Mechanistic insights into the spontaneous induction of bone formation.

The grand discovery of morphogens, or "form-generating substances", revealed that tissue morphogenesis is initiated by soluble molecular signals or morphogens primarily belonging to the transforming growth factor-ß (TGF-ß) supergene family. The regenerative potential of bone rests on its extracellular matrix, which is the repository of several morphogens that tightly control cellular differentiating pathways, cellular matrix deposition and remodeling. Alluringly, the matrix also contains specific factors transferred from the heterotopic implanted bone matrices initiating "Tissue Induction", as provocatively described in Nature in 1945. Later, it was found that selected genes and gene products of the TGF-ß supergene family singly, synchronously, and synergistically mastermind the induction of bone formation. This review describes the phenomenon of the spontaneous and/or intrinsic osteoinductivity of calcium phosphate-based biomaterials and titanium' constructs without the applications of soluble osteogenetic molecular signals. The review shows the spontaneous induction of bone formation initiated by Ca++ activating stem cell differentiation and up-regulation of bone morphogenetic proteins genes. Expressed gene products are embedded into the concavities of the calcium phosphate-based substrata, initiating bone formation as a secondary response. Pure titanium's substrata do not initiate the spontaneous induction of bone formation. The induction of bone is solely dependent on acid, alkali and heat treatments to form apatite layers on the treated titanium surfaces. The induction of bone formation is achieved exclusively by apatite-based biomaterial surfaces. The hydroxyapatite, in its various forms and geometric configurations, finely tunes the induction of bone formation in heterotopic sites. Cellular differentiation by fine-tuning of the cellular molecular machinery is initiated by specific geometric modularity of the hydroxyapatite substrata that push cellular buttons that start the ripple-like cascade of "Tissue Induction", generating newly formed ossicles with bone marrow in heterotopic extraskeletal sites. The highlighted mechanistic insights into the spontaneous induction of bone formation are a research platform invocating selected molecular elements to construct the induction of bone formation.

Pediatric Mandibular Defect Rehabilitation by Human Transforming Growth Factor-ß3 with an Implant-Supported Prosthesis.

Large mandibular defects in children are an uncommon but challenging problem for surgeons to solve. The time-honored options of autologous bone grafts are seldom a viable option, as suitable donor sites are unavailable. Osteoinductive morphogens may yet provide a solution in these cases. A large mandibular tumor in a child 10 years of age necessitated the resection of the entire dentate portion of the mandible. The defect was reconstructed at a second stage with a composite graft of human transforming growth factor-ß3 (hTGF-ß3), human demineralized bone matrix, and 12 g of autologous bone harvested from the posterior iliac crest. A mature ossicle suitable for the placement of osseointegrated implants developed in the erstwhile defect, and an implant-supported dental prosthesis was placed. The patient has been followed up into adulthood. Facial growth has proceeded unhindered, and the patient has maintained full oral and dental function. This case reports the long-term result of an uncommon condition treated with a novel method. The longterm follow-up of this patient provides evidence to dispel some of the concerns for the use of osteoinductive proteins in children. A composite graft of osteogenic morphogens, osteocompetent autologous cells, and mineralized and demineralized matrices-as opposed to osteogenic morphogens used solo-may improve clinical bone regeneration.

Osteogenic Competence and Potency of the Bone Induction Principle: Inductive Substrates That Initiate "Bone: Formation by Autoinduction".

ABSTRACT: The de novo induction of bone has always been a fascinating phenomenon, keeping skeletal reconstructionists and cellular developmental biologists continuously engaged to finally provide a molecular and cellular approach to the induction of bone formation. A significant advancement was made by the purification and cloning of the human recombinant bone morphogenetic proteins, members of the transforming growth factor-ß supergene family. Human bone morphogenetic proteins are powerful inducers of bone in animal models including nonhuman primates. Translation in clinical contexts has however, proven to be surprisingly difficult. This review also describes the significant induction of bone formation by the human transforming growth factor-ß3 when implanted in heterotopic intramuscular sites of the Chacma baboon Papio ursinus. Large mandibular defects implanted with 250 mg human transforming growth factor-ß3 in human patients showed significant osteoinduction; however, the induction of bone was comparatively less than the induction of bone in P ursinus once again highlighting the conundrum of human osteoinduction: is the bone induction principle failing clinical translation?

Long Term Follow-Up of Pediatric Mandibular Reconstruction With Human Transforming Growth Factor-ß3.

Translating bone regeneration induced by recombinant human bone morphogenetic proteins from animal models to human patients has proven inexplicably inconsistent. This prompted us to test in 5 pediatric patients, an alternative osteoinductive morphogen, recombinant human transforming growth factor ß3 (hTGF-ß3), to reconstruct mandibular defects of such a size to preclude reconstruction with autologous bone. An osteoinductive implant of human demineralized bone matrix (DBM) loaded with 125 µg hTGF-ß3 per gram of DBM was implanted into one defect, and 250 µg hTGF-ß3 per gram of DBM in another. Thereafter in 3 patients limited amounts of particulate cortico-cancellous bone graft harvested from the posterior iliac crest were combined with 250 µg hTGF-ß3 per gram of DBM. Patients were followed up for 3 to 6 years. Three patients achieved clinically significant osteoinduction, 1 patient with hTGF-ß3 only, and 2 by combining hTGF-ß3 with a small supplement of autologous bone. One patient with hTGF-ß3 only and followed up for 5 years retains a viable reconstruction but has had sub-optimal bone regeneration. One patient had osteoinductive failure due to sepsis although the plate reconstruction remains viable. Recombinant human TGF-ß3 initiates osteoinduction in humans and potentiates autologous bone graft activity allowing the reconstruction of large mandibular defects in pediatric patients.

Developmental pathways of periodontal tissue regeneration: Developmental diversities of tooth morphogenesis do also map capacity of periodontal tissue regeneration?

Nothing is known on the impact of developmental divergence on periodontal tissue regeneration in vertebrate animals. Molecularly, the induction of tooth morphogenesis is highly conserved deploying across animal phyla a constant and reproducible set of gene pathways, which result in morphogenesis of multiple odontode forms and shapes. Genetic mutations positively affect animal speciation via evolving biting and masticatory forces as well as dietary habits selectively imprinted in animal phyla during evolutionary speciation. The geometry of the attachment apparatus of a tooth is important for the interpretation of the induction of cementogenesis with de novo Sharpey's fibres as in thecodonty, ie, a tripartite attachment of alveolar bone, periodontal ligament and cementum. This review addresses the tooth implantation in different animal clades from the fibrous attachment of the Elasmobranch Carcharinus obscurus dusky shark, reviewing the evolution and functional significance of cementum with functionally inserted Sharpey's fibres. In sharks there is a continuous tooth replacement mechanistically supported by the continuously erupting dental lamina. We show that the arching of the continuously erupting dental lamina, a critical step for the selachians' tooth differentiation, is prominently characterized by transforming growth factor-ß3 (TGF-ß3 ) expression not only within the dental lamina but also in cellular condensations in the mesenchymal tissues of the erupting tooth. Such findings indicate the pleiotropic multifaceted activity of a highly conserved mammalian gene across genera, masterminding tooth morphogenesis in both selachians and mammals as well as periodontal tissue induction in the non-human primate Papio ursinus. In P. ursinus, the induction of cementogenesis entails the expression of TGF-ß3 and osteocalcin with fine-tuning and regulation of bone morphogenetic proteins BMP-2 and BMP-7, and upregulation of TGF-ß3 . TGF-ß3 autoinduction and upregulation during the induction of cementogenesis and osteogenesis in P. ursinus provide novel insights into the induction of cementogenesis. It is hypothesized that the evolutionary expression and upregulation of the TGF-ß3 gene may provide the mechanistic insights into the induction of extensive cementogenesis as seen in stem mammals and the induction of trabecular-like cementum formation in mosasaurs' tooth attachment. Aspidin, the precursor of cementum, was reported to appear 310-330 million years ago (Ma) in Odontostraci armoured fish. Studies showed that the differentiation of cementum with inserted Sharpey's fibres is also present in lower amniotes such as Diatectomorpha or Diadectidae, the first herbivorous tetrapods, 323 Ma. In mosasaurs, 168-165 Ma, there is the induction of extensive trabeculation of cementum though nothing is known on the phylogenetic temporo-spatial evolution of cementum before Diadectidae and stem mammals. The large trabeculations of cementum as seen in the attachment of extinct mosasaurs invocates a pleiotropic capacity of cemental growth previously unknown. The appearance of cementum facing a vascularized and innervated periodontal ligament space with Sharpey's fibres inserting on to mineralized cementum provides a multiform pleiotropic masticatory apparatus adapted to multiple biting and lacerating forces as well as finely tuned and controlled forces beyond mastication and deglutition. The remarkable cementogenesis as seen in stem mammals but particularly in mosasaurs with cemental trabeculations across the ligament space invocates the developmental capacity of cementum. The large cemental trabeculations as seen in mosasaurs and the cemental growth in stem mammals, together with regenerating scenarios in P. ursinus with large seams of cellular cementum and cementoid populated by contiguous cementoblasts indicate the continuous molecular cross-talk between cementum, newly formed cementoid matrix, cementoblasts and extracellular matrix soluble molecular signals. This molecular cross-talk may control the biomolecular homeostasis of both cementum and periodontal ligament, including angiogenesis. A further molecular scenario is invocated by the tight and exquisite anatomical relationships between the cementoid surfaces and the newly formed capillaries. The primitiveness of the craniate masticatory mineralized craniofacial apparatus has been controlled by several yet ancestral common genes not lastly the TGF-ß3 gene. The TGF-ß3 might have been responsible for the induction of cementogenesis not only in extant P. ursinus but also in Diatectomorpha and mosasaurs, thus providing continuous evolutionary mechanisms for the induction of tissue morphogenesis across animal phyla for almost a billion years of evolution, epitomizing Nature's parsimony in controlling tissue induction and morphogenesis. TGF-ß receptor II regulates osterix expression via Smad-dependent pathways indicating that TGF-ß signalling acts as an upstream regulator of osterix during cementoblast differentiation. The presence of morphogenetic signals within the cemental matrix capable of inducing bone formation needs now to be assigned: bone induction initiated by extracted and partially purified cemental matrices may be the result of a slow release of embryonic remnants of osteogenic signals required and deployed during cementogenesis. The cementum may thus rule the periodontal ligament space homeostasis, remodelling and repair by releasing sequestered morphogenetic signals that were deployed during embryogenesis.

Biomimetic Functionalized Surfaces and the Induction of Bone Formation.

Tissue engineering still needs to assign the molecular basis of pattern formation, tissue induction, and morphogenesis: What next to morphogens and stem cells? Macroporous biomimetic matrices per se, without the addition of the soluble osteogenic molecular signals of the transforming growth factor-ß (TGF-ß) supergene family, remarkably initiate the induction of bone formation. Carving geometries within different calcium phosphate-based macroporous bioreactors we show that geometric cues imprinted within the macroporous spaces initiate the spontaneous induction of bone. Concavities biomimetize the remodeling cycle of the primate osteonic bone and are endowed with functionalized smart geometric cues that per se initiate osteoblasts' differentiation with the expression and secretion of osteogenic molecular signals that induce bone as a secondary response. To study the role of calcium ions (Ca++) and osteoclastogenesis, coral-derived calcium carbonate (CC)/hydroxyapatite (HA) bioreactors with limited conversion to HA (7% HA/CC) were preloaded with 500 µg of the L-type voltage gated calcium channel blocker verapamil hydrochloride. Bioreactors were also loaded with 240 µg of the bisphosphonate zoledronate, an osteoclast inhibitor, and implanted in heterotopic sites of the rectus abdominis muscle of Papio ursinus. Bisphosphonate-treated specimens were characterized by a delayed profoundly inhibited induction of tissue patterning with limited induction of bone. Macroporous constructs pretreated with verapamil hydrochloride yielded limited bone formation. Similarly, 125 or 150 µg human Noggin previously adsorbed onto the macroporous bioreactors resulted in minimal bone formation by induction, indirectly showing that the initiation of bone formation is through the bone morphogenetic protein (BMP) pathway. Downregulation of BMP-2 and osteogenic protein-1 (OP-1) with upregulation of Noggin correlated with limited bone induction. Angiogenesis, capillary sprouting, and Ca++ provide chemotactic signals for myoendothelial, myoblastic, and pericytic stem cell differentiation into osteoblastic-like cells expressing the osteogenic soluble molecular signals of the TGF-ß supergene family. Secreted gene products are embedded directly onto the substratum within its regulatory concavities. The protected microenvironment of the concavities biomimetizes the phylogenetically ancient repetitive multitested designs and topographies of Nature. Migrating cells onto the primed substratum by osteoclastic nanotopographical geometric inductive modifications convert geometrical cues set by osteoclastogenesis into BMP gene expression pathways that ultimately set into motion the spontaneous induction of bone formation.

Cementogenesis and osteogenesis in periodontal tissue regeneration by recombinant human transforming growth factor-ß3 : a pilot study in Papio ursinus.

OBJECTIVES: The aim of this study was to investigate cementogenesis and alveolar bone induction during in vivo periodontal tissue regeneration upon implantation of hTGF-ß3 in furcation defects of Papio ursinus and to evaluate the feasibility of gene expression studies. MATERIALS AND METHODS: Class II furcation defects (day 0) were prepared in mandibular first and second molars of three P. ursinus and on day 30 implanted with and without 75 µg hTGF-ß3 in Matrigel® matrix. On day 0, 30 and 90, cementum and alveolar bone were harvested for gene expression analyses. Coral-derived bioreactors with and without 250 µg hTGF-ß3 were implanted in the rectus abdominis to monitor tissue induction. RESULTS: hTGF-ß3 induced cementogenesis with TGF-ß3 , Cementum Protein-1 (Cemp1) and Osteocalcin (OC) up-regulation, and down-regulation of BMP-2 and OP-1. Matrigel® matrix specimens showed up-regulation of BMP-2, TGF-ß3 , and OC, with down-regulation of OP-1 and Cemp1. hTGF-ß3 induced alveolar bone with down-regulation of OP-1, TGF-ß3 , OC, and Cemp1. hTGF-ß3 bioreactors induced bone at the periphery only. BMP-3, BMP-4, TGF-ß1 and TGF-ß3 were up-regulated in the adjacent muscle with TGF-ß2 down-regulation. CONCLUSIONS: Cementogenesis and osteogenesis by hTGF-ß3 entail the expression and up-regulation of TGF-ß3 and OC with fine tuning and modulation of BMP-2 and OP-1.

Functionalized Surface Geometries Induce: "Bone: Formation by Autoinduction".

The induction of tissue formation, and the allied disciplines of tissue engineering and regenerative medicine, have flooded the twenty-first century tissue biology scenario and morphed into high expectations of a fulfilling regenerative dream of molecularly generated tissues and organs in assembling human tissue factories. The grand conceptualization of deploying soluble molecular signals, first defined by Turing as forms generating substances, or morphogens, stemmed from classic last century studies that hypothesized the presence of morphogens in several mineralized and non-mineralized mammalian matrices. The realization of morphogens within mammalian matrices devised dissociative extractions and chromatographic procedures to isolate, purify, and finally reconstitute the cloned morphogens, found to be members of the transforming growth factor-ß (TGF-ß) supergene family, with insoluble signals or substrata to induce de novo tissue induction and morphogenesis. Can we however construct macroporous bioreactors per se capable of inducing bone formation even without the exogenous applications of the osteogenic soluble molecular signals of the TGF-ß supergene family? This review describes original research on coral-derived calcium phosphate-based macroporous constructs showing that the formation of bone is independent of the exogenous application of the osteogenic soluble signals of the TGF-ß supergene family. Such signals are the molecular bases of the induction of bone formation. The aim of this review is to primarily describe today's hottest topic of biomaterials' science, i.e., to construct and define osteogenetic biomaterials' surfaces that per se, in its own right, do initiate the induction of bone formation. Biomaterials are often used to reconstruct osseous defects particularly in the craniofacial skeleton. Edentulism did spring titanium implants as tooth replacement strategies. No were else that titanium surfaces require functionalized geometric nanotopographic cues to set into motion osteogenesis independently of the exogenous application of the osteogenic soluble molecular signals. Inductive morphogenetic surfaces are the way ahead of biomaterials' science: the connubium of stem cells on primed functionalized surfaces precisely regulates gene expression and the induction of the osteogenic phenotype.

Redundancy and Molecular Evolution: The Rapid Induction of Bone Formation by the Mammalian Transforming Growth Factor-ß3 Isoform.

The soluble osteogenic molecular signals of the transforming growth factor-ß (TGF-ß) supergene family are the molecular bases of the induction of bone formation and postnatal bone tissue morphogenesis with translation into clinical contexts. The mammalian TGF-ß3 isoform, a pleiotropic member of the family, controls a vast array of biological processes including the induction of bone formation. Recombinant hTGF-ß3 induces substantial bone formation when implanted with either collagenous bone matrices or coral-derived macroporous bioreactors in the rectus abdominis muscle of the non-human primate Papio ursinus. In marked contrast, the three mammalian TGF-ßs do not initiate the induction of bone formation in rodents and lagomorphs. The induction of bone by hTGF-ß3/preloaded bioreactors is orchestrated by inducing fibrin-fibronectin rings that structurally organize tissue patterning and morphogenesis within the macroporous spaces. Induced advancing extracellular matrix rings provide the structural anchorage for hyper chromatic cells, interpreted as differentiating osteoblasts re-programmed by hTGF-ß3 from invading myoblastic and/or pericytic differentiated cells. Runx2 and Osteocalcin expression are significantly up-regulated correlating to multiple invading cells differentiating into the osteoblastic phenotype. Bioreactors pre-loaded with recombinant human Noggin (hNoggin), a BMPs antagonist, show down-regulation of BMP-2 and other profiled osteogenic proteins' genes resulting in minimal bone formation. Coral-derived macroporous constructs preloaded with binary applications of hTGF-ß3 and hNoggin also show down-regulation of BMP-2 with the induction of limited bone formation. The induction of bone formation by hTGF-ß3 is via the BMPs pathway and it is thus blocked by hNoggin. Our systematic studies in P. ursinus with translational hTGF-ß3 in large cranio-mandibulo-facial defects in humans are now requesting the re-evaluation of "Bone: formation by autoinduction" in primate models including humans.

The synergistic induction of bone formation by the osteogenic proteins of the TGF-ß supergene family.

The momentum to compose this Leading Opinion on the synergistic induction of bone formation suddenly arose when a simple question was formulated during a discussion session on how to boost the often limited induction of bone formation seen in clinical contexts. Re-examination of morphological and molecular data available on the rapid induction of bone formation by the recombinant human transforming growth factor-ß3 (hTGF-ß3) shows that hTGF-ß3 replicates the synergistic induction of bone formation as invocated by binary applications of hOP-1:hTGF-ß1 at 20:1 by weight when implanted in heterotopic sites of the rectus abdominis muscle of the Chacma baboon, Papio ursinus. The rapid induction of bone formation in primates by hTGF-ß3 may stem from bursts of cladistic evolution, now redundant in lower animal species but still activated in primates by relatively high doses of hTGF-ß3. Contrary to rodents, lagomorphs and canines, the three mammalian TGF-ß isoforms induce rapid and substantial bone formation when implanted in heterotopic rectus abdominis muscle sites of P. ursinus, with unprecedented regeneration of full thickness mandibular defects with rapid mineralization and corticalization. Provocatively, thus providing potential molecular and biological rationales for the apparent redundancy of osteogenic molecular signals in primates, binary applications of recombinant human osteogenic protein-1 (hOP-1) with low doses of hTGF-ß1 and -ß3, synergize to induce massive ossicles in heterotopic rectus abdominis, orthotopic calvarial and mandibular sites of P. ursinus. The synergistic binary application of homologous but molecularly different soluble molecular signals has indicated that per force several secreted molecular signals are required singly, synchronously and synergistically to induce optimal osteogenesis. The morphological hallmark of the synergistic induction of bone formation is the rapid differentiation of large osteoid seams enveloping haematopoietic bone marrow that forms by day 15 in heterotopic rectus abdominis sites. Synergistic binary applications also induce the morphogenesis of rudimentary embryonic growth plates indicating that the "memory" of developmental events in embryo can be redeployed postnatally by the application of morphogen combinations. Synergistic binary applications or single relatively high doses of hTGF-ß3 have shown that hTGF-ß3 induces bone by expressing a variety of inductive morphogenetic proteins that result in the rapid induction of bone formation. Tissue induction thus invocated singly by hTGF-ß3 recapitulates the synergistic induction of bone formation by binary applications of hTGF-ß1 and -ß3 isoforms with hOP-1. Both synergistic strategies result in the rapid induction and expansion of the transformed mesenchymal tissue into large corticalized heterotopic ossicles with osteoblast-like cell differentiation at the periphery of the implanted reconstituted specimens with "tissue transfiguration" in vivo. Molecularly, the rapid induction of bone formation by binary applications of hOP-1 and hTGF-ß3 or by hTGF-ß3 applied singly resides in the up-regulation of selected genes involved in tissue induction and morphogenesis, Osteocalcin, RUNX-2, OP-1, TGF-ß1 and -ß3 with however the noted lack of TGF-ß2 up-regulation.

Reconstruction of 56 mandibular defects with autologous compressed particulate corticocancellous bone grafts.

We retrospectively evaluated the results of particulate corticocancellous bone grafting of mandibular defects. Patients with deficits of mandibular continuity as a result of injuries or resection of disease had the affected segment debrided or resected, followed by placement of a patient-specific reconstruction plate. Eight weeks after resection, it was reconstructed with an autotransplant from the posterior iliac crest. Grafts were deemed successful if the regenerated ossicle (after 6 months' maturation) was adequate to take an osseointegrated fixture at least 10mm long. Fifty-six patients were treated, of whom 5 were lost to follow-up. The remaining 51 patients were followed up for a mean (SD) of 29 (18) months. The mean (SD) length of the defect was 12.4 (8.4) cm. Of the 51 reconstructions, 43 healed uneventfully and the grafts were deemed successful. Two healed grafts developed recurrent tumour, which required resection of the entire reconstructed area in one, and partial resection in the other. Three patients lost the complete graft from sepsis, and five developed sepsis that required debridement with partial loss of the graft. Two patients in the latter group required a second graft. One patient required an augmentation graft, as the ossicle was not sufficient to take an implant. The technique of staged grafting with particulate corticocancellous bone after moulding of the recipient site with a spacer produces unmatched restitution of mandibular anatomy with low morbidity.

Profiling bone morphogenetic proteins and transforming growth factor-ßs by hTGF-ß3 pre-treated coral-derived macroporous bioreactors: the power of one.

To study the expression profile of bone morphogenetic proteins and transforming growth factor-ßs (BMPs and TGFßs), coral-derived calcium carbonate-based macroporous bioreactors with limited conversion to hydroxyapatite (7% HA/CC) were pre-loaded with and without 250 µg hTGF-ß3 and implanted in the rectus abdominis of 3 non-human primates Papio ursinus euthanized on day 60. To investigate the required dose of hNoggin, a BMPs antagonist that controls the induction of bone formation, 7% HA/CC were pre-loaded with 150 µg hNoggin, with 125 µg hTGF-ß3/150 µg hNoggin, with or without 125 µg hTGF-ß3 and implanted in the r. abdominis of 3 additional animals euthanized on day 90. Quantitative reverse-transcriptase polymerase chain reaction (qRT-PCR) evaluated the expression' profile of BMP-2, BMP-3, BMP-4, BMP-6, BMP-7 and TGF-ß1, -ß2, and -ß3 in tissue generating bioreactors as well as in the adjacent r. abdominis muscle. On day 60, 250 µg hTGF-ß3 induced bone formation at the periphery of the implanted bioreactors only. On day 90, 125 µg hTGF-ß3/treated bioreactors showed the induction of bone formation throughout the macroporous spaces. Untreated bioreactors induced bone, 4.11% vs. 2.00% on days 60 and 90, respectively. In hTGF-ß3/treated bioreactors, BMP-2 and BMP-3 were up-regulated at both time periods, both in the homogenized constructs and in the adjacent r. abdominis muscle whilst BMP-4 in the homogenized construct only. In untreated 7% HA/CC constructs, BMP-2 was up-regulated in the macroporous construct only. On day 60, 250 µg hTGF-ß3/treated and untreated macroporous constructs showed up-regulation of TGF-ß1 with a six fold increase vs. TGF-ß1 expression in adjacent muscle of untreated constructs. TGF-ß2 was down regulated in both untreated and 250 µg hTGF-ß3/treated bioreactors. On day 60, 250 µg hTGF-ß3/treated bioreactors showed TGF-ß3 expression in untreated, treated and adjacent muscle tissues. On day 90, BMP-2 was up-regulated, with BMP-3 up-regulation both with and without hNoggin. BMP-4 expression was reduced, with down-regulation of BMP-6 and BMP-7 with and without hNoggin. BMPs expression is finely tuned by hNoggin that blocks BMPs resulting in minimal if any induction of bone formation. The primary differentiating events in untreated bioreactors develop within the macroporous spaces after stem cells invasion and differentiation with lack of BMP-2 expression in the adjacent muscle. In hTGF-ß3/treated bioreactors, both the adjacent muscle and the macroporous bioreactor show BMP-2 up-regulation, correlating with the temporo/spatial rapid induction of bone formation at the periphery of the bioreactors only. The latter may be the result of the initiation of a sequential chain of cellular induction rapidly recruiting pericytic perivascular myoblastic cells adjacent to the implanted bioreactors directly transformed into secreting osteoblasts.

Re-evaluating the induction of bone formation in primates.

The molecular cloning of the osteogenic proteins of the transforming growth factor-ß (TGF-ß) supergene family and the results of numerous pre-clinical studies in several mammalian species including non-human primates, have prematurely convinced molecular biologists, tissue engineers and skeletal reconstructionists alike to believe that single recombinant human bone morphogenetic/osteogenic proteins (hBMPs/OPs) would result in tissue induction when translated in clinical contexts. This theoretical potential has not been translated to acceptable clinical results. Clinical trials in craniofacial and orthopedic applications such as mandibular reconstruction and sinus-lift operations have indicated that supra physiological doses of a single recombinant human protein are needed to induce unacceptable tissue regeneration whilst incurring significant costs without achieving equivalence to autogenous bone grafts. The acid test for clinically relevant bone tissue engineering should now become the concept of clinically significant osteoinduction, whereby the regenerated bone is readily identifiable on radiographic examination by virtue of its opacity and trabecular architecture. The need for alternatives to the hBMPs/OPs is now felt more acutely following reported complications and performance failure associated with the clinical use of hBMP-2 and hOP-1 (BMP-7). Because of the often substandard regeneration of clinical defects implanted with hBMPs/OPs, we now need to finally deal with the provocative question: are the hBMPs/OPs the only initiators of the induction of bone formation in pre-clinical and clinical contexts? The rapid induction of bone formation by the hTGF-ß3 isoform in heteropic intramuscular sites of the Chacma baboon Papio ursinus together with TGF-ß1, TGF-ß3, BMP-2, BMP-3, OP-1, RUNX-2 and Osteocalcin up-regulation and expression, hyper cellular osteoblastic activity, osteoid synthesis, angiogenesis and capillary sprouting are the molecular and morphological foundation for the induction of bone formation in clinical contexts. The induction of bone as initiated by hTGF-ß3 when implanted in the rectus abdominis muscle of P. ursinus is via the BMPs/OPs pathway with hTGF-ß3 controlling the induction of bone formation by regulating the expression of BMPs/OPs via Noggin expression, eliciting the induction of bone formation by up-regulating endogenous BMPs/OPs and it is blocked by hNoggin, providing insights into performance failure of hBMPs/OPs in clinical contexts. Physiological expression of BMPs/OPs genes upon implantation of hTGF-ß3 may escape the antagonist expression of Noggin and other inhibitors, whereas direct application of hBMPs/OPs, representing a later by-product step of the bone induction cascade as set by the TGF-ß3 master gene in primates, sets into motion Noggin' antagonist action, as shown by the limited effectiveness of hBMPs/OPs in clinical contexts. The unprecedented induction of bone formation by 250 µg hTGF-ß3 when combined with coral-derived macroporous constructs is the novel molecular and morphological frontier for the induction of bone formation in man. The induction of bone by hTGF-ß3 has been thus translated in clinical contexts to treat a large mandibular defect in a pediatric patient; 30 months after implantation of 250 µg hTGF-ß3 per gram of human demineralized bone matrix, radiographic analyses show the reconstruction of the avulsed large mandibular segment including the induction of the avulsed coronoid process.

The induction of bone formation by the recombinant human transforming growth factor-ß3.

Implantation of recombinant human transforming growth factor-ß3 (hTGF-ß3) with coral-derived calcium carbonate-based macroporous bioreactors with limited conversion to hydroxyapatite (7% HA/CC) in the rectus abdominis muscle of the non-human primate Chacma baboon Papio ursinus induces endochondral bone formation. The exact mechanisms by which hTGF-ß3 signalling induces bone in heterotopic sites of P. ursinus are not known. Coral-derived 7% HA/CC bioreactors with and without 125 µg hTGF-ß3 were implanted in triplicate in the rectus abdominis muscle of 6 adult baboons. 7% HA/CC bioreactors either with or without hTGF-ß3 were loaded with 125 µg of recombinant human Noggin (hNoggin), a bone morphogenetic proteins (BMPs) antagonist. Tissues on day 15, 60 and 90 were analysed by histomorphometry and quantitative reverse-transcriptase polymerase chain reaction (qRT-PCR). Down-regulation of BMP-2 characterized 7% HA/CC constructs preloaded with 125 µg hNoggin with Noggin down-regulated on day 60 and 90 together with lack of TGF-ß3 expression. Down-regulation of BMP-2 correlated with minimal bone formation by induction. hTGF-ß3/hNoggin pre-treated bioreactors up-regulated BMP-2 but only on day 90 together with a significant down-regulation of Noggin on day 60 and 90, correlating with the induction of bone formation, albeit limited, on day 90 at the periphery of the macroporous bioreactors only. hTGF-ß3 treated bioreactors significantly down-regulated BMP-2 on day 15 whilst up-regulating BMP-2 on day 60 and 90, together with down-regulation of Noggin on day 60 and 90 correlating with the prominent induction of bone formation. hTGF-ß3 significantly up-regulated RUNX-2 and Osteocalcin expression on day 15 controlling the differentiation of progenitor stem cells into the osteoblastic lineage. The induction of bone as initiated by hTGF-ß3 in the rectus abdominis muscle of P. ursinus is via the BMPs pathway with hTGF-ß3 controlling the induction of bone formation by regulating the expression of BMPs via Noggin expression. These results unequivocally demonstrate that hTGF-ß3 elicits bone induction by up-regulation of endogenous BMP-2 and is blocked by hNoggin.

Calcium ions and osteoclastogenesis initiate the induction of bone formation by coral-derived macroporous constructs.

Coral-derived calcium carbonate/hydroxyapatite macroporous constructs of the genus Goniopora with limited hydrothermal conversion to hydroxyapatite (7% HA/CC) initiate the induction of bone formation. Which are the molecular signals that initiate pattern formation and the induction of bone formation? To evaluate the role of released calcium ions and osteoclastogenesis, 7% HA/CC was pre-loaded with either 500 µg of the calcium channel blocker, verapamil hydrochloride, or 240 µg of the osteoclast inhibitor, biphosphonate zoledronate, and implanted in the rectus abdominis muscle of six adult Chacma baboons Papio ursinus. Generated tissues on days 15, 60 and 90 were analysed by histomorphometry and qRT-PCR. On day 15, up-regulation of type IV collagen characterized all the implanted constructs correlating with vascular invasion. Zoledronate-treated specimens showed an important delay in tissue patterning and morphogenesis with limited bone formation. Osteoclastic inhibition yielded minimal, if any, bone formation by induction. 7% HA/CC pre-loaded with the Ca(++) channel blocker verapamil hydrochloride strongly inhibited the induction of bone formation. Down-regulation of bone morphogenetic protein-2 (BMP-2) together with up-regulation of Noggin genes correlated with limited bone formation in 7% HA/CC pre-loaded with either verapamil or zoledronate, indicating that the induction of bone formation by coral-derived macroporous constructs is via the BMPs pathway. The spontaneous induction of bone formation is initiated by a local peak of Ca(++) activating stem cell differentiation and the induction of bone formation.

A Macroporous Bioreactor Super Activated by the Recombinant Human Transforming Growth Factor-ß(3).

Macroporous single phase hydroxyapatite (HA) and biphasic HA/ß-tricalcium phosphate with 33% post-sinter hydroxyapatite (HA/ß-TCP) were combined with 25 or 125 µg recombinant human transforming growth factor-ß3 (hTGF-ß(3)) to engineer a super activated bioreactor implanted in orthotopic calvarial and heterotopic rectus abdominis muscle sites and harvested on day 30 and 90. Coral-derived calcium carbonate fully converted (100%) and partially converted to 5 and 13% hydroxyapatite/calcium carbonate (5 and 13% HA/CC) pre-loaded with 125 and 250 µg hTGF-ß(3), and 1:5 and 5:1 binary applications of hTGF-ß(3): hOP-1 by weight, were implanted in the rectus abdominis and harvested on day 20 and 30, respectively, to monitor spatial/temporal morphogenesis by high doses of hTGF-ß(3). Bone formation was assessed on decalcified paraffin-embedded sections by measuring the fractional volume of newly formed bone. On day 30 and 90, single phase HA implants showed greater amounts of bone when compared to biphasic specimens; 5 and 13% HA/CC pre-loaded with 125 and 250 µg hTGF-ß(3) showed substantial induction of bone formation; 250 µg hTGF-ß(3) induced as yet unreported massive induction of bone formation as early as 20 days prominently outside the profile of the macroporous constructs. The induction of bone formation is controlled by the implanted ratio of the recombinant morphogens, i.e., the 1:5 hTGF-ß(3):hOP-1 ratio by weight was greater than the inverse ratio. The unprecedented tissue induction by single doses of 250 µg hTGF-ß(3) resulting in rapid bone morphogenesis of vast mineralized ossicles with multiple trabeculations surfaced by contiguous secreting osteoblasts is the novel molecular and morphological frontier for the induction of bone formation in clinical contexts.

Osteoinductive hydroxyapatite-coated titanium implants.

Previous studies have shown that heterotopic induction of bone formation by calcium phosphate-based macroporous constructs is set into motion by the geometry of the implanted substrata, i.e. a sequence of repetitive concavities assembled within the macroporous spaces. The aim of this study was to construct osteoinductive titanium implants that per se, and without the exogenous application of the osteogenic soluble molecular signals of the transforming growth factor-ß supergene family, would initiate the induction of bone formation. To generate intrinsically osteoinductive titanium implants for translation in clinical contexts, titanium grade Ti-6A1-4V cylinders of 15 mm in length and 3.85 mm in diameter, with or without concavities, were plasma sprayed with crystalline hydroxyapatite resulting in a uniform layer of 30 µm in thickness. Before coating, experimental titanium implants were prepared with a sequence of 36 repetitive concavities 1600 µm in diameter and 800 µm in depth, spaced a distance of 1000 µm apart. Mandibular molars and premolars were extracted to prepare edentulous mandibular ridges for later implantation. Planar and geometric hydroxyapatite-coated titanium constructs were implanted in the left and right edentulized hemi-mandibles, respectively, after a healing period of 7-8 months, 3 per hemi-mandible. Three planar and three geometric implants were implanted in the left and right tibiae, respectively; additionally, planar and geometric constructs were also inserted in the rectus abdominis muscle. Six animals were euthanized at 30 and 90 days after implantation; one animal had to be euthanized 5 days after surgery and the remaining animal was euthanized 31 months after implantation. Undecalcified longitudinal sections were precision-sawed, ground and polished to 40-60 µm; all sections were stained with a modified Goldner's trichrome. Undecalcified specimen block preparation was performed using the EXAKT precision cutting and grinding system. Histomorphometric analyses of bone in contact (BIC) showed that on day 30 there was no difference between the geometric vs. planar control implants; on day 90, the ratio of BIC to surface within the geometric implants was greater than on the standard planar implants in both mandibular and tibial sites; 31 months after implantation, selected concavities cut into the geometric implants harvested from the rectus abdominis muscle showed the spontaneous induction of bone formation with mineralized bone surfaced by osteoid seams. These data in non-human primates indicate that geometrically-constructed plasma-sprayed titanium implants are per se osteogenic, the concavities providing a unique microenvironment to initiate bone differentiation by induction.

Biomimetic matrices self-initiating the induction of bone formation.

The new strategy of tissue engineering, and regenerative medicine at large, is to construct biomimetic matrices to mimic nature's hierarchical structural assemblages and mechanisms of simplicity and elegance that are conserved throughout genera and species. There is a direct spatial and temporal relationship of morphologic and molecular events that emphasize the biomimetism of the remodeling cycles of the osteonic corticocancellous bone versus the "geometric induction of bone formation," that is, the induction of bone by "smart" concavities assembled in biomimetic matrices of macroporous calcium phosphate-based constructs. The basic multicellular unit of the corticocancellous bone excavates a trench across the bone surface, leaving in its wake a hemiosteon rather than an osteon, that is, a trench with cross-sectional geometric cues of concavities after cyclic episodes of osteoclastogenesis, eventually leading to osteogenesis. The concavities per se are geometric regulators of growth-inducing angiogenesis and osteogenesis as in the remodeling processes of the corticocancellous bone. The concavities act as a powerful geometric attractant for myoblastic/myoendothelial and/or endothelial/pericytic stem cells, which differentiate into bone-forming cells. The lacunae, pits, and concavities cut by osteoclastogenesis within the biomimetic matrices are the driving morphogenetic cues that induce bone formation in a continuum of sequential phases of resorption/dissolution and formation. To induce the cascade of bone differentiation, the soluble osteogenic molecular signals of the transforming growth factor ß supergene family must be reconstituted with an insoluble signal or substratum that triggers the bone differentiation cascade. By carving a series of repetitive concavities into solid and/or macroporous biomimetic matrices of highly crystalline hydroxyapatite or biphasic hydroxyapatite/ß-tricalcium phosphate, we were able to embed smart biologic functions within intelligent scaffolds for tissue engineering of bone. The concavities assembled in the bioceramic constructs biomimetize the remodeling cycle of the corticocancellous bone and are endowed with multifunctional pleiotropic self-assembly capacities, initiating angiogenesis and bone formation by induction without the exogenous applications of the osteogenic-soluble molecular signals of the transforming growth factor ß supergene family. The incorporation of specific biologic activities into biomimetic matrices by manipulating the geometry of the substratum, defined as geometric induction of bone formation, is now helping to engineer therapeutic osteogenesis in clinical contexts.

Perspectives in regenerative medicine and tissue engineering of bone.

This final review in a series of three focuses on how the advances made in the realm of molecular and cellular bone biology have been translated into experimental animal and human surgery.