaluated with regard to technical, regulatory, and commercial challenges. ABG, autologous bone grafting; ADSCs, adipose-derived stem cells; BMA, bone marrow aspirate; DBM, demineralized bone matrix; DCC, decellularized cartilage; GMP, good manufacturing practice. Color images available online at www.liebertpub.com/teb

FIG. 3.

Top-down approaches to mimic native ECM tissue require less processing before implantation and retain more of the ECM's physicochemical composition, structure, and function compared with bottom-up re-synthesis approaches. Color images available online at www.liebertpub.com/teb