Elucidating multiscale periosteal mechanobiology: a key to unlocking the smart properties and regenerative capacity of the periosteum?

Regulatory effects of stress release from decellularized periosteum on proliferation, migration, and osteogenic differentiation of periosteum-derived cells

Bone injury is often associated with tears in the periosteum and changes in the internal stress microenvironment of the periosteum. In this study, we investigated the biological effects of periosteal prestress release on periosteum-derived cells (PDCs) and the potential mechanisms of endogenous stem cell recruitment. Decellularized periosteum with natural extracellular matrix (ECM) components was obtained by a combination of physical, chemical, and enzymatic decellularization. The decellularized periosteum removed immunogenicity while retaining the natural network structure and composition of the ECM. The Young's modulus has no significant difference between the periosteum before and after decellularization. The extracted PDCs were further composited with the decellularized periosteum and subjected to 20% stress release. It was found that the proliferative capacity of PDCs seeded on decellularized periosteum was significantly enhanced 6 h after stress release of the periosteum. The cell culture supernatant obtained after periosteal prestress release was able to significantly promote the migration ability of PDCs within 24 h. Enzyme-linked immunosorbnent assay (ELISA) experiments showed that the expression of stroma-derived factor-1α (SDF-1α) and vascular endothelial growth factor (VEGF) in the supernatant increased significantly after 3 h and 12 h of stress release, respectively. Furthermore, periosteal stress release promoted the high expression of osteogenic markers osteocalcin (OCN), osteopontin (OPN), and collagen type I of PDCs. The change in stress environment caused by the release of periosteal prestress was sensed by integrin β1, a mechanoreceptor on the membrane of PDCs, which further stimulated the expression of YAP in the nucleus. These investigations provided a novel method to evaluate the importance of mechanical stimulation in periosteum, which is also of great significance for the design and fabrication of artificial periosteum with mechanical regulation function.

Electrospun Biomimetic Periosteum Capable of Controlled Release of Multiple Agents for Programmed Promoting Bone Regeneration

The effective repair of large bone defects remains a major challenge due to its limited self-healing capacity. Inspired by the structure and function of the natural periosteum, an electrospun biomimetic periosteum is constructed to programmatically promote bone regeneration using natural bone healing mechanisms. The biomimetic periosteum is composed of a bilayer with an asymmetric structure in which an aligned electrospun poly(ε-caprolactone)/gelatin/deferoxamine (PCL/GEL/DFO) layer mimics the outer fibrous layer of the periosteum, while a random coaxial electrospun PCL/GEL/aspirin (ASP) shell and PCL/silicon nanoparticles (SiNPs) core layer mimics the inner cambial layer. The bilayer controls the release of ASP, DFO, and SiNPs to precisely regulate the inflammatory, angiogenic, and osteogenic phases of bone repair. The random coaxial inner layer can effectively antioxidize, promoting cell recruitment, proliferation, differentiation, and mineralization, while the aligned outer layer can promote angiogenesis and prevent fibroblast infiltration. In particular, different stages of bone repair are modulated in a rat skull defect model to achieve faster and better bone regeneration. The proposed biomimetic periosteum is expected to be a promising candidate for bone defect healing.

Collagen molecular organization preservation in human fascia lata and periosteum after tissue engineering

Large bone defect regeneration remains a major challenge for orthopedic surgeons. Tissue engineering approaches are therefore emerging in order to overcome this limitation. However, these processes can alter some of essential native tissue properties such as intermolecular crosslinks of collagen triple helices, which are known for their essential role in tissue structure and function. We assessed the persistence of extracellular matrix (ECM) properties in human fascia lata (HFL) and periosteum (HP) after tissue engineering processes such as decellularization and sterilization. Harvested from cadaveric donors (N = 3), samples from each HFL and HP were decellularized following five different chemical protocols with and without detergents (D1-D4 and D5, respectively). D1 to D4 consisted of different combinations of Triton, Sodium dodecyl sulfate and Deoxyribonuclease, while D5 is routinely used in the institutional tissue bank. Decellularized HFL tissues were further gamma-irradiated (minimum 25 kGy) in order to study the impact of sterilization on the ECM. Polarized light microscopy (PLM) was used to estimate the thickness and density of collagen fibers. Tissue hydration and content of hydroxyproline, enzymatic crosslinks, and non-enzymatic crosslinks (pentosidine) were semi-quantified with Raman spectroscopy. ELISA was also used to analyze the maintenance of the decorin (DCN), an important small leucine rich proteoglycan for fibrillogenesis. Among the decellularization protocols, detergent-free treatments tended to further disorganize HFL samples, as more thin fibers (+53.7%) and less thick ones (-32.6%) were recorded, as well as less collagen enzymatic crosslinks (-25.2%, p = 0.19) and a significant decrease of DCN (p = 0.036). GAG content was significantly reduced in both tissue types after all decellularization protocols. On the other hand, HP samples were more sensitive to the D1 detergent-based treatments, with more disrupted collagen organization and greater, though not significant loss of enzymatic crosslinks (-37.4%, p = 0.137). Irradiation of D5 HFL samples, led to a further and significant loss in the content of enzymatic crosslinks (-29.4%, p = 0.037) than what was observed with the decellularization process. Overall, the results suggest that the decellularization processes did not significantly alter the matrix. However, the addition of a gamma-irradiation is deleterious to the collagen structural integrity of the tissue.

Approximation of extracted features enabling 3D design tuning for reproducing the mechanical behaviour of biological soft tissues

This article describes a new method, inspired by machine learning, to mimic the mechanical behaviour of target biological soft tissues with 3D printed materials. The principle is to optimise the structure of a 3D printed composite consisting of a geometrically tunable fibre embedded in a soft matrix. Physiological features are extracted from experimental stress-strain curves of several biological soft tissues. Then, using a cubic Bézier curve as the composite inner fibre, we optimised its geometric parameters, amplitude and height, to generate a specimen that exhibits a stress-strain curve in accordance with the extracted features of tensile tests. From this first phase, we created a database of specimen geometries that can be used to reproduce a wide variety of biological soft tissues. We applied this process to two soft tissues with very different behaviours: the mandibular periosteum and the calvarial periosteum. The results show that our method can successfully reproduce the mechanical behaviour of these tissues. This highlights the versatility of this approach and demonstrates that it can be extended to mimic various biological soft tissues.

Recent Advances in Cell Sheet-Based Tissue Engineering for Bone Regeneration

In conventional bone tissue engineering, cells are seeded onto scaffolds to create three-dimensional (3D) tissues, but the cells on the scaffolds are unable to effectively perform their physiological functions due to their low density and viability. Cell sheet (CS) engineering is expected to be free from this limitation. CS engineering uses the principles of self-assembly and self-organization of endothelial and mesenchymal stem cells to prepare CSs as building blocks for engineering bone grafts. This process recapitulates the native tissue development, thus attracting significant attention in the field of bone regeneration. However, the method is still in the prebasic experimental stage in bone defect repair. To make the method clinically applicable and valuable in personalized and precision medicine, current research is focused on the preparation of multifunctionalized building blocks using CS technologies, such as 3D layered CSs containing microvascular structures. Considering the great potential of CS engineering in repairing bone defects, in this review, the types of cell technologies are first outlined. We then summarize the various types of CSs as building blocks for engineering bone grafts. Furthermore, the specific applications of CSs in bone repair are discussed. Finally, we present specific suggestions for accelerating the application of CS engineering in the clinical treatment of bone defects.

Analysis of Cellular Crosstalk and Molecular Signal between Periosteum-Derived Precursor Cells and Peripheral Cells During Bone Healing Process Using a Paper-Based Osteogenesis-On-A-Chip Platform

Periosteum-derived progenitor cells (PDPCs) are highly promising cell sources that are indispensable in the bone healing process. Adipose-derived stem cells (ADSCs) are physiologically close to periosteum tissue and release multiple growth factors to promote the bone healing process. Co-culturing PDPCs and ADSCs can construct periosteum-bone tissue microenvironments for the study of cellular crosstalk and molecular signal in the bone healing process. In the current work, a paper-based osteogenesis-on-a-chip platform was successfully developed to provide an in vitro three-dimensional coculture model. The platform was a paper substrate sandwiched between PDPC-hydrogel and ADSC-hydrogel suspensions. Cell secretion could be transferred through the paper substrate from one side to another side. Growth factors including BMP2, TGF-β, POSTN, Wnt proteins, PDGFA, and VEGFA were directly analyzed by a paper-based immunoassay. Cellular crosstalk was studied by protein expression on the paper substrate. Moreover, osteogenesis of PDPCs was investigated by examining the mRNA expressions of PDPCs after culture. Neutralizing and competitive assays were conducted to understand the correlation between growth factors secreted from ADSCs and the osteogenesis of PDPCs. In vitro periosteum-bone tissue microenvironment was established by the paper-based osteogenesis-on-a-chip platform. The proposed approach provides a promising assay of cellular crosstalk and molecular signal in 3D coculture microenvironment that may potentially lead to the development of effective bone regeneration therapy.

A spike in circulating cytokines TNF-α and TGF-β alters barrier function between vascular and musculoskeletal tissues

Molecular transport between the circulatory and musculoskeletal systems regulates articular joint physiology in health and disease. Osteoarthritis (OA) is a degenerative joint disease linked to systemic and local inflammation. Inflammatory events involve cytokines, which are secreted by cells of the immune system and modulate molecular transport across tissue interfaces (referred to as tight junction [TJ] barrier function). In a previous study from our group, OA knee joint tissues were shown to exhibit size separation of different sized molecules delivered as a single bolus to the heart (Ngo et al. in Sci. Rep. 8:10254, 2018). Here, in a follow up study of parallel design, we test the hypothesis that two common cytokines, with multifaceted roles in the etiology of osteoarthritis as well as immune state in general, modulate the barrier function properties of joint tissue interfaces. Specifically, we probe the effect of an acute cytokine increase (spike) on molecular transport within tissues and across tissue interfaces of the circulatory and musculoskeletal systems. A single bolus of fluorescent-tagged 70 kDa dextran, was delivered intracardially, either alone, or with either the pro-inflammatory cytokine TNF-α or the anti-inflammatory cytokine TGF-β, to skeletally mature (11 to 13-month-old) guinea pigs (Dunkin-Hartley, a spontaneous OA animal model). After five minutes' circulation, whole knee joints were serial sectioned and fluorescent block face cryo-imaged at near-single-cell resolution. The 70 kDa fluorescent-tagged tracer is analogous in size to albumin, the most prevalent blood transporter protein, and quantification of tracer fluorescence intensity gave a measure of tracer concentration. Within five minutes, a spike (acute doubling) in circulating cytokines TNF-α or TGF-β significantly disrupted barrier function between the circulatory and musculoskeletal systems, with barrier function essentially abrogated in the TNF-α group. In the entire volume of the joint (including all tissue compartments and the bounding musculature), tracer concentration was significantly decreased in the TGF-β- and TNF-α- compared to the control-group. These studies implicate inflammatory cytokines as gatekeepers for molecular passage within and between tissue compartments of our joints and may open new means to delay the onset and mitigate the progression of degenerative joint diseases such as OA, using pharmaceutical and/or physical measures.

Role of periosteum in alveolar bone regeneration comparing with collagen membrane in a buccal dehiscence model of dogs

To investigate the role of periosteum on the treatment of buccal dehiscence defects comparing with collagen membrane in canine model. Bilateral dehiscence-type defects at the buccal side on the distal root of the lower 3rd/4th premolars were created in six beagle dogs with a total of 24 defects and assigned into three groups: Group A: blood clot in an untreated defect; Group B: deproteinized bovine bone material (DBBM) covered with an absorbable membrane; Group C: DBBM covered with the periosteum. The structural parameters for trabecular architecture and vertical bone regeneration were evaluated. Histological and histomorphometric evaluation were carried out to observe new bone formation and mineralization in the graft site. Immunohistochemical analysis was performed to identify the expression of osteopontin (OPN) and osteocalcin (OCN) at postoperative 3 months. Group C achieved greater vertical alveolar bone gain than that of group A and group B. The periosteum-covered group showed significantly greater new bone formation and accelerated mineralization. The greater immunolabeling for OPN and OCN was observed in group C than in group A. Periosteal coverage has explicit advantages over collagen membranes for the quality and quantity of new bone regeneration in dehiscence defects repairing.

Current strategies in biomaterial-based periosteum scaffolds to promote bone regeneration: A review

The role of periosteum rich in a variety of bone cells and growth factors in the treatment of bone defects has gradually been discovered. However, due to the limited number of healthy transplantable periosteum, there are still major challenges in the clinical treatment of critical-size bone defects. Various techniques for preparing biomimetic periosteal scaffolds that are similar in composition and structure to natural periosteal scaffold have gradually emerged. This article reviews the current preparation methods of biomimetic periosteal scaffolds based on various biomaterials, which are mainly divided into natural periosteal materials and various polymer biomaterials. Several preparation methods of biomimetic periosteal scaffolds with different principles are listed, their strengths and weaknesses are also discussed. It aims to provide a more systematic perspective for the preparation of biomimetic periosteal scaffolds in the future.

A New Osteogenic Membrane to Enhance Bone Healing: At the Crossroads between the Periosteum, the Induced Membrane, and the Diamond Concept

The lack of viability of massive bone allografts for critical-size bone defect treatment remains a challenge in orthopedic surgery. The literature has reviewed the advantages of a multi-combined treatment with the synergy of an osteoconductive extracellular matrix (ECM), osteogenic stem cells, and growth factors (GFs). Questions are still open about the need for ECM components, the influence of the decellularization process on the latter, the related potential loss of function, and the necessity of using pre-differentiated cells. In order to fill in this gap, a bone allograft surrounded by an osteogenic membrane made of a decellularized collagen matrix from human fascia lata and seeded with periosteal mesenchymal stem cells (PMSCs) was analyzed in terms of de-/recellularization, osteogenic properties, PMSC self-differentiation, and angiogenic potential. While the decellularization processes altered the ECM content differently, the main GF content was decreased in soft tissues but relatively increased in hard bone tissues. The spontaneous osteogenic differentiation was necessarily obtained through contact with a mineralized bone matrix. Trying to deepen the knowledge on the complex matrix-cell interplay could further propel these tissue engineering concepts and lead us to provide the biological elements that allow bone integration in vivo.

Hallmarks of peripheral nerve function in bone regeneration

Skeletal tissue is highly innervated. Although different types of nerves have been recently identified in the bone, the crosstalk between bone and nerves remains unclear. In this review, we outline the role of the peripheral nervous system (PNS) in bone regeneration following injury. We first introduce the conserved role of nerves in tissue regeneration in species ranging from amphibians to mammals. We then present the distribution of the PNS in the skeletal system under physiological conditions, fractures, or regeneration. Furthermore, we summarize the ways in which the PNS communicates with bone-lineage cells, the vasculature, and immune cells in the bone microenvironment. Based on this comprehensive and timely review, we conclude that the PNS regulates bone regeneration through neuropeptides or neurotransmitters and cells in the peripheral nerves. An in-depth understanding of the roles of peripheral nerves in bone regeneration will inform the development of new strategies based on bone-nerve crosstalk in promoting bone repair and regeneration.

Periosteum and fascia lata: Are they so different?

Introduction: The human fascia lata (HFL) is used widely in reconstructive surgery in indications other than fracture repair. The goal of this study was to compare microscopic, molecular, and mechanical properties of HFL and periosteum (HP) from a bone tissue engineering perspective.

Material and Methods: Cadaveric HP and HFL (N = 4 each) microscopic morphology was characterized using histology and immunohistochemistry (IHC), and the extracellular matrix (ECM) ultrastructure assessed by means of scanning electron microscopy (SEM). DNA, collagen, elastin, glycosaminoglycans, major histocompatibility complex Type 1, and bone morphogenetic protein (BMP) contents were quantified. HP (N = 6) and HFL (N = 11) were submitted to stretch tests.

Results: Histology and IHC highlighted similarities (Type I collagen fibers and two-layer organization) but also differences (fiber thickness and compaction and cell type) between both tissues, as confirmed using SEM. The collagen content was statistically higher in HFL than HP (735 vs. 160.2 μg/mg dry weight, respectively, p < 0.0001). On the contrary, DNA content was lower in HFL than HP (404.75 vs. 1,102.2 μg/mg dry weight, respectively, p = 0.0032), as was the immunogenic potential (p = 0.0033). BMP-2 and BMP-7 contents did not differ between both tissues (p = 0.132 and p = 0.699, respectively). HFL supported a significantly higher tension stress than HP.

Conclusion: HP and HFL display morphological differences, despite their similar molecular ECM components. The stronger stretching resistance of HFL can specifically be explained by its higher collagen content. However, HFL contains many fewer cells and is less immunogenic than HP, as latter is rich in periosteal stem cells. In conclusion, HFL is likely suitable to replace HP architecture to confer a guide for bone consolidation, with an absence of osteogenicity. This study could pave the way to a bio-engineered periosteum built from HFL.

Biomimicking design of artificial periosteum for promoting bone healing

Background

Periosteum is a vascularized tissue membrane covering the bone surface and plays a decisive role in bone reconstruction process after fracture. Various artificial periosteum has been developed to assist the allografts or bionic bone scaffolds in accelerating bone healing. Recently, the biomimicking design of artificial periosteum has attracted increasing attention due to the recapitulation of the natural extracellular microenvironment of the periosteum and has presented unique capacity to modulate the cell fates and ultimately enhance the bone formation and improve neovascularization.

Methods

A systematic literature search is performed and relevant findings in biomimicking design of artificial periosteum have been reviewed and cited.

Results

We give a systematical overview of current development of biomimicking design of artificial periosteum. We first summarize the universal strategies for designing biomimicking artificial periosteum including biochemical biomimicry and biophysical biomimicry aspects. We then discuss three types of novel versatile biomimicking artificial periosteum including physical-chemical combined artificial periosteum, heterogeneous structured biomimicking periosteum, and healing phase-targeting biomimicking periosteum. Finally, we comment on the potential implications and prospects in the future design of biomimicking artificial periosteum.

Conclusion

This review summarizes the preparation strategies of biomimicking artificial periosteum in recent years with a discussion of material selection, animal model adoption, biophysical and biochemical cues to regulate the cell fates as well as three types of latest developed versatile biomimicking artificial periosteum. In future, integration of innervation, osteochondral regeneration, and osteoimmunomodulation, should be taken into consideration when fabricating multifunctional artificial periosteum.

The Translational Potential of this Article: This study provides a holistic view on the design strategy and the therapeutic potential of biomimicking artificial periosteum to promote bone healing. It is hoped to open a new avenue of artificial periosteum design with biomimicking considerations and reposition of the current strategy for accelerated bone healing.

Small Intestinal Submucosa Biomimetic Periosteum Promotes Bone Regeneration

Background: Critical bone defects are a significant problem in clinics. The periosteum plays a vital role in bone regeneration. A tissue-engineered periosteum (TEP) has received increasing attention as a novel strategy for bone defect repairs. Methods: In this experiment, a biomimetic periosteum was fabricated by using coaxial electrospinning technology with decellularized porcine small intestinal submucosa (SIS) as the shell and polycaprolactone (PCL) as the core. In vitro, the effects of the biomimetic periosteum on Schwann cells, vascular endothelial cells, and bone marrow mesenchymal stem cells were detected by a scratch test, an EdU, a tube-forming test, and an osteogenesis test. In vivo, we used HE staining to evaluate the effect of the biomimetic periosteum on bone regeneration. Results: In vitro experiments showed that the biomimetic periosteum could significantly promote the formation of angiogenesis, osteogenesis, and repaired Schwann cells (SCs). In vivo experiments showed that the biomimetic periosteum could promote the repair of bone defects. Conclusions: The biomimetic periosteum could simulate the structural function of the periosteum and promote bone repair. This strategy may provide a promising method for the clinical treatment of skull bone defects.

Progress of Periosteal Osteogenesis: The Prospect of In Vivo Bioreactor

Repairing large segment bone defects is still a clinical challenge. Bone tissue prefabrication shows great translational potentials and has been gradually accepted clinically. Existing bone reconstruction strategies, including autologous periosteal graft, allogeneic periosteal transplantation, xenogeneic periosteal transplantation, and periosteal cell tissue engineering, are all clinically valuable treatments and have made significant progress in research. Herein, we reviewed the research progress of these techniques and briefly explained the relationship among in vivo microenvironment, mechanical force, and periosteum osteogenesis. Moreover, we also highlighted the importance of the critical role of periosteum in osteogenesis and explained current challenges and future perspective.

Targeted activation of androgen receptor signaling in the periosteum improves bone fracture repair

Low testosterone level is an independent predictor of osteoporotic fracture in elderly men as well as increased fracture risk in men undergoing androgen deprivation. Androgens and androgen receptor (AR) actions are essential for bone development and homeostasis but their linkage to fracture repair remains unclear. Here we found that AR is highly expressed in the periosteum cells and is co-localized with a mesenchymal progenitor cell marker, paired-related homeobox protein 1 (Prrx1), during bone fracture repair. Mice lacking the AR gene in the periosteum expressing Prrx1-cre (AR^-/Y;Prrx1::Cre) but not in the chondrocytes (AR^-/Y;Col-2::Cre) exhibits reduced callus size and new bone volume. Gene expression data analysis revealed that the expression of several collagens, integrins and cell adhesion molecules were downregulated in periosteum-derived progenitor cells (PDCs) from AR^-/Y;Prrx1::Cre mice. Mechanistically, androgens-AR signaling activates the AR/ARA55/FAK complex and induces the collagen-integrin α2β1 gene expression that is required for promoting the AR-mediated PDCs migration. Using mouse cortical-defect and femoral graft transplantation models, we proved that elimination of AR in periosteum of host mice impairs fracture healing, regardless of AR existence of transplanted donor graft. While testosterone implanted scaffolds failed to complete callus bridging across the fracture gap in AR^-/Y;Prrx1::Cre mice, cell-based transplantation using DPCs re-expressing AR could lead to rescue bone repair. In conclusion, targeting androgen/AR axis in the periosteum may provide a novel therapy approach to improve fracture healing.

Osteogenic and Chondrogenic Potential of Periosteum-Derived Mesenchymal Stromal Cells: Do They Hold the Key to the Future?

The periosteum, with its outer fibrous and inner cambium layer, lies in a dynamic environment with a niche of pluripotent stem cells for their reparative needs. The inner cambium layer is rich in mesenchymal progenitors, osteogenic progenitors, osteoblasts, and fibroblasts in a scant collagen matrix environment. Their role in union and remodeling of fracture is well known. However, the periosteum as a source of mesenchymal stem cells has not been explored in detail. Moreover, with the continuous expansion of techniques, newer insights have been acquired into the roles and regulation of these periosteal cells. From a therapeutic standpoint, the periosteum as a source of tissue engineering has gained much attraction. Apart from its role in bone repair, analysis of the bone-forming potential of periosteum-derived stem cells is lacking. Hence, this article elucidates the role of the periosteum as a potential source of mesenchymal stem cells along with their capacity for osteogenic and chondrogenic differentiation for therapeutic application in the future.

The cytoskeleton and connected elements in bone cell mechano-transduction

Bone is a mechano-responsive tissue that adapts to changes in its mechanical environment. Increases in strain lead to increased bone mass acquisition, whereas decreases in strain lead to a loss of bone mass. Given that mechanical stress is a regulator of bone mass and quality, it is important to understand how bone cells sense and transduce these mechanical cues into biological changes to identify druggable targets that can be exploited to restore bone cell mechano-sensitivity or to mimic mechanical load. Many studies have identified individual cytoskeletal components - microtubules, actin, and intermediate filaments - as mechano-sensors in bone. However, given the high interconnectedness and interaction between individual cytoskeletal components, and that they can assemble into multiple discreet cellular structures, it is likely that the cytoskeleton as a whole, rather than one specific component, is necessary for proper bone cell mechano-transduction. This review will examine the role of each cytoskeletal element in bone cell mechano-transduction and will present a unified view of how these elements interact and work together to create a mechano-sensor that is necessary to control bone formation following mechanical stress.

Periosteal Tissue Engineering: Current Developments and Perspectives

Periosteum, a highly vascularized bilayer connective tissue membrane plays an indispensable role in the repair and regeneration of bone defects. It is involved in blood supply and delivery of progenitor cells and bioactive molecules in the defect area. However, sources of natural periosteum are limited, therefore, there is a need to develop tissue-engineered periosteum (TEP) mimicking the composition, structure, and function of natural periosteum. This review explores TEP construction strategies from the following perspectives: i) different materials for constructing TEP scaffolds; ii) mechanical properties and surface topography in TEP; iii) cell-based strategies for TEP construction; and iv) TEP combined with growth factors. In addition, current challenges and future perspectives for development of TEP are discussed.

Isolation and Culture of Periosteum-Derived Progenitor Cells from Mice

This chapter describes the methods of isolation of mouse periosteal progenitor cells. There are three basic methods utilized. The bone grafting method was developed utilizing the fracture healing process to expand the progenitor populations. Bone capping methods requires enzymatic digestion and purification of cells from the native periosteum, while the Egression/Explant method requires the least manipulation with placement of cortical bone fragments with attached periosteum in a culture dish. Various cell surface antibodies have been employed over the years to characterize periosteum derived progenitor cells, but the most consistent minimal criteria was recommended by the International Society for Cellular Therapy. Confirmation of the multipotent status of these isolated cells can be achieved by differentiation into the three basic mesodermal lineages in vitro.

Matrix metalloproteinase (MMP)-degradable tissue engineered periosteum coordinates allograft healing via early stage recruitment and support of host neurovasculature

Despite serving as the clinical "gold standard" treatment for critical size bone defects, decellularized allografts suffer from long-term failure rates of ~60% due to the absence of the periosteum. Stem and osteoprogenitor cells within the periosteum orchestrate autograft healing through host cell recruitment, which initiates the regenerative process. To emulate periosteum-mediated healing, tissue engineering approaches have been utilized with mixed outcomes. While vascularization has been widely established as critical for bone regeneration, innervation was recently identified to be spatiotemporally regulated together with vascularization and similarly indispensable to bone healing. Notwithstanding, there are no known approaches that have focused on periosteal matrix cues to coordinate host vessel and/or axon recruitment. Here, we investigated the influence of hydrogel degradation mechanism, i.e. hydrolytic or enzymatic (cell-dictated), on tissue engineered periosteum (TEP)-modified allograft healing, especially host vessel/nerve recruitment and integration. Matrix metalloproteinase (MMP)-degradable hydrogels supported endothelial cell migration from encapsulated spheroids whereas no migration was observed in hydrolytically degradable hydrogels in vitro, which correlated with increased neurovascularization in vivo. Specifically, ~2.45 and 1.84-fold, and ~3.48 and 2.58-fold greater vessel and nerve densities with high levels of vessel and nerve co-localization was observed using MMP degradable TEP (MMP-TEP) -modified allografts versus unmodified and hydrolytically degradable TEP (Hydro-TEP)-modified allografts, respectively, at 3 weeks post-surgery. MMP-TEP-modified allografts exhibited greater longitudinal graft-localized vascularization and endochondral ossification, along with 4-fold and 2-fold greater maximum torques versus unmodified and Hydro-TEP-modified allografts after 9 weeks, respectively, which was comparable to that of autografts. In summary, our results demonstrated that the MMP-TEP coordinated allograft healing via early stage recruitment and support of host neurovasculature.

Advanced Design and Manufacture of Mechanoactive Materials Inspired by Skin, Bones, and Skin-on-Bones

Life is mechanobiological. Natural living materials exhibit remarkable, emergent and smart properties under mechanical loading. Such materials are classified as mechanoactive, in contrast to electroactive polymers and materials that exhibit advanced properties when subjected to electrical stimulation. Cutting edge, multiscale imaging technologies have proven enabling for the elucidation of molecular to meso-scale structure and function of natural mechanoactive materials. Using Microscopy-Aided Design And ManufacturE, (MADAME) this perspective article describes mechanoactive properties of natural materials including skin-on-bones (periosteum) and bone itself. In so doing, it demonstrates the principle to emulate natural smart properties using recursive logic, the basis of many computer algorithms, and to design and manufacture mechanoactive materials and products using advanced manufacturing methods that also incorporate principles of recursive logic. In sum, the MADAME approach translates physically the computer science paradigm of recursion by implementing Jacquard textile methods, which themselves form a historical basis for computing machines, together with additive manufacturing methods including multidimensional printing, stereolithography, laser sintering, etc. These integrated methods provide a foundation and translational pathway for scaled-up manufacture of disruptive mechanoactive materials that will find use in fields as varied as medicine, safety, transport and sports, for internal (implants) and external (wearables) applications.

Mapping human serum-induced gene networks as a basis for the creation of biomimetic periosteum for bone repair

BACKGROUND

The periosteum is a highly vascularized, collagen-rich tissue that plays a crucial role in directing bone repair. This is orchestrated primarily by its resident progenitor cell population. Indeed, preservation of periosteum integrity is critical for bone healing. Cells extracted from the periosteum retain their osteochondrogenic properties and as such are a promising basis for tissue engineering strategies for the repair of bone defects. However, the culture expansion conditions and the way in which the cells are reintroduced to the defect site are critical aspects of successful translation. Indeed, expansion in human serum and implantation on biomimetic materials has previously been shown to improve in vivo bone formation.

AIM

This study aimed to develop a protocol to allow for the expansion of human periosteum derived cells (hPDCs) in a biomimetic periosteal-like environment.

METHODS

The expansion conditions were defined through the investigation of the bioactive cues involved in augmenting hPDC proliferative and multipotency characteristics, based on transcriptomic analysis of cells cultured in human serum.

RESULTS

Master regulators of transcriptional networks were identified, and an optimized periosteum-derived growth factor cocktail (PD-GFC; containing β-estradiol, FGF2, TNFα, TGFβ, IGF-1 and PDGF-BB) was generated. Expansion of hPDCs in PD-GFC resulted in serum mimicry with regard to the cell morphology, proliferative capacity and chondrogenic differentiation. When incorporated into a three-dimensional collagen type 1 matrix and cultured in PD-GFC, the hPDCs migrated to the surface that represented the matrix topography of the periosteum cambium layer. Furthermore, gene expression analysis revealed a down-regulated WNT and TGFβ signature and an up-regulation of CREB, which may indicate the hPDCs are recreating their progenitor cell signature.

CONCLUSION

This study highlights the first stage in the development of a biomimetic periosteum, which may have applications in bone repair.

Three-Dimensional-Printed Poly-L-Lactic Acid Scaffolds with Different Pore Sizes Influence Periosteal Distraction Osteogenesis of a Rabbit Skull

The repair of bone defects is a big challenge in reconstructive surgery. Periosteal distraction osteogenesis (PDO), as a promising technique used for bone regeneration, forms a space between the periosteum and bone cortex to regenerate the new bone merely by distracting the periosteum. In order to investigate the influence of distractor framework on the PDO, we utilized three-dimensional (3D) printing technology to fabricate three kinds of poly-L-lactic acid (PLLA) scaffolds with different pore sizes in this study. The in vitro experiments showed that the customized PLLA scaffolds had different-sized microchannels with low toxicity, good biocompatibility, and enough mechanical strength. Then, we built up an in vivo bioreactor under the skull periosteum of New Zealand white rabbits. The distractors with different pore sizes all could satisfy the demand of periosteal distraction in the animal experiments. After 8 weeks of consolidation period, the quality and quantity of the newly formed bone were improved with the increasing pore sizes of the distractors. Moreover, the newly formed bone also displayed an increasing degree of vascularization. In conclusion, 3D printing technology could promote the innovation of PDO devices and fabricate optimized scaffolds with appropriate pore sizes, shapes, and structures. It would help us regenerate more functional tissue-engineered bone and provide new ideas for further clinical application of the PDO technique.

Bone Regeneration, Reconstruction and Use of Osteogenic Cells; from Basic Knowledge, Animal Models to Clinical Trials

The deterioration of the human skeleton's capacity for self-renewal occurs naturally with age. Osteoporosis affects millions worldwide, with current treatments including pharmaceutical agents that target bone formation and/or resorption. Nevertheless, these clinical approaches often result in long-term side effects, with better alternatives being constantly researched. Mesenchymal stem cells (MSCs) derived from bone marrow and adipose tissue are known to hold therapeutic value for the treatment of a variety of bone diseases. The following review summarizes the latest studies and clinical trials related to the use of MSCs, both individually and combined with other methods, in the treatment of a variety of conditions related to skeletal health. For example, some of the most recent works noted the advantage of bone grafts based on biomimetic scaffolds combined with MSC and growth factor delivery, with a greatly increased regeneration rate and minimized side effects for patients. This review also highlights the continuing research into the mechanisms underlying bone homeostasis, including the key transcription factors and signalling pathways responsible for regulating the differentiation of osteoblast lineage. Paracrine factors and specific miRNAs are also believed to play a part in MSC differentiation. Furthering the understanding of the specific mechanisms of cellular signalling in skeletal remodelling is key to incorporating new and effective treatment methods for bone disease.

Porous Titanium Granules in comparison with Autogenous Bone Graft in Femoral Osseous Defects: A Histomorphometric Study of Bone Regeneration and Osseointegration in Rabbits

Background

The high resorption rate of autogenous bone is a well-documented phenomenon that can lead to insufficient bone quality and quantity in an augmented area. Nonresorbable bone substitutes might perform better than autogenous bone in certain applications if they are able to provide adequate bone formation and graft osseointegration.

Purpose

The aim of this study was to compare the osseous regeneration and graft integration in standardized defects in the rabbit femur treated either with porous titanium granules or autogenous osseous graft.

Materials and Methods

Standardized femoral osseous defects were surgically induced in 45 New Zealand rabbits. Fifteen were treated with porous titanium granules (TIGRAN™-PTG) and membrane (PTGM), 15 with autogenous graft and membrane (AGM), and 15 with membrane alone (CM, control). At six weeks, the defects were assessed histologically and histomorphometrically.

Results

PTGM as compared to AGM presented similar percentages of newly formed bone tissue, but a significantly higher fraction of the region of interest was filled with the bone substitute material. Accordingly, the composite of new bone plus bone substitute material showed significantly higher volumes for PTGM. Yet, the smaller amount of remaining autogenous bone was far better osseointegrated than the titanium granules, which in large regions showed no connection to newly formed bone. Both PTGM and AGM as compared to CM presented higher values of newly formed bone.

Conclusions

This study demonstrated that PTG was similarly effective as autogenous osseous graft in achieving osseous regeneration while PTG performed markedly better in graft volume stability. The resulting higher total percentage of new bone combined with the bone substitute material in PTG could provide a superior foundation for implant placement.

In vitro biocompatibility and biomechanics study of novel, Microscopy Aided Designed and ManufacturEd (MADAME) materials emulating natural tissue weaves and their intrinsic gradients

This study conducted biomechanical and biocompatibility tests of textiles and textile composites, created using recursive logic to emulate the properties of natural tissue weaves and their intrinsic mechanical stiffness gradients. Two sets of samples were created, first to test feasibility on textile samples designed as periosteum substitutes with elastane fibers mimicking periosteum's endogenous elastin and nylon fibers substituting for collagen, and then on composites comprising other combinations of suture materials before and after sterilization. In the first part, the bulk tensile mechanical stiffness of elastane-nylon textiles were tuned through respective fiber composition and orientation, i.e., aligned with and orthogonal to loading direction. Cell culture biocompatibility studies revealed no significant differences in proliferation rates of embryonic murine stem cells seeded on textiles compared to collagen membrane controls. Until the 15th day of culture, cells were rarely observed in direct contact with the elastane fibers, similar to previous observations with elastomeric sheets used in periosteum substitute implants. In the second part of the study textile samples were created from FDA-approved medical sutures comprising silk, expanded polytetrafluoroethylene, and polybutester. Biocompatibility and mechanical stiffness were assessed as a function of sterilization/disinfection mode (steam, ethylene oxide, and serial disinfection with ethanol). Cell proliferation rates did not differ significantly from controls, except for silk-suture containing textiles, which showed bacterial contamination and no viable cells after 15 days' culture for all sterilization methods. Sterilization had mixed (mostly not significant) effects on textile stiffness, except for the case of polybutester suture-based textiles that showed a significant increase in stiffness with ethylene oxide sterilization. In general, all textile combinations exhibited significantly higher stiffness than periosteum. Textiles comprising medical sutures of different stiffnesses arranged in engineered patterns offer a novel means to achieve mechanical gradients in medical device materials, emulating those of nature's own.

Quality Analysis of Minerals Formed by Jaw Periosteal Cells under Different Culture Conditions

Previously, we detected a higher degree of mineralization in fetal calf serum (FCS) compared to serum-free cultured jaw periosteum derived osteoprogenitor cells (JPCs). By Raman spectroscopy, we detected an earlier formation of mineralized extracellular matrix (ECM) of higher quality under serum-free media conditions. However, mineralization potential remained too low. In the present study, we aimed to investigate the biochemical composition and subsequent biomechanical properties of the JPC-formed ECM and minerals under human platelet lysate (hPL) and FCS supplementation. JPCs were isolated (n = 4 donors) and expanded under FCS conditions and used in passage five for osteogenic induction under both, FCS and hPL media supplementation. Raman spectroscopy and Alizarin Red/von Kossa staining were employed for biochemical composition analyses and for visualization and quantification of mineralization. Osteocalcin gene expression was analyzed by quantitative PCR. Biomechanical properties were assessed by using atomic force microscopy (AFM). Raman spectroscopic measurements showed significantly higher (p < 0.001) phosphate to protein ratios and in the tendency, lower carbonate to phosphate ratios in osteogenically induced JPCs under hPL in comparison to FCS culturing. Furthermore, higher crystal sizes were detected under hPL culturing of the cells. With respect to the ECM, significantly higher ratios of the precursor protein proline to hydroxyproline were detected in hPL-cultured JPC monolayers (p < 0.001). Additionally, significantly higher levels (p < 0.001) of collagen cross-linking were calculated, indicating a higher degree of collagen maturation in hPL-cultured JPCs. By atomic force microscopy, a significant increase in ECM stiffness (p < 0.001) of FCS cultured JPC monolayers was observed. The reverse effect was measured for the JPC formed precipitates/minerals. Under hPL supplementation, JPCs formed minerals of significantly higher stiffness (p < 0.001) when compared to the FCS setting. This study demonstrates that hPL culturing of JPCs leads to the formation of an anorganic material of superior quality in terms of biochemical composition and mechanical properties.

Prospective Design, Rapid Prototyping, and Testing of Smart Dressings, Drug Delivery Patches, and Replacement Body Parts Using Microscopy Aided Design and ManufacturE (MADAME)

Natural materials exhibit smart properties including gradients in biophysical properties that engender higher order functions, as well as stimuli-responsive properties which integrate sensor and/or actuator capacities. Elucidation of mechanisms underpinning such smart material properties (i), and translation of that understanding (ii), represent two of the biggest challenges in emulating natural design paradigms for design and manufacture of disruptive materials, parts, and products. Microscopy Aided Design And ManufacturE (MADAME) stands for a computer-aided additive manufacturing platform that incorporates multidimensional (multi-D) printing and computer-controlled weaving. MADAME enables the creation of composite design motifs emulating e.g., patterns of woven protein fibers as well as gradients in different caliber porosities, mechanical, and molecular properties, found in natural tissues, from the skin on bones (periosteum) to tree bark. Insodoing, MADAME provides a means to manufacture a new genre of smart materials, products and replacement body parts that exhibit advantageous properties both under the influence of as well as harnessing dynamic mechanical loads to activate material properties (mechanoactive properties). This Technical Report introduces the MADAME technology platform and its associated machine-based workflow (pipeline), provides basic technical background of the novel technology and its applications, and discusses advantages and disadvantages of the approach in context of current 3 and 4D printing platforms.

Primary cilia are necessary for Prx1-expressing cells to contribute to postnatal skeletogenesis

Although Prx1 (also known as PRRX1)-expressing cells and their primary cilia are critical for embryonic development, they have yet to be studied in the context of postnatal skeletogenesis owing to the lethality of mouse models. A tamoxifen-inducible Prx1 model has been developed, and we determined that expression directed by this promoter is highly restricted to the cambium layers in the periosteum and perichondrium after birth. To determine the postnatal role of these cambium layer osteochondroprogenitors (CLOPs) and their primary cilia, we developed models to track the fate of CLOPs (Prx1CreER-GFP;Rosa26^tdTomato) and selectively disrupt their cilia (Prx1CreER-GFP;Ift88^fl/fl). Our tracking studies revealed that CLOPs populate cortical and trabecular bone, the growth plate and secondary ossification centers during the normal program of postnatal skeletogenesis. Furthermore, animals lacking CLOP cilia exhibit stunted limb growth due to disruptions in endochondral and intramembranous ossification. Histological examination indicates that growth is stunted due to limited differentiation, proliferation and/or abnormal hypertrophic differentiation in the growth plate. Collectively, our results suggest that CLOPs are programmed to rapidly populate distant tissues and produce bone via a primary cilium-mediated mechanism in the postnatal skeleton.

Contribution of the periosteum to mandibular distraction

Mandibular distraction is a surgical process that progressively lengthens bone. To improve the distraction procedure and devices, the load of distraction and the mechanical strain of soft tissues during the process must be determined. We tested the assumption that it could be the periosteum primarily opposing distraction. Therefore we assessed the mechanical properties of the human mandibular periosteum and compared the stress-strain data with the torque measured on the activator during a cadaveric mandibular distraction. A 20 mm horizontal mandibular distraction was performed in 7 cadavers using standard distractors. Torque was measured with a torquemeter placed on the activation rods of the devices, providing a load (L_t) for each millimeter of distraction. In parallel, 18 periosteum samples were harvested from 9 cadaver mandibles. Uniaxial tensile tests were performed on the specimens and an estimated load (L_c) was calculated using periosteal stress-strain data and mandibular dimensions. During the distraction process, we observed an increase of the load L_t from 11.6 to 50.6 N. The periosteum exhibited a nonlinear viscoelastic stress-strain relationship, typical of biological tissues composed of collagen and elastin. The median L_c and L_t were not significantly different for the first millimeter of distraction. We demonstrated the periosteum is primarily responsible for opposing the distraction load.

Ischaemia, healing and outcomes in proximal humeral fractures

Interfragmental ischaemia is a prerequisite for the initiation of the inflammatory and immunological response to fracturing of bone.Intrafragmental ischaemia is inevitable: the extent of the initial ischaemic insult does not, however, directly relate to the outcome for healing of the fracture zones and avascular necrosis of the humeral head. The survival of distal regions of fragments with critical perfusion may be the result of a type of inosculation (blood vessel contact), which establishes reperfusion before either revascularization or neo-angiogenesis has occurred.Periosteum has a poorly defined role in fracture healing in the proximal humerus. The metaphyseal periosteal perfusion may have a profound effect, as yet undefined, on the healing of most metaphyseal fractures of the proximal humerus, and may be disturbed further by inadvertent surgical manipulation.The metaphysis can be considered as a 'torus' or ring of bone, its surface covered by periosteum antero- and posterolaterally, through which the tuberosity segments gain perfusion and capsular reflections antero- and posteromedially, through which the humeral head (articular) fragment gains perfusion.The torus is broken in relatively simple primary patterns: a fracture line at the upper surface of the torus is an anatomical 'neck' fracture; a fracture line at the lower surface of the torus is the surgical 'neck' fracture. Secondary fragmentation (through compression and/or distraction) of the torus itself creates complexity for analysis (classification), alters the capacity and outcome for healing (by variable interruption of the fragmental blood supply) and influences interfragmental stability. Cite this article: EFORT Open Rev 2018;3 DOI: 10.1302/2058-5241.3.180005.

The periosteum-like effect of fresh human amniotic membrane on bone regeneration in a rabbit critical-sized defect model

OBJECTIVE

The purpose of this study was to investigate the effect of fresh human amniotic membrane (FHAM) as a substitute of periosteum to enhance bone regeneration in critical-sized defects.

METHODS

Tibial diaphyseal bone defects were created in forty New Zealand white rabbits and treated with FHAM or left empty. Treatment groups consisted of: FHAM implanted in the place of removed periosteum (FHAMP group); FHFAM implanted to fill the entire defect (FHAMF group) compared to negative control group; empty defect with removing the periosteum (NC group) and positive control group; and empty defect without removing the periosteum (PC group). Bone regeneration was evaluated by radiographic, micro-computed tomography (μ-CT) and histological analyses at 4 and 8weeks post-surgery.

RESULTS

Radiographic and μ-CT analysis demonstrated clearly enhanced new bone formation in positive control group (PC) and FHAMP group compared to negative control group (NC) and FHAMF group. Histological staining exhibited remaining woven bones and cartilage matrix in the FHAMP group, immature lamellar bone with medellury cavity and marrow bone formation in PC group from 4weeks post-operatively. For FHAMF group, a little new bone formation was detected only from 8weeks post-operatively, and an absence of any sign of healing in NC group at both time points.

CONCLUSION

The results provide that FHAM increases bone regeneration in critical-sized defects when it is implanted in the place of the removed periosteum, but its additive effect does not have the same effect of the natural periosteum.

Muscle-Bone Crosstalk: Emerging Opportunities for Novel Therapeutic Approaches to Treat Musculoskeletal Pathologies

Osteoporosis and sarcopenia are age-related musculoskeletal pathologies that often develop in parallel. Osteoporosis is characterized by a reduced bone mass and an increased fracture risk. Sarcopenia describes muscle wasting with an increasing risk of injuries due to falls. The medical treatment of both diseases costs billions in health care per year. With the impact on public health and economy, and considering the increasing life expectancy of populations, more efficient treatment regimens are sought. The biomechanical interaction between both tissues with muscle acting on bone is well established. Recently, both tissues were also determined as secretory endocrine organs affecting the function of one another. New exciting discoveries on this front are made each year, with novel signaling molecules being discovered and potential controversies being described. While this review does not claim completeness, it will summarize the current knowledge on both the biomechanical and the biochemical link between muscle and bone. The review will highlight the known secreted molecules by both tissues affecting the other and finish with an outlook on novel therapeutics that could emerge from these discoveries.

Musculoskeletal Health in the Context of Spinal Cord Injury

PURPOSE OF REVIEW

This review assembles recent understanding of the profound loss of muscle and bone in spinal cord injury (SCI). It is important to try to understand these changes, and the context in which they occur, because of their impact on the wellbeing of SC-injured individuals, and the urgent need for viable preventative therapies.

RECENT FINDINGS

Recent research provides new understanding of the effects of age and systemic factors on the response of bone to loading, of relevance to attempts to provide load therapy for bone in SCI. The rapidly growing dataset describing the biochemical crosstalk between bone and muscle, and the cell and molecular biology of myokines signalling to bone and osteokines regulating muscle metabolism and mass, is reviewed. The ways in which this crosstalk may be altered in SCI is summarised. Therapeutic approaches to the catabolic changes in muscle and bone in SCI require a holistic understanding of their unique mechanical and biochemical context.

Engineering mechanical gradients in next generation biomaterials - Lessons learned from medical textile design

Nonwoven and textile membranes have been applied both externally and internally to prescribe boundary conditions for medical conditions as diverse as oedema and tissue defects. Incorporation of mechanical gradients in next generation medical membrane design offers great potential to enhance function in a dynamic, physiological context. Yet the gradient properties and resulting mechanical performance of current membranes are not well described. To bridge this knowledge gap, we tested and compared the mechanical properties of bounding membranes used in both external (compression sleeves for oedema, exercise bands) and internal (surgical membranes) physiological contexts. We showed that anisotropic compression garment textiles, isotropic exercise bands and surgical membranes exhibit similar ranges of resistance to tension under physiologic strains. However, their mechanical gradients and resulting stress-strain relationships show differences in work capacity and energy expenditure. Exercise bands' moduli of elasticity and respective thicknesses allow for controlled, incremental increases in loading to facilitate healing as injured tissues return to normal structure and function. In contrast, the gradients intrinsic to compression sleeve design exhibit gaps in the middle range (1-5N) of physiological strains and also inconsistencies along the length of the sleeve, resulting in less than optimal performance of these devices. These current shortcomings in compression textile and garment design may be addressed in the future through implementation of novel approaches. For example, patterns, fibre compositions, and fibre anisotropy can be incorporated into biomaterial design to achieve seamless mechanical gradients in structure and resulting dynamic function, which would be particularly useful in physiological contexts. These concepts can be applied further to biomaterial design to deliver pressure gradients during movement of oedematous limbs (compression garments) and facilitate transport of molecules and cells during tissue genesis within tissue defects (surgical membranes).

STATEMENT OF SIGNIFICANCE

External and internal biomaterial membranes prescribe boundary conditions for treatment of medical disorders, from oedema to tissue defects. Studies are needed to guide the design of next generation biomaterials and devices that incorporate gradient engineering approaches, which offer great potential to enhance function in a dynamic and physiological context. Mechanical gradients intrinsic to currently implemented biomaterials such as medical textiles and surgical interface membranes are poorly understood. Here we characterise quantitatively the mechanics of textile and nonwoven biomaterial membranes for external and internal use. The lack of seamless gradients in compression medical textiles contrasts with the graded mechanical effects achieved by elastomeric exercise bands, which are designed to deliver controlled, incremental increases in loading to facilitate healing as injured tissues return to normal structure and function. Engineering textiles with a prescient choice of fibre composition/size, type of knit/weave and inlay fibres, and weave density/anisotropy will enable creation of fabrics that can deliver spatially and temporally controlled mechanical gradients to maintain force balances at tissue boundaries, e.g. to treat oedema or tissue defects.

Engineering biomimetic periosteum with β-TCP scaffolds to promote bone formation in calvarial defects of rats

Background

There is a critical need for the management of large bone defects. The purpose of this study was to engineer a biomimetic periosteum and to combine this with a macroporous β-tricalcium phosphate (β-TCP) scaffold for bone tissue regeneration.

Methods

Rat bone marrow-derived mesenchymal stem cells (rBMSCs) were harvested and cultured in different culture media to form undifferentiated rBMSC sheets (undifferentiated medium (UM)) and osteogenic cell sheets (osteogenic medium (OM)). Simultaneously, rBMSCs were differentiated to induced endothelial-like cells (iECs), and the iECs were further cultured on a UM to form a vascularized cell sheet. At the same time, flow cytometry was used to detect the conversion rates of rBMSCs to iECs. The pre-vascularized cell sheet (iECs/UM) and the osteogenic cell sheet (OM) were stacked together to form a biomimetic periosteum with two distinct layers, which mimicked the fibrous layer and cambium layer of native periosteum. The biomimetic periostea were wrapped onto porous β-TCP scaffolds (BP/β-TCP) and implanted in the calvarial bone defects of rats. As controls, autologous periostea with β-TCP (AP/β-TCP) and β-TCP alone were implanted in the calvarial defects of rats, with a no implantation group as another control. At 2, 4, and 8 weeks post-surgery, implants were retrieved and X-ray, microcomputed tomography (micro-CT), histology, and immunohistochemistry staining analyses were performed.

Results

Flow cytometry results showed that rBMSCs were partially differentiated into iECs with a 35.1% conversion rate in terms of CD31. There were still 20.97% rBMSCs expressing CD90. Scanning electron microscopy (SEM) results indicated that cells from the wrapped cell sheet on the β-TCP scaffold apparently migrated into the pores of the β-TCP scaffold. The histology and immunohistochemistry staining results from in vivo implantation indicated that the BP/β-TCP and AP/β-TCP groups promoted the formation of blood vessels and new bone tissues in the bone defects more than the other two control groups. In addition, micro-CT showed that more new bone tissue formed in the BP/β-TCP and AP/β-TCP groups than the other groups.

Conclusions

Inducing rBMSCs to iECs could be a good strategy to obtain an endothelial cell source for prevascularization. Our findings indicate that the biomimetic periosteum with porous β-TCP scaffold has a similar ability to promote osteogenesis and angiogenesis in vivo compared to the autologous periosteum. This function could result from the double layers of biomimetic periosteum. The prevascularized cell sheet served a mimetic fibrous layer and the osteogenic cell sheet served a cambium layer of native periosteum. The biomimetic periosteum with a porous ceramic scaffold provides a new promising method for bone healing.

Scale-up of nature's tissue weaving algorithms to engineer advanced functional materials

We are literally the stuff from which our tissue fabrics and their fibers are woven and spun. The arrangement of collagen, elastin and other structural proteins in space and time embodies our tissues and organs with amazing resilience and multifunctional smart properties. For example, the periosteum, a soft tissue sleeve that envelops all nonarticular bony surfaces of the body, comprises an inherently "smart" material that gives hard bones added strength under high impact loads. Yet a paucity of scalable bottom-up approaches stymies the harnessing of smart tissues' biological, mechanical and organizational detail to create advanced functional materials. Here, a novel approach is established to scale up the multidimensional fiber patterns of natural soft tissue weaves for rapid prototyping of advanced functional materials. First second harmonic generation and two-photon excitation microscopy is used to map the microscopic three-dimensional (3D) alignment, composition and distribution of the collagen and elastin fibers of periosteum, the soft tissue sheath bounding all nonarticular bone surfaces in our bodies. Then, using engineering rendering software to scale up this natural tissue fabric, as well as multidimensional weaving algorithms, macroscopic tissue prototypes are created using a computer-controlled jacquard loom. The capacity to prototype scaled up architectures of natural fabrics provides a new avenue to create advanced functional materials.

Periosteum mechanobiology and mechanistic insights for regenerative medicine

Periosteum is a smart mechanobiological material that serves as a habitat and delivery vehicle for stem cells as well as biological factors that modulate tissue genesis and healing. Periosteum's remarkable regenerative capacity has been harnessed clinically for over two hundred years. Scientific studies over the past decade have begun to decipher the mechanobiology of periosteum, which has a significant role in its regenerative capacity. This integrative review outlines recent mechanobiological insights that are key to modulating and translating periosteum and its resident stem cells in a regenerative medicine context.

Determining the critical size of a rabbit rib segmental bone defect model

In order to establish and standardize the rabbit rib segmental bone defect model, it is of vital importance to determine rabbit rib critical size defect (CSD). According to the general time needed for spontaneous long-bone regeneration, three-month observation period was set to determine the CSD. The rabbit rib segmental bone defects with different sizes from 1 to 5 cm with or without periosteum were performed in the eighth rib of 4-month-old male New Zealand rabbits and underwent X-ray examinations at the 4th, 8th and 12th weeks postoperatively. The gross and histological examinations at postoperative week 12 were evaluated, which showed that the critical sizes in the rabbit rib models with and without periosteum were 5 and 2 cm, respectively. This study provides prerequisite data for establishing rabbit rib CSD model and evaluating bone materials using this model.

Non-invasive diffuse correlation tomography reveals spatial and temporal blood flow differences in murine bone grafting approaches

Longitudinal blood flow during murine bone graft healing was monitored non-invasively using diffuse correlation tomography. The system utilized spatially dense data from a scanning set-up, non-linear reconstruction, and micro-CT anatomical information. Weekly in vivo measurements were performed. Blood flow changes in autografts, which heal successfully, were localized to graft regions and consistent across mice. Poor healing allografts showed heterogeneous blood flow elevation and high inter-subject variabilities. Allografts with tissue-engineered periosteum showed responses intermediate to both autografts and allografts, consistent with healing observed. These findings suggest that spatiotemporal blood flow changes can be utilized to differentiate the degree of bone graft healing.

Live Tissue Imaging to Elucidate Mechanical Modulation of Stem Cell Niche Quiescence

The periosteum, a composite cellular connective tissue, bounds all nonarticular bone surfaces. Like Velcro, collagenous Sharpey's fibers anchor the periosteum in a prestressed state to the underlying bone. The periosteum provides a niche for mesenchymal stem cells. Periosteal lifting, as well as injury, causes cells residing in the periosteum (PDCs) to change from an immobile, quiescent state to a mobile, active state. The physical cues that activate PDCs to home to and heal injured areas remain a conundrum. An understanding of these cues is key to unlocking periosteum's remarkable regenerative power. We hypothesized that changes in periosteum's baseline stress state modulate the quiescence of its stem cell niche. We report, for the first time, a three-dimensional, high-resolution live tissue imaging protocol to observe and characterize ovine PDCs and their niche before and after release of the tissue's endogenous prestress. Loss of prestress results in abrupt shrinkage of the periosteal tissue. At the microscopic scale, loss of prestress results in significantly increased crimping of collagen of periosteum's fibrous layer and a threefold increase in the number of rounded nuclei in the cambium layer. Given the body of published data describing the relationships between stem cell and nucleus shape, structure and function, these observations are consistent with a role for mechanics in the modulation of periosteal niche quiescence. The quantitative characterization of periosteum as a stem cell niche represents a critical step for clinical translation of the periosteum and periosteum substitute-based implants for tissue defect healing. Stem Cells Translational Medicine 2017;6:285-292.

Translating Periosteum's Regenerative Power: Insights From Quantitative Analysis of Tissue Genesis With a Periosteum Substitute Implant

: An abundance of surgical studies during the past 2 centuries provide empirical evidence of periosteum's regenerative power for reconstructing tissues as diverse as trachea and bone. This study aimed to develop quantitative, efficacy-based measures, thereby providing translational guidelines for the use of periosteum to harness the body's own healing potential and generate target tissues. The current study quantitatively and qualitatively demonstrated tissue generation modulated by a periosteum substitute membrane that replicates the structural constituents of native periosteum (elastin, collagen, progenitor cells) and its barrier, extracellular, and cellular properties. It shows the potentiation of the periosteum's regenerative capacity through the progenitor cells that inhabit the tissue, biological factors intrinsic to the extracellular matrix of periosteum, and mechanobiological factors related to implant design and implementation. In contrast to the direct intramembranous bone generated in defects surrounded by patent periosteum in situ, tissue generation in bone defects bounded by the periosteum substitute implant occurred primarily via endochondral mechanisms whereby cartilage was first generated and then converted to bone. In addition, in defects treated with the periosteum substitute, tissue generation was highest along the major centroidal axis, which is most resistant to prevailing bending loads. Taken together, these data indicate the possibility of designing modular periosteum substitute implants that can be tuned for vectorial and spatiotemporal delivery of biological agents and facilitation of target tissue genesis for diverse surgical scenarios and regenerative medicine approaches. It also underscores the potential to develop physical therapy protocols to maximize tissue genesis via the implant's mechanoactive properties.

SIGNIFICANCE

In the past 2 centuries, the periosteum, a niche for stem cells and super-smart biological material, has been used empirically in surgery to repair tissues as diverse as trachea and bone. In the past 25 years, the number of articles indexed in PubMed for the keywords "periosteum and tissue engineering" and "periosteum and regenerative medicine" has burgeoned. Yet the biggest limitation to the prescriptive use of periosteum is lack of easy access, giving impetus to the development of periosteum substitutes. Recent studies have opened up the possibility to bank periosteal tissues (e.g., from the femoral neck during routine resection for implantation of hip replacements). This study used an interdisciplinary, quantitative approach to assess tissue genesis in modular periosteum substitute implants, with the aim to provide translational strategies for regenerative medicine and tissue engineering.

Engineering and commercialization of human-device interfaces, from bone to brain

Cutting edge developments in engineering of tissues, implants and devices allow for guidance and control of specific physiological structure-function relationships. Yet the engineering of functionally appropriate human-device interfaces represents an intractable challenge in the field. This leading opinion review outlines a set of current approaches as well as hurdles to design of interfaces that modulate transfer of information, i.a. forces, electrical potentials, chemical gradients and haptotactic paths, between endogenous and engineered body parts or tissues. The compendium is designed to bridge across currently separated disciplines by highlighting specific commonalities between seemingly disparate systems, e.g. musculoskeletal and nervous systems. We focus on specific examples from our own laboratories, demonstrating that the seemingly disparate musculoskeletal and nervous systems share common paradigms which can be harnessed to inspire innovative interface design solutions. Functional barrier interfaces that control molecular and biophysical traffic between tissue compartments of joints are addressed in an example of the knee. Furthermore, we describe the engineering of gradients for interfaces between endogenous and engineered tissues as well as between electrodes that physically and electrochemically couple the nervous and musculoskeletal systems. Finally, to promote translation of newly developed technologies into products, protocols, and treatments that benefit the patients who need them most, regulatory and technical challenges and opportunities are addressed on hand from an example of an implant cum delivery device that can be used to heal soft and hard tissues, from brain to bone.

Non-Invasive Monitoring of Temporal and Spatial Blood Flow during Bone Graft Healing Using Diffuse Correlation Spectroscopy

Vascular infiltration and associated alterations in microvascular blood flow are critical for complete bone graft healing. Therefore, real-time, longitudinal measurement of blood flow has the potential to successfully predict graft healing outcomes. Herein, we non-invasively measure longitudinal blood flow changes in bone autografts and allografts using diffuse correlation spectroscopy in a murine femoral segmental defect model. Blood flow was measured at several positions proximal and distal to the graft site before implantation and every week post-implantation for a total of 9 weeks (autograft n = 7 and allograft n = 10). Measurements of the ipsilateral leg with the graft were compared with those of the intact contralateral control leg. Both autografts and allografts exhibited an initial increase in blood flow followed by a gradual return to baseline levels. Blood flow elevation lasted up to 2 weeks in autografts, but this duration varied from 2 to 6 weeks in allografts depending on the spatial location of the measurement. Intact contralateral control leg blood flow remained at baseline levels throughout the 9 weeks in the autograft group; however, in the allograft group, blood flow followed a similar trend to the graft leg. Blood flow difference between the graft and contralateral legs (ΔrBF), a parameter defined to estimate graft-specific changes, was elevated at 1–2 weeks for the autograft group, and at 2–4 weeks for the allograft group at the proximal and the central locations. However, distal to the graft, the allograft group exhibited significantly greater ΔrBF than the autograft group at 3 weeks post-surgery (p < 0.05). These spatial and temporal differences in blood flow supports established trends of delayed healing in allografts versus autografts.

Pressure-activated microsyringe (PAM) fabrication of bioactive glass-poly(lactic-co-glycolic acid) composite scaffolds for bone tissue regeneration

The aim of this work was the fabrication and characterization of bioactive glass-poly(lactic-co-glycolic acid) (PLGA) composite scaffolds mimicking the topological features of cancellous bone. Porous multilayer PLGA-CEL2 composite scaffolds were innovatively produced by a pressure-activated microsyringe (PAM) method, a CAD/CAM processing technique originally developed at the University of Pisa. In order to select the optimal formulations to be extruded by PAM, CEL2-PLGA composite films (CEL2 is an experimental bioactive SiO₂ -P₂ O₅ -CaO-MgO-Na₂ O-K₂ O glass developed at Politecnico di Torino) were produced and mechanically tested. The elastic modulus of the films increased from 30 to > 400 MPa, increasing the CEL2 amount (10-50 wt%) in the composite. The mixture containing 20 wt% CEL2 was used to fabricate 2D and 3D bone-like scaffolds composed by layers with different topologies (square, hexagonal and octagonal pores). It was observed that the increase of complexity of 2D topological structures led to an increment of the elastic modulus from 3 to 9 MPa in the composite porous monolayer. The elastic modulus of 3D multilayer scaffolds was intermediate (about 6.5 MPa) between the values of the monolayers with square and octagonal pores (corresponding to the lowest and highest complexity, respectively). MG63 osteoblast-like cells and periosteal-derived precursor cells (PDPCs) were used to assess the biocompatibility of the 3D bone-like scaffolds. A significant increase in cell proliferation between 48 h and 7 days of culture was observed for both cell phenotypes. Moreover, qRT-PCR analysis evidenced an induction of early genes of osteogenesis in PDPCs. Copyright © 2015 John Wiley & Sons, Ltd.

Osteoblast biocompatibility of premineralized, hexamethylene-1,6-diaminocarboxysulfonate crosslinked chitosan fibers

Biopolymer-ceramic composites are thought to be particularly promising materials for bone tissue engineering as they more closely mimic natural bone. Here, we demonstrate the fabrication by electrospinning of fibrous chitosan-hydroxyapatite composite scaffolds with low (1 wt %) and high (10 wt %) mineral contents. Scanning electron microscopy (FESEM), energy dispersive spectroscopy (EDS) and unidirectional tensile testing were performed to determine fiber surface morphology, elemental composition, and tensile Young's modulus (E) and ultimate tensile strength (σUTS ), respectively. EDS scans of the scaffolds indicated that the fibers, crosslinked with either hexamethylene-1,6-diaminocarboxysulfonate (HDACS) or genipin, have a crystalline hydroxyapatite mineral content at 10 wt % additive. Moreover, FESEM micrographs showed that all electrospun fibers have diameters (122-249 nm), which fall within the range of those of fibrous collagen found in the extracellular matrix of bone. Young's modulus and ultimate tensile strength of the various crosslinked composite compositions were in the range of 116-329 MPa and 2-15 MPa, respectively. Osteocytes seeded onto the mineralized fibers were able to demonstrate good biocompatibility enhancing the potential use for this material in future bone tissue engineering applications.