N.L. Davison

Zinc in calcium phosphate mediates bone induction: in vitro and in vivo model.

Zinc-containing tricalcium phosphate (Zn-TCP) was synthesized to investigate the role of zinc in osteoblastogenesis, osteoclastogenesis and in vivo bone induction in an ectopic implantation model. Zinc ions were readily released in the culture medium. Zn-TCP with the highest zinc content enhanced the alkaline phosphatase activity of human bone marrow stromal cells and tartrate-resistant acid phosphatase activity, as well as multinuclear giant cell formation of RAW264.7 monocyte/macrophages. RAW264.7 cultured with different dosages of zinc supplements in medium with or without zinc-free TCP showed that zinc could influence both the activity and the formation of multinuclear giant cells. After a 12-week implantation in the paraspinal muscle of canines, de novo bone formation and bone incidence increased with increasing zinc content in Zn-TCP - up to 52% bone in the free space. However, TCP without zinc induced no bone formation. Although the observed bone induction cannot be attributed to zinc release alone, these results indicate that zinc incorporated in TCP can modulate bone metabolism and render TCP osteoinductive, indicating to a novel way to enhance the functionality of this synthetic bone graft material.

The foreign body giant cell cannot resorb bone, but dissolves hydroxyapatite like osteoclasts

Foreign body multinucleated giant cells (FBGCs) and osteoclasts share several characteristics, like a common myeloid precursor cell, multinuclearity, expression of tartrate-resistant acid phosphatase (TRAcP) and dendritic cell-specific transmembrane protein (DC-STAMP). However, there is an important difference: osteoclasts form and reside in the vicinity of bone, while FBGCs form only under pathological conditions or at the surface of foreign materials, like medical implants. Despite similarities, an important distinction between these cell types is that osteoclasts can resorb bone, but it is unknown whether FBGCs are capable of such an activity. To investigate this, we differentiated FBGCs and osteoclasts in vitro from their common CD14+ monocyte precursor cells, using different sets of cytokines. Both cell types were cultured on bovine bone slices and analyzed for typical osteoclast features, such as bone resorption, presence of actin rings, formation of a ruffled border, and characteristic gene expression over time. Additionally, both cell types were cultured on a biomimetic hydroxyapatite coating to discriminate between bone resorption and mineral dissolution independent of organic matrix proteolysis. Both cell types differentiated into multinucleated cells on bone, but FBGCs were larger and had a higher number of nuclei compared to osteoclasts. FBGCs were not able to resorb bone, yet they were able to dissolve the mineral fraction of bone at the surface. Remarkably, FBGCs also expressed actin rings, podosome belts and sealing zones—cytoskeletal organization that is considered to be osteoclast-specific. However, they did not form a ruffled border. At the gene expression level, FBGCs and osteoclasts expressed similar levels of mRNAs that are associated with the dissolution of mineral (e.g., anion exchange protein 2 (AE2), carbonic anhydrase 2 (CAII), chloride channel 7 (CIC7), and vacuolar-type H+-ATPase (v-ATPase)), in contrast the matrix degrading enzyme cathepsin K, which was hardly expressed by FBGCs. Functionally, the latter cells were able to dissolve a biomimetic hydroxyapatite coating in vitro, which was blocked by inhibiting v-ATPase enzyme activity. These results show that FBGCs have the capacity to dissolve the mineral phase of bone, similar to osteoclasts. However, they are not able to digest the matrix fraction of bone, likely due to the lack of a ruffled border and cathepsin K.

In vivo performance of microstructured calcium phosphate formulated in novel water-free carriers

Osteoinductive calcium phosphate (CaP) ceramics can be combined with polymeric carriers to make shapeable bone substitutes as an alternative to autologous bone; however, carriers containing water may degrade the ceramic surface microstructure, which is crucial to bone formation. In this study five novel tricalcium phosphate (TCP) formulations were designed from water-free polymeric binders and osteoinductive TCP granules of different particle sizes (500-1000 μm for moldable putty forms, and 150-500 μm for flowable paste forms). The performance of these novel TCP formulations was studied and compared with control TCP granules alone (both 150-500 and 500-1000 μm). In vitro the five TCP formulations were characterized by their carrier dissolution times and TCP mineralization kinetic profiles in simulated body fluid. In vivo the formulations were implanted in the dorsal muscle and a unicortical femoral defect (Ø=5 mm) of dogs for 12 weeks. The TCP formulation based on a xanthan gum-glycerol carrier exhibited fast carrier dissolution (1 h) and TCP mineralization (7 days) in vitro, but induced inflammation and showed little ectopic bone formation. This carrier chemistry was thus found to disrupt the early cellular response related to osteoinduction by microstructured TCP. TCP formulations based on carboxymethyl cellulose-glycerol and Polyoxyl 15-hydroxystearate-Pluronic(®) F127 allowed the in vitro surface mineralization of TCP by day 7 and produced the highest level of orthotopic bone bridging and ectopic bone formation, which was equivalent to the control. These results demonstrate that water-free carriers can preserve the chemistry, microstructure, and performance of osteoinductive CaP ceramics.

Degradation of Biomaterials

The tissue engineering approach requires suitable biomaterials to serve as three-dimensional scaffolds to support cell growth and differentiation into functional tissues. Depending on the type of tissue in need of repair, a biomaterial must be designed with specific performance criteria in mind. Several excellent books and review articles (e.g., Ratner et al. (2013), Temenoff and Mikos (2008)) on biomaterials have appeared. Essential characteristics of biomaterial scaffolds for tissue engineering applications are described by Williams (2014). For instance, biomaterials used as load-bearing prostheses for hips and knees should retain their mechanical function for the lifetime of the patient. In large bone defects, where load-bearing is not critical (e.g., the skull), biomaterials—used alone or with cells as tissue engineering constructs—need not be so mechanically strong (Chapter 10). In this case, a degradable biomaterial scaffold would be ideal to allow newly formed bone tissue to gradually take the place of the implanted construct resulting in seamless bone repair and no residual material. In this way, the manner in which the biomaterial is degraded—broken down in the body—is a primary consideration. When a biomaterial is implanted in the body, it is immediately exposed to physiologic fluid and shortly after, cells whose main purpose is to clear it from the host (Chapter 15). Thus, the degradation of biomaterials involves multiple physiologic processes at the same time making it a science to its own. This chapter reviews the degradation mechanisms of the two most established classes of biomaterials—ceramics and polymers—and how these degradation properties can be beneficial in their primary application, bone tissue engineering.

Chapter 6: Degradation of Biomaterials

The tissue engineering approach requires suitable biomaterials to serve as three-dimensional scaffolds to support cell growth and differentiation into functional tissues. Depending on the type of tissue in need of repair, a biomaterial must be designed with specific performance criteria in mind. Several excellent books and review articles (e.g., Ratner et al. (2013), Temenoff and Mikos (2008)) on biomaterials have appeared. Essential characteristics of biomaterial scaffolds for tissue engineering applications are described by Williams (2014). For instance, biomaterials used as load-bearing prostheses for hips and knees should retain their mechanical function for the lifetime of the patient. In large bone defects, where load-bearing is not critical (e.g., the skull), biomaterials—used alone or with cells as tissue engineering constructs—need not be so mechanically strong (Chapter 10). In this case, a degradable biomaterial scaffold would be ideal to allow newly formed bone tissue to gradually take the place of the implanted construct resulting in seamless bone repair and no residual material. In this way, the manner in which the biomaterial is degraded—broken down in the body—is a primary consideration. When a biomaterial is implanted in the body, it is immediately exposed to physiologic fluid and shortly after, cells whose main purpose is to clear it from the host (Chapter 15). Thus, the degradation of biomaterials involves multiple physiologic processes at the same time making it a science to its own. This chapter reviews the degradation mechanisms of the two most established classes of biomaterials—ceramics and polymers—and how these degradation properties can be beneficial in their primary application, bone tissue engineering.

Chapter 6 – Degradation of Biomaterials

The tissue engineering approach requires suitable biomaterials to serve as three-dimensional scaffolds to support cell growth and differentiation into functional tissues. Depending on the type of tissue in need of repair, a biomaterial must be designed with specific performance criteria in mind. Several excellent books and review articles (e.g., Ratner et al. (2013), Temenoff and Mikos (2008)) on biomaterials have appeared. Essential characteristics of biomaterial scaffolds for tissue engineering applications are described by Williams (2014). For instance, biomaterials used as load-bearing prostheses for hips and knees should retain their mechanical function for the lifetime of the patient. In large bone defects, where load-bearing is not critical (e.g., the skull), biomaterials—used alone or with cells as tissue engineering constructs—need not be so mechanically strong (Chapter 10). In this case, a degradable biomaterial scaffold would be ideal to allow newly formed bone tissue to gradually take the place of the implanted construct resulting in seamless bone repair and no residual material. In this way, the manner in which the biomaterial is degraded—broken down in the body—is a primary consideration. When a biomaterial is implanted in the body, it is immediately exposed to physiologic fluid and shortly after, cells whose main purpose is to clear it from the host (Chapter 15). Thus, the degradation of biomaterials involves multiple physiologic processes at the same time making it a science to its own. This chapter reviews the degradation mechanisms of the two most established classes of biomaterials—ceramics and polymers—and how these degradation properties can be beneficial in their primary application, bone tissue engineering.