This work was supported by a Grant-in-Aid for Scientific Research (KAKENHI) from the Japan Society for the Promotion of Science (JSPS) (grant nos. JP19K10259 and 22K10032 to MAI).

This protocol uses 4-week-old male nude mice. House four mice in a cage under a 12 h light/dark cycle at 22&#8722;24 &#176;C and 50%&#8722;70% relative humidity. All animal experiments were conducted in accordance with the guidelines approved by the Institutional Animal Care and Use Committee of Tokyo Medical and Dental University (approval ID: A2019-204C, A2020-116A, and A2021-121A).
1. Preparation of buffers and reagents
<ol>
	<li>Prepare mesenchymal stem cell growth mediu...

Artificial Cartilage Implants for Bone Regeneration via Endochondral Ossification

Using appropriate scaffold materials that promote the transition from hypertrophic cartilage to bone is a promising approach to scale up MSC-based engineered hypertrophic cartilage grafts and treat bone defects of clinically significant size. Here, we show that HA is an excellent scaffold material to support the differentiation of MSC-based hypertrophic cartilage tissue in vitro and to promote endochondral bone formation in vivo38. Furthermore, in vivo, HA constructs wer...

Autologous bone is still the gold standard for repairing bone defects due to trauma, congenital defects, and surgical resection. However, autogenous bone grafting has significant limitations, including donor pain, risk of infection, and limited bone volume that can be isolated from the patients1,2,3,4. Numerous biomaterials have been developed as bone substitutes, combining natural or synthetic polymers with mineralized materials such as calcium phosphate or hydroxyapatite5,6. Bone formation in these engineered materials is usually achieved using the mineralized material as a priming material to allow stem cells to differentiate directly into osteoblasts through the intramembrane ossification (IMO) process7. This process lacks the angiogenic step, resulting in insufficient in vivo vascularization of the graft after implantation8,9,10, and therefore, approaches using such a process may not be optimal for treating large bone defects11.
Strategies applied to recapitulate the endochondral ossification (ECO) process, an innate mechanism in skeletogenesis during development, have been shown to overcome significant problems associated with traditional IMO-based approaches. In ECO, chondrocytes in the cartilage template release vascular endothelial growth factor (VEGF), which promotes vascular infiltration and remodeling of the cartilage template into bone12. The ECO-mediated approach to osteogenesis via cartilage remodeling and angiogenesis, which is also activated during fracture repair, uses artificially created cartilage tissue derived from MSCs as a priming material. Chondrocytes can tolerate hypoxia in bone defects, induce angiogenesis, and convert a vascular-free cartilage graft into angiogenic tissue. Numerous studies have reported that MSC-based cartilage grafts generate bone in vivo by implementing such an ECO program13,14,15,16,17,18,19,20,21.
An essential requirement for the clinical application of this ECO-mediated approach is how to prepare the desired amount of cartilage graft in a clinical setting. Preparing clinical cartilage of a size that fits the actual bone defect is not practical. Therefore, graft cartilage must form bone integrally when multiple fragments are implanted22. Hydrogels may be an attractive tool for scaling up tissue-engineered grafts for endochondral ossification. Many naturally derived hydrogels support MSC cartilage formation in vitro and ECO in vivo23,24,25,26,27,28,29,30,31,32; however, the optimal support material to meet the clinical application requirements has remained undetermined. Hyaluronic acid (HA) is a biodegradable and biocompatible polysaccharide present in the extracellular matrix of cartilage33. HA interacts with MSCs via surface receptors such as CD44 to support chondrogenic differentiation25,26,28,30,31,32,34. In addition, HA scaffolds promote IMO-mediated osteogenic differentiation of human dental pulp stem cells35, and scaffolds combined with collagen promote ECO-mediated osteogenesis36,37.
Here, we present a method for preparing HA hydrogels using bone marrow-derived adult human MSCs and their use for hypertrophic chondrogenesis in vitro and subsequent endochondral ossification in vivo38. We compared the characteristics of HA with those of collagen, a material widely applied in bone tissue engineering with MSCs and a useful material for scaling up artificial grafts for endochondral ossification17. In an immunocompromised mouse model, HA and collagen constructs seeded with human MSCs were evaluated for in vivo ECO potential by subcutaneous implantation. The results show that HA hydrogels are excellent as a scaffold for MSCs to create artificial cartilage grafts that allow bone formation through ECO.
The protocol is divided into two steps. First, constructs of human MSCs seeded on hyaluronan hydrogel are prepared and differentiated into hypertrophic cartilage in vitro. Next, the differentiated constructs are implanted subcutaneously in a nude model to induce endochondral ossification in vivo (Figure 1).

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integrated bone formation through in vivo endochondral ossification using mesenchymal stem cells

Conventional bone regeneration therapy using mesenchymal stem cells (MSCs) is difficult to apply to bone defects larger than the critical size because it does not have a mechanism to induce angiogenesis. Implanting artificial cartilage tissue fabricated from MSCs induces angiogenesis and bone formation in vivo via endochondral ossification (ECO). Therefore, this ECO-mediated approach may be a promising bone regeneration therapy in the future. An important aspect of the clinical application of this ECO-mediated approach is establishing a protocol for preparing enough cartilage to be implanted to repair the bone defect. It is especially not practical to design a single mass of grafted cartilage of a size that conforms to the shape of the actual bone defect. Therefore, the cartilage to be transplanted must have the property of forming bone integrally when multiple pieces are implanted. Hydrogels may be an attractive tool for scaling up tissue-engineered grafts for endochondral ossification to meet clinical requirements. Although many naturally derived hydrogels support MSC cartilage formation in vitro and ECO in vivo, the optimal scaffold material to meet the needs of clinical applications has yet to be determined. Hyaluronic acid (HA) is a crucial component of the cartilage extracellular matrix and is a biodegradable and biocompatible polysaccharide. Here, we show that HA hydrogels have excellent properties to support in vitro differentiation of MSC-based cartilage tissue and promote endochondral bone formation in vivo.

Author Spotlight: Enhancing Bone Regeneration with Vascularized Artificial Cartilage Integration 

Integrated Bone Formation Through In Vivo Endochondral Ossification Using Mesenchymal Stem Cells

Our research intends to develop a method that improves the lack of blood flow and can be applied to large bone defects. The protocol presented here is one with the approaches we have taken to this end. The approach to bone formation using artificial cartilage the structure that mimics the natural process of skeletal development during the embryogenesis.This approach is relatively new in bone regeneration research. The advantage we inducing cartilage neogenesis, which is not an existent method currently used in clinical practice exists for vascularized bone grafts. Traditionally, bone regeneration research has used the techniques that combine stem cells, biomaterials, and growth factors alone or in combination.The method of bone formation by implanting artificial cartilage uses a techniques that makes artificial cartilage by mixing stem cells with biomaterials that serve the scaffolding material, and inducing cartilage differentiation. Healing our patients'bone defect clinically requires a large quantity of high quality artificial cartilage. Our protocol addresses the challenge of producing high quality artificial cartilage, and how to make multiple cartilage fuse with each other to form a unified bone.Our protocol helps generate larger, higher quality cartilage with fewer cells than protocols that use or do not use other scaffold materials to create artificial cartilage. Furthermore, the protocol can generate one integrated piece of bone, even with multiple pieces implanted in vivo. This is important because preparing clinical cartilage of a shape and size that fits the bone defect is impractical.

MSC-encapsulated HA hydrogels were cultured in chondrogenic medium supplemented with TGF&#946;3, an inducer of chondrogenesis41 (step 4.1). We compared the properties of HA with those of collagen, which has been shown to be effective in creating MSC-based artificial cartilage grafts for endochondral ossification, as described previously38. Undifferentiated MSCs were not included as negative controls in this study because it has been demonstrated that undifferentiated MSCs r...

Watch this Scientific Journal Video about Enhancing Bone Regeneration with Vascularized Artificial Cartilage Integration at JoVE.com

Bone therapy via endochondral ossification by implanting artificial cartilage tissue produced from mesenchymal stem cells has the potential to circumvent the drawbacks of conventional therapies. Hyaluronic acid hydrogels are effective in scaling up uniformly differentiated cartilage grafts as well as creating integrated bone with vascularization between fused grafts in vivo.

Conventional bone regeneration therapy using mesenchymal stem cells (MSCs) is difficult to apply to bone defects larger than the critical size because it ...

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743 Views.  Tokyo Medical and Dental University. Our research intends to develop a method that improves the lack of blood flow and can be applied to large bone defects. The protocol presented here is one with the approaches we have taken to this end. The approach to bone formation using artificial cartilage the structure that mimics the natural process of skeletal development during the embryogenesis.This approach is relatively new in bone regeneration research. The advantage we inducing cartilage neogenesis, which is not an existent...