Platelets are mitogenic for periosteum-derived cells.

The early stages of bone regeneration are associated with a high mitogenic activity of periosteal cells. Here we addressed the question of whether platelets that accumulate within the developing haematoma can account for this tissue response. Addition of platelets, platelet-released supernatants, platelet membranes, and microparticles to bovine periosteum-derived cells resulted in an increase in 3H-thymidine incorporation; lipid extracts had no effect. Platelet-released supernatants retained their activity after incubation at 56 degrees C, but not at 100 degrees C. Gel chromatographic analysis revealed the highest mitogenic activity at approximately 35 kD. Of the factors released from activated platelets, basic fibroblast growth factor (bFGF) and platelet-derived growth factor (PDGF) increased 3H-thymidine incorporation. The mitogenic activity of platelet-released supernatants was decreased by anti-PDGF, and anti-bFGF antibodies. Platelet-released supernatants increased the number of proliferating periosteum-derived cells as determined by the expression pattern of Ki67. Platelet-released supernatants also resulted in a stimulation of cell proliferation in periosteal explants. These results suggest that platelets have the potential to stimulate the mitogenic response of the periosteum during bone repair.     

Induction of ectopic bone formation by using human periosteal cells in combination with a novel scaffold technology

Due to their osteogenic germination potential, periosteum-derived osteoprogenitor cells are a potential source for tissue engineering a bone graft that could be used to regenerate skeletal defects. In this study we evaluated if ectopic bone formation could be induced by a construct made of human periosteal cells and a novel scaffold architecture whose mechanical properties are in the range of cancellous bone. Biopsies from human calvarial periosteum were harvested and cells were isolated from the inner cambial layer. Fifty thousand periosteal cells were seeded into the scaffolds measuring 6 x 6 x 2 mm. The cell-scaffold constructs were cultured for a period of 3 weeks prior to implantation into balb C nude mice. Mice were sacrificed and implants were analyzed 6 and 17 weeks postoperatively. Immunohistochemical analysis confirmed the osteoblastic phenotype of the seeded cells. Formation of focal adhesions and stress fibers could be observed in both scaffold architectures. Three-dimensional cell proliferation was observed after 2 weeks of culturing with centripetal growth pattern inside the pore network. The deposition of calcified extracellular matrix was observed after 3 weeks of culturing. In vivo, endochondral bone formation with osteoid production was detectable via von Kossa and Osteocalcin staining after 6 and 17 weeks. Histology and SEM revealed that the entire scaffold/bone grafts were penetrated by a vascular network. This study showed the potential of bone tissue engineering by using human periosteal cells in combination with a novel scaffold technology.     

Transforming growth factor beta 1 stimulates type II collagen expression in cultured periosteum-derived cells.

Chondrogenesis can occur during a bone repair process, which is related to several growth factors. Transforming growth factor beta 1 (TGF-beta 1) downregulates the expression of type II collagen by chondrocytes in vitro, but injection of TGF-beta 1 into the periosteum in vivo increases type II collagen mRNA levels and initiates chondrogenesis. We examined the effect of TGF-beta 1 on collagen gene expression in a bovine periosteum-derived cell culture system to evaluate its direct effect on the periosteum. Cultured cells expressed alkaline phosphatase and collagen pro alpha 1(I) and pro alpha 1(II) mRNAs. A low level of type II collagen synthesis was demonstrated by immunoprecipitation. TGF-beta 1 had no effect on periosteal cell proliferation. Expression of collagen pro alpha 1(I) mRNA did not change with TGF-beta 1 treatment, but alkaline phosphatase mRNA showed a dose-dependent decrease. Expression of collagen pro alpha 1(II) mRNA was stimulated 2.7-fold by TGF-beta 1. TGF-beta 1 also caused a 2.6-fold increase in type II collagen synthesis by immunoprecipitation. These findings indicate that TGF-beta 1 is an enhancer of the expression of the chondrocyte phenotype of the periosteal cells and suggest that TGF-beta 1 is important in initiating and promoting cartilage formation in vivo.     

Hydraulic elevation of the periosteum: a novel technique for periosteal harvest.

Periosteum has been promoted as a potential substrate for tissue engineering. Its principal virtues are that it has a source of pluripotential mesenchymal cells and chondrogenic growth factors located in the cambium layer, and it can serve as a template for directional evolution of neo-tissue. The clinical use and in vitro study of periosteum-derived neo-tissue has been limited by the level of surgical skill required for harvest. Precise surgical technique, task-specific experience, adequate volume of procedures, and general surgical expertise are required for optimal harvest using the traditional periosteal elevator method. This report describes an easily mastered technique that preserves viability while providing the harvest of relatively large amounts of periosteum. Skeletally mature New Zealand white rabbits (11 males/20 tibias; 4 females/8 tibias; approximate weight 3.5 kg) and one Yucatan miniature pig were used for harvest of periosteum from the tibia using the traditional periosteal elevator and the developed hydraulic elevation approach. Histologic examination of the periosteal explants obtained by the developed method showed preservation of the cambium layer containing the progenitor cells necessary for the generation of neo-cartilage. This technique provides a simple method of harvesting large segments (>5 cm x 1 cm) of periosteum in a single procedure and may facilitate better exploitation of periosteum in tissue engineering.     

The periosteum: its function reassessed

A review of experimental work relating to the function of the periosteum, beginning with that of Macewen in 1912, is of special interest to clinicians. Macewen's failure to obtain new-bone formation following the subperiosteal removal of the radius was due to a technique that obliterated the space, an occurrence that is inevitable in total excision. The periosteum without an associated hematoma in contact with bone has no evident bone-forming properties. Except in avulsion fractures, the periosteum provides mechanical connection between the two bone fragments and is the vehicle for the revascularization of the contents of the periosteal tube. Loss of this continuity is one of the causes of fibrous union in avulsion fractures. In developing countries, delayed and unsatisfactory treatment of fractures provides valuable clinical information in the role of the periosteum. Destruction of the periosteum along with the bony shaft in extensive untreated osteomyelitis results in a missing segment of a major bone.     

Current insights on the regenerative potential of the periosteum: Molecular,cellular, and endogenous engineering approaches

While century old clinical reports document the periosteum's remarkable regenerative capacity, only in the past decade have scientists undertaken mechanistic investigations of its regenerative potential. At a Workshop at the 2012 Annual Meeting of Orthopaedic Research Society, we reviewed the molecular, cellular, and tissue scale approaches to elucidate the mechanisms underlying the periosteum's regenerative potential as well as translational therapies engineering solutions inspired by its remarkable regenerative capacity. The entire population of osteoblasts within periosteum, and at endosteal and trabecular bone surfaces within the bone marrow, derives from the embryonic perichondrium. Periosteal cells contribute more to cartilage and bone formation within the callus during fracture healing than do cells of the bone marrow or endosteum, which do not migrate out of the marrow compartment. Furthermore, a current healing paradigm regards the activation, expansion, and differentiation of periosteal stem/progenitor cells as an essential step in building a template for subsequent neovascularization, bone formation, and remodeling. The periosteum comprises a complex, composite structure, providing a niche for pluripotent cells and a repository for molecular factors that modulate cell behavior. The periosteum's advanced, “smart” material properties change depending on the mechanical, chemical, and biological state of the tissue. Understanding periosteum development, progenitor cell‐driven initiation of periosteum's endogenous tissue building capacity, and the complex structure–function relationships of periosteum as an advanced material are important for harnessing and engineering ersatz materials to mimic the periosteum's remarkable regenerative capacity. © 2012 Orthopaedic Research Society. Published by Wiley Periodicals, Inc. J Orthop Res 30:1869–1878, 2012     

Effect of cortisol on periosteal and nonperiosteal collagen and DNA synthesis in cultured rat calvariae

Summary The effects of cortisol on bone formation are complex and may be modulated by the presence of periosteal cells or by factors released by the periosteal tissue. To test these possibilities, cortisol was examined for its effects on the incorporation of³H-proline into collagenase-digestible protein (CDP) and noncollagen protein (NCP), on DNA synthesis and on alkaline phosphatase activity in intact and in the periosteum and nonperiosteal bone of dissected calvariae from 21-day-old fetal rats. After 24 h of treatment, cortisol increased the incorporation of³H-proline into CDP in intact bones and in the nonperiosteal bone of calvariae dissected after the culture. Cortisol inhibited the incorporation of³H-thymidine into calvarial DNA but it caused a small increase in nonperiosteal DNA content. Cortisol did not affect the incorporation of³H-proline into CDP in calvariae dissected prior to the culture if the periosteum and nonperiosteal central bone were incubated separately; the stimulatory effect was observed only if the two tissues were cultured in the same vial and were in contact. In contrast, cortisol stimulated alkaline phosphatase activity in the central nonperiosteal bone of calvariae dissected before or after the culture. After 72–96 h of treatment, cortisol inhibited the labeling of CDP, NCP, and DNA and the DNA content in intact bones and in both periosteal and nonperiosteal central bone of calvariae dissected after the culture. In contrast, when the periosteum was removed before the incubation, these inhibitory effects were observed in the periosteum and not in the nonperiosteal bone. Cortisol inhibited alkaline phosphatase activity in intact bones treated for 96 h, but removal of the periosteum resulted in a stimulatory effect in the nonperiosteal central bone. These studies indicate that 24 h treatment with cortisol stimulates collagen synthesis in nonperiosteal bone, an effect that requires the presence of periosteal tissue. Exposure to cortisol for 72–96 h inhibits collagen, noncollagen protein, and DNA synthesis, an effect that is secondary to an inhibition of periosteal cell replication. Cortisol does not cause a direct inhibition of osteoblastic function.     

MMP9 regulates the cellular response to inflammation after skeletal injury

Like other tissue injuries, bone fracture triggers an inflammatory response, which plays an important role in skeletal repair. Inflammation is believed to have both positive and negative effects on bone repair, but the underlying cellular mechanisms are not well understood. To assess the role of inflammation on skeletal cell differentiation, we used mouse models of fracture repair that stimulate either intramembranous or endochondral ossification. In the first model, fractures are rigidly stabilized leading to direct bone formation, while in the second model, fracture instability causes cartilage and bone formation. We compared the inflammatory response in these two mechanical environments and found changes in the expression patterns of inflammatory genes and in the recruitment of inflammatory cells and osteoclasts. These results suggested that the inflammatory response could influence skeletal cell differentiation after fracture. We then exploited matrix metalloproteinase 9 (MMP9) that is expressed in inflammatory cells and osteoclasts, and which we previously showed is a potential regulator of cell fate decisions during fracture repair. Mmp9^?/? mice heal stabilized fractures via endochondral ossification, while wild type mice heal via intramembranous ossification. In parallel, we observed increases in macrophages and T cells in the callus of Mmp9^?/? compared to wild type mice. To assess the link between the profile of inflammatory cells and skeletal cell fate functionally, we transplanted Mmp9^?/? mice with wild type bone marrow, to reconstitute a wild type hematopoietic lineage in interaction with the Mmp9^?/? stroma and periosteum. Following transplantation, Mmp9^?/? mice healed stabilized fractures via intramembranous ossification and exhibited a normal profile of inflammatory cells. Moreover, Mmp9^?/? periosteal grafts healed via intramembranous ossification in wild type hosts, but healed via endochondral ossification in Mmp9^?/? hosts. We observed that macrophages accumulated at the periosteal surface in Mmp9^?/? mice, suggesting that cell differentiation in the periosteum is influenced by factors such as BMP2 that are produced locally by inflammatory cells. Taken together, these results show that MMP9 mediates indirect effects on skeletal cell differentiation by regulating the inflammatory response and the distribution of inflammatory cells, leading to the local regulation of periosteal cell differentiation.     

Skeletal Stem/Progenitor Cells in Periosteum and Skeletal Muscle Share a Common Molecular Response to Bone Injury

Bone regeneration involves skeletal stem/progenitor cells (SSPCs) recruited from bone marrow, periosteum, and adjacent skeletal muscle. To achieve bone reconstitution after injury, a coordinated cellular and molecular response is required from these cell populations. Here, we show that SSPCs from periosteum and skeletal muscle are enriched in osteochondral progenitors, and more efficiently contribute to endochondral ossification during fracture repair as compared to bone-marrow stromal cells. Single-cell RNA sequencing (RNAseq) analyses of periosteal cells reveal the cellular heterogeneity of periosteum at steady state and in response to bone fracture. Upon fracture, both periosteal and skeletal muscle SSPCs transition from a stem/progenitor to a fibrogenic state prior to chondrogenesis. This common activation pattern in periosteum and skeletal muscle SSPCs is mediated by bone morphogenetic protein (BMP) signaling. Functionally, Bmpr1a gene inactivation in platelet-derived growth factor receptor alpha (Pdgfra)-derived SSPCs impairs bone healing and decreases SSPC proliferation, migration, and osteochondral differentiation. These results uncover a coordinated molecular program driving SSPC activation in periosteum and skeletal muscle toward endochondral ossification during bone regeneration. © 2022 The Authors. Journal of Bone and Mineral Research published by Wiley Periodicals LLC on behalf of American Society for Bone and Mineral Research (ASBMR).     

Regulation of the expression of the type-II collagen gene in periosteum-derived cells by three members of the transforming growth factor–β superfamily

Although transforming growth factors-β and bone morphogenetic proteins are both capable of inducing bone formation in vivo, the target cells of their osteoinductive actions may be different. To evaluate periosteal cells as potential targets of the actions of transforming growth factor–β and bone morphogenetic protein, we investigated the ability of three members of the transforming growth factor–β superfamily to modulate expression of the gene encoding the α1(II) chain of type-ll collagen in periosteum-derived cells in Vitro. The results demonstrate that transforming growth factor–β mRNA is expressed by periosteum-derived cells and that exogenous transforming growth factor–β1 acts to upregulate expression of the gene encoding collagen α1(II). This effect was observed as early as 12 hours after administration of transforming growth factor–β1 but was not observed in response to bone morphogenetic proteins 3 or 4. No synergy was demonstrated between transforming growth factor–β1 and bone morphogenetic protein-3 in the ability to upregulate expression of the collagen α1(II) gene. These results support the hypothesis that committed periosteal mesenchymal cells are cellular targets of the action of transforming growth factor–β.     

The behaviour of the periosteum during callotasis

Periosteal behaviour during callotasis has been studied in animals but not in humans. Markers were inserted into the periosteum of seven patients who underwent callotasis. All of them had tibial callotasis, five patients had bone transport procedure and two had leg lengthening due to congenital disorder. They were followed up radiologically at regular intervals and during the distraction the movement of the periosteal markers was recorded. This enabled observation of the way the periosteum elongates during the procedure and also the calculation of periosteal strain at different stages at the points where the periosteum is fixed to bone. The study indicated that in most cases the periosteum acts as an elastic sleeve surrounding the newly formed tissue during lengthening. The site of attachment between sleeve and cortex became established early during lengthening, and hardly changed position at later stages. There was a wide spread of attachment sites and periosteal strains. Attachment sites were not related to pin or wire entry points, and strains were not predictive of callus shape or healing time.     

Periosteum,bone's “smart” bounding membrane,exhibits direction‐dependent permeability

The periosteum serves as bone's bounding membrane, exhibits hallmarks of semipermeable epithelial barrier membranes, and contains mechanically sensitive progenitor cells capable of generating bone. The current paucity of data regarding the periosteum's permeability and bidirectional transport properties provided the impetus for the current study. In ovine femur and tibia samples, the periosteum's hydraulic permeability coefficient, k, was calculated using Darcy's Law and a custom‐designed permeability tester to apply controlled, volumetric flow of phosphate‐buffered saline through periosteum samples. Based on these data, ovine periosteum demonstrates mechanically responsive and directionally dependent (anisotropic) permeability properties. At baseline flow rates comparable to interstitial fluid flow (0.5 µL/s), permeability is low and does not exhibit anisotropy. In contrast, at high flow rates comparable to those prevailing during traumatic injury, femoral periosteum exhibits an order of magnitude higher permeability compared to baseline flow rates. In addition, at high flow rates permeability exhibits significant directional dependence, with permeability higher in the bone to muscle direction than vice versa. Furthermore, compared to periosteum in which the intrinsic tension (pre‐stress) is maintained, free relaxation of the tibial periosteum after resection significantly increases its permeability in both flow directions. Hence, the structure and mechanical stress state of periosteum influences its role as bone's bounding membrane. During periods of homeostasis, periosteum may serve as a barrier membrane on the outer surface of bone, allowing for equal albeit low quiescent molecular communication between tissue compartments including bone and muscle. In contrast, increases in pressure and baseline flow rates within the periosteum resulting from injury, trauma, and/or disease may result in a significant increase in periosteum permeability and consequently in increased molecular communication between tissue compartments. Elucidation of the periosteum's permeability properties is key to understanding periosteal mechanobiology in bone health and healing, as well as to elucidate periosteum structure and function as a smart biomaterial that allows bidirectional and mechanically responsive fluid transport. © 2013 American Society for Bone and Mineral Research.     

Osteogenic potential of primed periosteum graft in the rat calvarial model

Repair of bone defects remains a major concern in plastic and maxillofacial surgery. Based on modern concepts of tissue engineering, periosteum has gained attention as a suitable osteogenic material. We tested the hypothesis that surgically released and immediately repositioned periosteum would exhibit high osteogenic capacity upon grafting in a rat calvarial defect. Seven days after periosteum was released from the tibia and immediately repositioned, the "primed periosteum graft" (PPG; n = 15) was placed into a critical-sized defect of rat calvaria and the process of bone formation was evaluated histologically, immunohistologically, and radiographically at 7, 14, and 21 days after grafting. Findings were compared with a nonprimed periosteal graft (NPG; n = 15).Endochondral ossification was observed in both the PPG and NPG. The PPG showed higher expression of proliferative cell nuclear antigen, bone morphogenetic protein, and vascular endothelial growth factor than the NPG. Three-dimensional radiographic examination revealed significantly increased bone formation in the PPG than in the NPG (P < 0.01). These findings suggested that surgical stimulation of the periosteum enhanced the osteogenic potential of periosteal cells. This method may be suitable for the clinical repair of bone defects.     

In vivo osteochondrogenic potential of cultured cells derived from the periosteum

Periosteal cells were isolated from young chicks, introduced into cell culture, subcultured, and then inoculated into athymic, nude mice to test the in vivo osteochondrogenic potential of cultured periosteal cells. In monolayer cultures, the adherent periosteal cells showed fibroblastlike morphology and overtly expressed neither osteogenic nor chondrogenic phenotypes. Cultured cells inoculated heterotopically into nude mice eventually gave rise to bone tissue at the subcutaneous injection site. The process of bone formation occurred through two different mechanisms: intramembranous bone formation at the peripheral portion of the inoculum early and endochondral bone formation in the central portion later. Frozen, preserved periosteal cells also formed bone after introduction into nude mice in the same temporal histologic sequence as the unfrozen cells. Cultured chick muscle fibroblasts from donors that were the same age as controls did not form bone or cartilage when inoculated under identical conditions to those of cultured periosteal cells. These results suggest that periosteum of young chicks contains subsets of progenitor cells that possess the potential to differentiate directly into osteoblasts or chondrocytes when inoculated in vivo. Importantly, this potential is retained after enzymatic isolation, cell culture, subculturing, and freeze preservation.     

A Perspective: Engineering Periosteum for Structural Bone Graft Healing

Autograft is superior to both allograft and synthetic bone graft in repair of large structural bone defect largely due to the presence of multipotent mesenchymal stem cells in periosteum. Recent studies have provided further evidence that activation, expansion and differentiation of the donor periosteal progenitor cells are essential for the initiation of osteogenesis and angiogenesis of donor bone graft healing. The formation of donor cell-derived periosteal callus enables efficient host-dependent graft repair and remodeling at the later stage of healing. Removal of periosteum from bone autograft markedly impairs healing whereas engraftment of multipotent mesenchymal stem cells on bone allograft improves healing and graft incorporation. These studies provide rationale for fabrication of a biomimetic periosteum substitute that could fit bone of any size and shape for enhanced allograft healing and repair. The success of such an approach will depend on further understanding of the molecular signals that control inflammation, cellular recruitment as well as mesenchymal stem cell differentiation and expansion during the early phase of the repair process. It will also depend on multidisciplinary collaborations between biologists, material scientists and bioengineers to address issues of material selection and modification, biological and biomechanical parameters for functional evaluation of bone allograft healing.     

Periosteal progenitor cell fate in segmental cortical bone graft transplantations: implications for functional tissue engineering.

A murine segmental femoral bone graft model was used to show the essential role of donor periosteal progenitor cells in bone graft healing. Transplantation of live bone graft harvested from Rosa 26A mice showed that approximately 70% of osteogenesis on the graft was attributed to the expansion and differentiation of donor periosteal progenitor cells. Furthermore, engraftment of BMP-2-producing bone marrow stromal cells on nonvital allografts showed marked increases in cortical graft incorporation and neovascularization, suggesting that gene-enhanced, tissue engineered functional periosteum may improve allograft incorporation and repair. INTRODUCTION: The loss of cellular activity in a structural bone allograft markedly reduces its healing potential compared with a live autograft. To further understand the cellular mechanisms for structural bone graft healing and repair and to devise a therapeutic strategy aimed at enhancing the performance of allograft, we established a segmental femoral structural bone graft model in mice that permits qualitative and quantitative analyses of graft healing and neovascularization. MATERIALS AND METHODS: Using this segmental femoral bone graft model, we transplanted live isografts harvested from Rosa 26A mice that constitutively express beta-galactosidase into their wildtype control mice. In an attempt to emulate the osteogenic and angiogenic properties of periosteum, we applied a cell-based, adenovirus-mediated gene therapy approach to engraft BMP-2-producing bone marrow stromal cells onto devitalized allografts. RESULTS: X-gal staining for donor cells allowed monitoring the progression of periosteal progenitor cell fate and showed that 70% of osteogenesis was attributed to cellular proliferation and differentiation of donor progenitor cells on the surface of the live bone graft. Quantitative muCT analyses showed a 3-fold increase in new bone callus formation and a 6.8-fold increase in neovascularization for BMP-2/stromal cell-treated allograft compared with control acellular allografts. Histologic analyses showed the key features of autograft healing in the BMP-2/stromal cell-treated allografts, including the formation of a mineralized bone callus completely bridging the segmental defects, abundant neovascularization, and extensive resorption of bone graft. CONCLUSIONS: The marked improvement of healing in these cellularized allografts suggests a clinical strategy for engineering a functional periosteum to improve the osteogenic and angiogenic properties of processed allografts.     

Immunolocalization of vascular endothelial growth factor during heterotopic bone formation induced from grafted periosteum

Vessel invasion is an important step in cartilage replacement that leads to bone formation, and vascular endothelial growth factor (VEGF) has been implicated as a key player in this process. Although grafted periosteum undergoes endochondral ossification, little is known about the role of VEGF in this process. In the current study the authors investigated by immunohistochemical, histochemical, and ultrastructural techniques the localization of VEGF during bone formation in periosteal grafts. At day 14 after grafting the tibias of Japanese white rabbits, periosteal cells in the grafted tissue had differentiated into chondrocytes to form cartilage. Some chondrocytes were immunopositive for VEGF expression, and subsequent vessel invasion occurred predominantly in these VEGF-positive areas. At day 45, the cartilage invaded by blood vessels had been replaced by newly formed bone. These findings suggest that VEGF is associated with the process of blood vessel invasion into cartilage before bone replacement in endochondral ossification from grafted periosteum.     

Bone vs. fat: embryonic origin of progenitors determines response to androgen in adipocytes and osteoblasts

Although androgen is considered an anabolic hormone, the consequences of androgen receptor (AR) overexpression in skeletally-targeted AR-transgenic lines highlight the detrimental effect of enhanced androgen sensitivity on cortical bone quality. A compartment-specific anabolic response is observed only in male and not in female AR3.6-transgenic (tg) mice, with increased periosteal bone formation and calvarial thickening. To identify anabolic signaling cascades that have the potential to increase bone formation, qPCR array analysis was employed to define expression differences between AR3.6-tg and wild-type (WT) periosteal tissue. Notably, categories that were significantly different between the two genotypes included axonal guidance, CNS development and negative regulation of Wnt signaling with a node centered on stem cell pathways. Further, fine mapping of AR3.6-tg calvaria revealed that anabolic thickening in vivo is not uniform across the calvaria, occurring only in frontal and in not parietal bones. Multipotent fraction 1 progenitor populations from both genotypes were cultured separately as frontal bone neural crest stem-like cells (fNCSC) and parietal bone mesenchymal stem-like cells (pMSC). Both osteoblastic and adipogenic differentiation in these progenitor populations was influenced by embryonic lineage and by genotype. Adipogenesis was enhanced in WT fNCSC compared to pMSC, but transgenic cultures showed strong suppression of lipid accumulation only in fNCSC cells. Osteoblastogenesis was significantly increased in transgenic fNCSC cultures compared to WT, with elevated alkaline phosphatase (ALP) activity and induction of mineralization and nodule formation assessed by alizarin red and von Kossa staining. Osteocalcin (OC) and ALP mRNA levels were also increased in fNCSC cultures from AR3.6-tg vs. WT, but in pMSC cultures ALP mRNA levels, mineralization and nodule formation were decreased in AR3.6-tg cells. Expression differences identified by array in long bone periosteal tissue from AR3.6-tg vs. WT were recapitulated in the fNCSC samples while pMSC profiles reflected cortical expression. These observations reveal the opposing effects of androgen signaling on lineage commitment and osteoblast differentiation that is enhanced in cells derived from a neural crest origin but inhibited in cells derived from a mesodermal origin, consistent with in vivo compartment-specific responses to androgen. Combined, these results highlight the complex action of androgen in the body that is dependent on the embryonic lineage and developmental origin of the cell. Further, these data these data suggest that the periosteum surrounding long bone is derived from neural crest.     

Periosteum: biology, regulation, and response to osteoporosis therapies

Periosteum contains osteogenic cells that regulate the outer shape of bone and work in coordination with inner cortical endosteum to regulate cortical thickness and the size and position of a bone in space. Induction of periosteal expansion, especially at sites such as the lumbar spine and femoral neck, reduces fracture risk by modifying bone dimensions to increase bone strength. The cell and molecular mechanisms that selectively and specifically activate periosteal expansion, as well as the mechanisms by which osteoporosis drugs regulate periosteum, remain poorly understood. We speculate that an alternate strategy to protect human bones from fracture may be through targeting of the periosteum, either using current or novel agents. In this review, we highlight current concepts of periosteal cell biology, including their apparent differences from endosteal osteogenic cells, discuss the limited data regarding how the periosteal surface is regulated by currently approved osteoporosis drugs, and suggest one potential means through which targeting periosteum may be achieved. Improving our understanding of mechanisms controlling periosteal expansion will likely provide insights necessary to enhance current and develop novel interventions to further reduce the risk of osteoporotic fractures.     

Regeneration of the mandibular head from grafted periosteum

Grafted periosteum has a rich potential to induce heterotopic bone formation. In the current study the authors investigate whether autogenous periosteal grafts can regenerate the mandibular head in a rabbit model. They removed the mandibular head of Japanese white rabbits and grafted tibial periosteum to the cut surface of the mandible. Grafted periosteum was observed histologically and radiographically at day 7, 14, 21, and 45 after surgery. At day 7 after grafting, grafted tissue showed remarkable cell proliferation. By 14 days these cells had differentiated into chondrocytes to form cartilage, and endochondral ossification took place after 21 days. At 45 days after surgery, soft X-ray findings showed a newly formed mandibular head, which was similar histologically to that of a normal mandibular head. The cut mandible without periosteal graft showed no regeneration. These findings indicate that grafted periosteum can regenerate the mandibular head without special procedures such as bone fixation in a rabbit model, and suggest that this technique may be useful clinically.