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REVIEW |
1 Columbia University College of Dental Medicine and Biomedical Engineering, 630 W. 168 St. - PH7 CDM, New York, NY 10032, USA;
2 Michigan Center for Oral Health Research, University of Michigan Clinical Research Center, 24 Frank Lloyd Wright Drive, Lobby M, Box 422, Ann Arbor, MI 48106;
3 Department of Surgery, Stanford University School of Medicine, 257 Campus Drive, Palo Alto, CA 94305;
4 Department of Biomedical Engineering, University of Michigan, Ann Arbor, MI 48109;
5 Department of Biological and Materials Science, University of Michigan, 1011 N. University, Ann Arbor, MI 48109; and
6 National Institute of Dental and Craniofacial Research, National Institutes of Health, Building 30, Room 134, 30 Convent Dr. MSC 4320, Bethesda, MD 20892
* corresponding author, jmao{at}columbia.edu
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ABSTRACT |
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 TOP ABSTRACT (1) INTRODUCTION(2) MESENCHYMAL STEM CELLS...(3) TISSUE ENGINEERING OF...(4) PERIODONTAL TISSUE...(5) TISSUE ENGINEERING OF...(6) IMPACT OF CRANIOFACIAL...REFERENCES |
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KEY WORDS: stem cells • tissue engineering • biomaterials • wound healing • regenerative medicine
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(1) INTRODUCTION |
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TOPABSTRACT(1) INTRODUCTION(2) MESENCHYMAL STEM CELLS...(3) TISSUE ENGINEERING OF...(4) PERIODONTAL TISSUE...(5) TISSUE ENGINEERING OF...(6) IMPACT OF CRANIOFACIAL...REFERENCES |
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Mesenchymal cells undergo asymmetric division, with one offspring cell differentiating toward an end-stage cell, while the other replicates into an offspring mesenchymal cell. Residual offspring of mesenchymal cells, upon the completion of morphogenesis, continue to reside in various craniofacial tissues, and retain their status as stem cells. After birth, mesenchymal cells are called ’mesenchymal stem cells’ (MSCs). In the adult, MSCs maintain physiologically necessary tissue turnover and, upon injury or disease, differentiate to launch tissue regeneration.
MSCs have been experimentally differentiated into all mesenchymal or connective tissue lineages (Caplan, 1991; Pittenger et al., 1999) and, in many cases, have been used to engineer the craniofacial structures that their prenatal predecessors, mesenchymal cells, are capable of generating during development. The capacity of MSCs in the de novo formation and/or regeneration of craniofacial structures is too natural an endeavor to make one wonder why this potential has not been exploited substantially up until the past decade. As it turns out, our ability to grow human craniofacial tissues and organs is by no means a small task. Engineering craniofacial structures from stem cells was an insurmountable effort until advances from several seemingly unrelated disciplines—such as cell and molecular biology, polymer chemistry, molecular genetics, materials science, robotics, and mechanical engineering—converged into the self-assembling field of tissue engineering (Nerem, 1992; Langer and Vacanti, 1993). To engineer a functional biological structure, cells must be instructed to differentiate and receive positional cues, and to synthesize the appropriate extracellular matrix molecules in the overall shape and dimensions of the diseased or missing tissues/organs. Biomimetic scaffolds are frequently needed to enable cell growth and differentiation to occur in an environment that has been previously unfamiliar to either biologists or engineers. Craniofacial structures offer complex and, in many cases, unique challenges when they are being engineered.
Large-scale tissue-engineering research, craniofacial or otherwise, began to take place in the early 1990s and has grown exponentially ever since. The first overview of craniofacial tissue engineering appeared within the past few years and focused on primarily tissue-engineering principles (Hollinger and Winn, 1999; Alsberg et al., 2001). As outlined below, substantial advances have been made in craniofacial tissue engineering:
The tissue-engineered craniofacial structures to date are undoubtedly prototypes that warrant further development and refinements, but nonetheless were not even available a few years ago (Hollinger and Winn, 1999; Alsberg et al., 2001). Despite formidable challenges, this review offers the optimistic view that functional craniofacial tissues and organs can be grown via in vitro and/or in vivo approaches for ultimate therapeutic applications. This review is designed not only to serve as a timely and comprehensive synthesis of our current knowledge of craniofacial tissue engineering, but also to identify immediate challenges in this dynamic field. Craniofacial tissue engineering, like tissue engineering in general, is an interdisciplinary field by nature. A review such as this cannot possibly include some of the necessary technical details of those more individualized fields that have converged to form the foundation of craniofacial tissue engineering. The reader is referred to several recent in-depth reviews in the individual fields of stem cell biology, biomaterials, cell and molecular biology and genetics, and regenerative medicine (Drury and Mooney, 2003; Alhadlaq and Mao, 2004; Patel and Mikos, 2004; Atala, 2005: Gregory et al., 2005; Hollister, 2005; Kyba, 2005; Mao, 2005a; Rahaman and Mao, 2005; Shi et al., 2005).
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(2) MESENCHYMAL STEM CELLS AND CRANIOFACIAL-DERIVED STEM CELLS |
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TOPABSTRACT(1) INTRODUCTION(2) MESENCHYMAL STEM CELLS...(3) TISSUE ENGINEERING OF...(4) PERIODONTAL TISSUE...(5) TISSUE ENGINEERING OF...(6) IMPACT OF CRANIOFACIAL...REFERENCES |
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(2.1) Mesenchymal Stem Cells (MSCs)
Mesenchymal stem cells (MSCs) are self-renewable and can differentiate into all cell lineages that form mesenchymal and connective tissues (Conget and Minguell, 1999; Caplan, 1991; Pittenger et al., 1999; Alhadlaq and Mao, 2004). In addition, MSCs have been reported to differentiate into hepatic (Petersen et al., 1999), renal (Poulsom et al., 2003), cardiac (Orlic et al., 2001), and neural cells (Brazelton et al., 2000; Mezey et al., 2000).
The first successful isolation of bone marrow MSCs, then called colony-forming fibroblast-like cells, was described almost 4 decades ago (Friedenstein et al., 1970). The isolation method was based on the adherence of marrow-derived MSCs to the plastic substrate of the cell culture plates, and a concomitant lack of adherence of marrow-derived hematopoietic stem cells. To date, this straightforward protocol is widely used for the isolation of MSCs in multiple vertebrate species (Alhadlaq and Mao, 2004).
Previous approaches, utilizing flow cytometry, have investigated the isolation of increasingly homogenous populations of MSCs based on differential cellular features within the bone marrow stroma. Further purification and cloning of MSCs are desirable for stem cell biology, similar to the development of cell lines. Recently, several relatively straightforward MSC culture protocols have been developed. Size-dependent sieving of a cell population from human bone marrow aspirates through a porous membrane resulted in a relatively homogeneous population that had the capacity of self-renewal and multi-lineage differentiation (Hung et al., 2002). Positive selection of MSCs with microbeads, combined with fluorescence-activated cell-sorting (FACS) (Jones et al., 2002) or magnetic-activated cell-sorting (MACS) (Kinnaird et al., 2004), is an effective technique for the increasingly defined isolation and precise characterization of MSCs (Reese et al., 1999; Lee et al., 2001; Arbab et al., 2004).
Whether highly purified or cloned MSC populations are necessarily needed for the engineering of craniofacial structures is not clear. First, stem cell populations that generate native craniofacial structures, such as the mandibular joint, are heterogeneous and likely include both mesenchymal and hematopoietic stem cells. The morphogenesis of the articular condyle requires stem cells, chondrocytes, and osteoblasts in addition to angiogenesis (Mao, 2005a). Second, host cell invasion and stem cell homing are likely inevitable in porous biomimetic scaffolds that are used as carriers for delivering stem cells and/or stem-cell-derived tissue-forming cells (Christopherson et al., 2004; Hidalgo and Frenette, 2005).
(2.2) Dental Pulp Stem Cells
Although the regenerative capacity of the human dentin/pulp complex is not well-understood, it is known that, upon injury, reparative dentin is formed as a protective barrier for the pulp (Murray et al., 2001). Accordingly, one might anticipate that dental pulp contains the dentinogenic progenitors that are responsible for dentin repair. Previous work has shown that dental pulp contains proliferating cells that are analogous to bone cells, because they express osteogenic markers and respond to many growth factors for osteo/odontogenic differentiation (Hanks et al., 1998; Unda et al., 2000; Ueno et al., 2001). In addition, dental pulp cells are capable of forming mineral deposits with distinctive dentin-like crystalline structures (About et al., 2000; Couble et al., 2000; Gronthos et al., 2000). Recently, dental pulp stem cells (DPSCs) have been isolated from extracted human third molars (Gronthos et al., 2000; Shi et al., 2001).
To determine the colony-forming efficiency of the isolated DPSCs, investigators have prepared single-cell suspensions by collagenase/dispase treatment of pulp fragments filtered through a fine mesh to remove cell aggregates, and seeded at low plating densities (Gronthos et al., 2000). Approximately 40 single-colony clusters can be retrieved from 10,000 cells in culture (Gronthos et al., 2000, 2003). The clones from the initial primary culture demonstrated variable capacities for forming dentin-like structures following in vivo implantation (Shi and Gronthos, 2003). DPSCs showed a higher proliferation rate than bone-marrow-derived MSCs under the same culture conditions, potentially attributable to strong expression of cyclin-dependent kinase 6, a cell-cycle activator (Shi et al., 2001). Expression of various perivascular markers—such as STRO-1, VCAM-1, MUC-18, and 
-smooth-muscle actin—provides clues that DPSCs are a heterogeneous population of MSCs and likely located in the perivascular niche in the pulp (Gronthos et al., 2000; Shi and Gronthos, 2003).
To elucidate the self-renewal ability, investigators isolated cells from DPSC implants at two months post-subcutaneous implantation, by enzymatic digestion and subsequent expansion in vitro. The isolated cells from the DPSC implants were sorted by fluorescent-activated cell-sorting (FACS) with human beta1-integrin monoclonal antibody. The isolated human cells were re-implanted into immunodeficient mice for two months. The recovered secondary implants yielded the same dentin/pulp-like structures as the primary implants. Human dentin sialophosphoprotein (DSPP) was expressed in dentin-like structures, confirming the human origin of the odontoblast/pulp cells in the secondary DPSC implants (Batouli et al., 2003).
So that their multi-lineage differentiation capacity could be determined, DPSCs were cultured in adipogenic inductive agents. In a few weeks, DPSCs differentiated into adipocyte-like cells that were positive to Oil red O, and expressed PPAR
2 and lipoprotein lipase (LPL) (Batouli et al., 2003). Furthermore, DPSCs were shown to differentiate into neuron-like and glial-like cells by expressing both nestin, an early marker of neural precursor cells, and glial fibrillary acid protein (GFAP), an antigen characteristic of glial cells (Gronthos et al., 2000). These DPSC-derived cells developed long cytoplasmic processes, a departure from their usual bipolar fibroblastic appearance.
(2.3) Stem Cells from Human Exfoliated Deciduous Teeth (SHED)
The exfoliated deciduous tooth houses living pulp remnants consisting of connective tissue, blood vessels, and odontoblasts. We found that from 12 to 20 cells from each exfoliated incisor formed adherent colony clusters with extensive proliferative capacity (Miura et al., 2003). Ex vivo-expanded SHED expressed STRO-1 and CD146 (MUC18), two early cell-surface markers for bone-marrow-derived MSCs (Shi and Gronthos, 2003). In addition, SHED expressed a variety of osteoblast/odontoblastic markers, including Runx2, alkaline phosphatase (ALP), matrix extracellular phosphoglycoprotein (MEPE), bone sialoprotein (BSP), and DSPP. After implantation into immunocompromised mice, with hydroxyapatite/tricalcium phosphate (HA/TCP) as a carrier, SHED differentiated into odontoblast-like cells that formed small dentin-like structures (Miura et al., 2003). These results suggest that SHEDs are distinctive from DSPCs with respect to odontogenic differentiation and osteogenic induction (Miura et al., 2003).
(2.4) Periodontal Ligament Stem Cells (PDLSCs)
The periodontal ligament (PDL) connects the cementum to alveolar bone, and functions primarily to support the tooth in the alveolar socket. A recent report identified stem cells in human PDL (PDLSCs) and found that PDLSCs implanted into nude mice generated cementum/PDL-like structures that resemble the native PDL as a thin layer of cementum that interfaced with dense collagen fibers, similar to Sharpey’s fibers (Seo et al., 2004). After a three-week culture with an adipogenic-inductive cocktail, PDLSCs differentiated into Oil-red-O-positive, lipid-laden adipocytes (Seo et al., 2004). Upon four-week osteo/odontogenic inductions, alizarin-red-positive nodules formed in the PDLSC cultures, similar to MSCs and DPSCs (Shi et al., 2002; Seo et al., 2004). Thus, the PDLSCs have the potential for forming periodontal structures, including the cementum and PDL.
(2.5) Challenges in Stem Cell Biology Related to Craniofacial Tissue Engineering
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(3) TISSUE ENGINEERING OF THE TEMPOROMANDIBULAR JOINT FROM STEM CELLS |
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TOPABSTRACT(1) INTRODUCTION(2) MESENCHYMAL STEM CELLS...(3) TISSUE ENGINEERING OF...(4) PERIODONTAL TISSUE...(5) TISSUE ENGINEERING OF...(6) IMPACT OF CRANIOFACIAL...REFERENCES |
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In the past few years, we have reported the tissue engineering of a mandibular condyle exhibiting the shape and dimensions of a human cadaver TMJ. The engineered mandibular condyle had stratified layers of cartilage and bone from a single population of bone-marrow-derived MSCs, and was molded into the shape and dimensions of a human cadaver mandibular condyle at 11 x 7 x 9 mm (l x w x h) (Alhadlaq and Mao, 2003, 2005; Alhadlaq et al., 2004; Mao, 2005b).
(3.1) Cell Survival and Matrix Synthesis in the Tissue-engineered Mandibular Condyle
MSCs were isolated from femoral and tibial bone marrows of adult rats and exposed separately to either chondrogenic or osteogenic supplemented culture medium (Alhadlaq and Mao, 2003; Alhadlaq et al., 2004). Poly(ethylene glycol) diacrylate (PEGDA) was dissolved in PBS with a biocompatible ultraviolet photoinitiator (Alhadlaq and Mao, 2003; Alhadlaq et al., 2004). MSC-derived chondrogenic and osteogenic cells were encapsulated in PEGDA hydrogel into a negative mold of an adult human cadaver mandibular condyle in two stratified and yet integrated layers. The photopolymerized osteochondral construct was implanted into the dorsum of immunodeficient mice for up to 12 wks.
De novo formation of a structure having the same shape and dimensions as the cadaver human mandibular condyle was observed after 4 wks of in vivo implantation (Fig. 1
). The tissue-engineered mandibular joint condyles retained the macroscopic shape and dimensions of the cadaver mandibular condyle (Fig. 1A). The interface between the upper-layer PEGDA hydrogel encapsulating MSC-derived chondrogenic cells and the lower-layer PEGDA hydrogel encapsulating MSC-derived osteogenic cells demonstrated distinctive microscopic characteristics (Figs. 1B–1D). The chondrogenic and osteogenic portions remained in their respective layers (cf. interface in Fig. 1B). The chondrogenic layer contained sparse chondrocyte-like cells surrounded by abundant intercellular matrix (Fig. 1C). The intercellular matrix of the chondrogenic layer showed strong, intense staining with safranin O (Fig. 1C), a cationic marker that binds to cartilage-related glycosaminoglycans (GAG), such as chondroitin sulfate and keratan sulfate. Some of the MSC-derived chondrogenic cells were surrounded by pericellular matrix, characteristic of native chondrocytes (Fig. 1C). In contrast, the osteogenic layer contained mineral nodules, as revealed by von Kossa staining (lower portion of Fig. 1B). The osteogenic layer also showed multiple island structures occupied by osteoblast-like cells (Fig. 1D).
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) did not resemble the mutual infiltration of chondral and osseous tissues in the native osteochondral junction. We then attempted to improve tissue maturation and osteochondral integration by increasing the cell encapsulation density from the previous 5 x 106 cells/mL to 20 x 106 cells/mL.
At a density of 20 x 106 cells/mL, the tissue-engineered mandibular joint condyle again retained the pre-defined shape and dimensions of the human cadaver mandibular condyle (Alhadlaq and Mao, 2005). The cartilage layer was positively stained by safranin O, indicating the presence of cartilage-related glycosaminoglycans (Fig. 2A), and contained type II collagen (Fig. 2B). The deep portion of the cartilage layer, near the tissue-engineered osteochondral interface, contained chondrocyte-like cells with a hypertrophic appearance and characterized by the expression of type X collagen (data not shown, but cf. Alhadlaq and Mao, 2005), a marker for hypertrophic and degenerating chondrocytes. In contrast, the osseous layer demonstrated immunolocalization of bone markers such as osteopontin (Fig. 2C) and osteonectin (cf. Alhadlaq and Mao, 2005). The chondrogenic layer lacked immunolocalization of bone markers, whereas the osseous layer lacked immunolocalization of cartilaginous markers (Figs. 2A–2C). There was mutual infiltration of the cartilaginous and osseous components into each other’s territory (Figs. 2A, 2B). This mutual infiltration of chondral and osseous tissues, absent in our previous work, where we used a four-fold-lower cell encapsulation density (Alhadlaq and Mao, 2003; Alhadlaq et al., 2004), resembles the osteochondral interface in age-matched native rat mandibular condyle (Alhadlaq and Mao, 2005). The cell-free PEGDA hydrogel scaffold showed an intact border surrounded by fibrous capsule without host cell invasion (Fig. 2D), further substantiating the conclusion that the engineered mandibular joint condyle is formed solely by MSC-derived chondrogenic cells and osteogenic cells, rather than by host cells.
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The ultimate goal is to use integrated design/fabricated scaffolds for functional mandibular condyle and other craniofacial reconstruction. Integrated design/fabrication methods not only make functional reconstruction possible, but also permit the testing of design hypotheses concerning scaffold/cell carrier combinations, eventually leading to optimal reconstruction methods. Our group has begun to test anatomically designed scaffolds for mandibular reconstruction in a Yucatan minipig model. We designed and fabricated a mandibular condyle scaffold directly from a minipig mandibular CT scan. The initial design was a shell into which autologous bone marrow was packed at the time of surgery (Figs. 3A–3C). Initial results at 1 and 3 months showed that the minipigs masticated normally, as documented by video, and, furthermore, that bone regenerated in the desired condyle shape, as shown by CT (Figs. 3D, 3E). Specimens will be processed for histology so that cartilage formation can be assessed. These results demonstrate that the shell scaffold design could support functional load-bearing as well as tissue regeneration. Further work will investigate how scaffold design, material, and biologic/carrier combinations affect regeneration.
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(4) PERIODONTAL TISSUE ENGINEERING |
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(4.1) Microbial Influences on Periodontal Tissue Engineering
Periodontal infection is initiated by invasive oral pathogens that colonize dental plaque biofilms on the tooth root surface. This chronic challenge of virulent micro-organisms leads to the destruction of tooth-supporting soft and hard tissues, including alveolar bone, tooth root cementum, and the PDL. Thus, tissue-engineered constructs for periodontal reconstruction will likely face constant exposure to the oral bacterial flora, which contains 106–107 colony-forming units per milliliter (CFU/mL) of saliva (Motisuki et al., 2005). Even in the healthy oral cavity, the mucosal surfaces are colonized by approximately 90% anaerobic and 10% aerobic bacteria (Weber, 1997; Callender, 1999). Although the incidence of infection in oral surgical procedures is between 1 and 5% (Loukota, 1991), the consequences will, at least conceptually, compromise the outcome of periodontal tissue engineering.
(4.2) Growth and Amelogenin Factors in Periodontal Regeneration
Over the past decade, numerous polypeptide growth and amelogenin-like factors have been used in the repair of periodontal defects. Pre-clinical investigations have revealed encouraging results with respect to the repair of periodontal defects with growth factors such as platelet-derived growth factor (PDGF) (Lynch et al., 1991; Rutherford et al., 1992; Giannobile et al., 1994; Giannobile, 1996), bone morphogenetic proteins (BMPs) (Sigurdsson et al., 1995; Giannobile et al., 1998), and fibroblast growth factor-2 (Rossa et al., 2000; Takayama et al., 2001; Nakahara et al., 2003). PDGF has demonstrated the promise to regenerate bone in periodontal osseous defects, as shown in human clinical trials when used alone (Camelo et al., 2003; Nevins et al., 2003, 2005) or in combination with IGF-1 (Howell et al., 1997b). BMPs have been shown to induce bone regeneration in peri-implant and maxillary sinus-floor augmentation procedures (Boyne, 1996; Boyne et al., 1997; Hanisch et al., 1997), extraction socket defects (Howell et al., 1997a; Cochran et al., 2000; Fiorellini et al., 2005), and mandibular discontinuity defects (Boyne et al., 1999; Warnke et al., 2004). The use of BMPs for oral and craniofacial reconstruction has recently been reviewed (Nakashima and Reddi, 2003; Wikesjö et al., 2005).
Enamel matrix derivative (EMD) appears to promote periodontal regeneration by mimicking corresponding developmental events. EMD is composed of amelogenins with metallo-endoprotease and serine protease activity. Although their roles in epithelial-mesenchymal interactions remain to be clarified, EMDs have been shown to promote periodontal regeneration in multiple controlled human clinical trials (Heijl et al., 1997; Zetterström et al., 1997; Silvestri et al., 2000; Tonetti et al., 2002). In addition to EMD, a product that utilizes a biomimetic RGD cell-binding peptide fused to anorganic bone matrix has demonstrated efficacy in promoting periodontal bone repair in human clinical trials (Yukna et al., 2000, 2002).
(4.3) Gene Delivery in Periodontal Tissue Engineering
Gene transfer offers significant potential to improve growth factor delivery to tooth-supporting defects (Baum et al., 2002). Gene therapy has demonstrated strong potential to target, deliver, and improve the bioavailability of growth factors (such as BMPs and PDGFs) to stimulate the tissue engineering of periodontal defects (Fig. 4). Delivery of PDGF by gene transfer stimulates remarkable mitogenesis and proliferation of gingival fibroblasts, PDL, and cementoblasts, in comparison with continuous PDGF administration in vitro (Zhu et al., 2001; Chen and Giannobile, 2002; Jin et al., 2004a). Adenovirus-mediated PDGF-B gene transfer accelerated gingival wound healing in an ex vivo wound repair model (Anusaksathien et al., 2003). Surgically created alveolar bone wounds treated with AdPDGF-B revealed significant regeneration of cementum and PDL, along with extended growth factor expression, as demonstrated by in vivo biodistribution (Jin et al., 2004a). Application of a dominant-negative mutant of the PDGF-A gene (PDGF-1308) directly to periodontal osseous defects or to cementoblasts transplanted ex vivo in polymer scaffolds led to impaired periodontal regeneration (Anusaksathien et al., 2004; Jin et al., 2004b). BMP viral gene delivery has showed remarkable potential in the regeneration of both long bones and craniofacial bones (Lieberman et al., 1998, 1999; Alden et al., 2000; Baltzer et al., 2000; Krebsbach et al., 2000; Shen et al., 2004a,b; Dunn et al., 2005). Ex vivo gene delivery of BMP7-transduced fibroblasts has regenerated alveolar bone and cementum, whereas local knock-out of BMP bioactivity by Noggin, a BMP-antagonist, blocks cementogenesis of tissue-engineered cementum (Jin et al., 2003a, 2004b).
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(4.5) Challenges of Periodontal Tissue Engineering
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(5) TISSUE ENGINEERING OF CRANIOFACIAL BONE |
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(5.1) Adipose-derived Stem Cells Heal Calvarial Defects
Adipose-derived mesenchymal cells (AMCs) are readily obtained via lipo-aspirate, and can be expanded in culture (Zuk et al., 2001, 2002; De Ugarte et al., 2003). Several groups have reported that AMCs are multipotent and capable of differentiation into muscle, bone, and cartilage cells (Zuk et al., 2001, 2002; Gimble and Guilak, 2003; Hicok et al., 2004). Bone tissue engineering is perhaps the most widely investigated application of AMCs. For example, rat AMCs seeded in polyglycolic acid scaffolds and implanted subcutaneously have led to bone formation (Lee et al., 2003). Similar findings have been noted with human AMCs using HA-TCP scaffolds in immunodeficient mice (Hicok et al., 2004). Recently, human AMCs manipulated via BMP-2-mediated gene therapy in collagen I scaffolds induced significant bone formation in the hindlimb of immunodeficient mice (Dragoo et al., 2003, 2005). Finally, AMCs were used as a potential source for cell-based therapies for healing calvarial defects (Fig. 5, from Cowan et al., 2004). The adipose-derived stem cells were seeded in apatite-coated PLGA scaffolds and implanted into the surgically created, critical-size calvarial defects (Cowan et al., 2004). AMCs induced new bone formation similar in amount to that induced by either bone marrow MSCs or osteoblasts (Cowan et al., 2004).
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(5.2) Special Considerations of Cranial Skeletal Repair
The skeleton of the head has a complex developmental history. Whereas the appendicular skeleton is derived solely from the mesoderm and forms bone through endochondral ossification, the cranial skeleton is a patchwork of bony elements derived from the cranial neural crest and the paraxial mesoderm, which forms bone through both endochondral and intramembranous ossification (Le Lièvre and Douarin, 1975; Noden, 1988; Couly et al., 1993; Chai et al., 2000; Jiang et al., 2002). If adult tissue repair recapitulates fetal development, one might ask: Is cranial repair a different process from skeletal healing in other parts of the body?
Growing evidence indicates that at least one critical cellular player in the repair process—namely, the calvarial osteoblast—exhibits several distinctive characteristics, depending upon cellular derivation (for reviews, see Allen et al., 2004; Barry and Murphy, 2004; Taichman, 2005). The first unique feature among osteoblasts from various parts of the body is their response to molecular signals: Both in vitro and in vivo responses to growth factors are different between calvarial osteoblasts and osteoblasts derived from the appendicular and axial skeletons (Anderson and Danylchuk, 1978; Uddstromer and Ritsila, 1979; Miller et al., 1991; Sheng et al., 1999). A second noticeable difference is the response of osteoblasts from different sites of the body to mechanical stimuli (Hock et al., 1982; Allen et al., 2004; for reviews, see de la Fuente and Helms, 2005; Mao, 2005b). Even within a single skeletal element, periosteal osteoblasts are dissimilar to endosteal osteoblasts, both in their response to molecular signals and in their response to mechanical stress (for reviews, see Epker and Frost, 1965; Jones et al., 1991; Bord et al., 2001; Midura et al., 2003). Another difference is embryonic origin. The appendicular and axial osteoblasts are derived from mesoderm, whereas calvarial osteoblasts originate from the neural crest (Noden, 1988; Couly et al., 1993, 2002; Chai et al., 2000; Brault et al., 2001; Jiang et al., 2002; Ruhin et al., 2003; Brewer et al., 2004; Le Douarin et al., 2004). Does any of these differences affect osteoblast behavior during bone repair? This fundamental question has rarely been systematically addressed. Therefore, we are left with the question: Does the embryonic origin of a bone have an effect on how it heals? Most of our knowledge of skeletal repair comes from the analysis of long-bone fractures. To understand if the repair of cranial defects is equivalent to, or distinctive from, the healing of long bones, one must compare the two healing processes directly. Variables to keep in mind include the dual origins of the cranial skeleton and the process by which individual bones form: i.e., intramembranous vs. endochondral. The mechanical environment plays an indisputable role in long-bone healing (Rodriguez-Merchan and Forriol, 2004; Isaksson et al., 2006) and most likely influences cranial healing as well, but this important aspect remains to be further characterized (Carter et al., 1998; Mao, 2005b). Answers to these kinds of questions will have direct and profound implications for the treatment of skeletal injuries, diseases, and disfigurements.
(5.3) Craniofacial Bone Regeneration after Ablative Cancer Surgery and Radiation Therapy
Oral cancer represents about 5% of all malignancies in the United States (Greenlee et al., 2001). Despite advances in treatment strategies, the morbidity and mortality rates are significant, with a significant number of patients developing recurrence, distant metastasis, and second primary tumors (Denham et al., 2001; Myers et al., 2001). Approximately 50% of all cancer patients receive radiation therapy. Radiation toxicity adversely affects bone development, remodeling, and fracture healing (Mitchell and Logan, 1998; Spear et al., 1999). Radiation therapy also complicates reconstructive surgery, due to increased apoptosis and compromised vascularization (Okunieff et al., 1998a,b). Irradiation of up to 8 Gy did not affect the BMP-2-induced osteoblast differentiation of C2C12 cells in vitro (Ikeda et al., 2000). New bone was formed on BMP-2-treated hydroxyapatite disks implanted into a subperiosteal pocket in rabbits radiated pre-operatively with a fractionated dose of 20 Gy (Howard et al., 1998). Despite these encouraging findings, 3-mm rat calvarial defects pre-operatively treated with a 12-Gy radiation dose, and subsequently with BMP-2, had successful bone regeneration, but incomplete healing of the defect (Wurzler et al., 1998). In rat mandibular defects treated with a fractionated 45-Gy radiation dose, followed by transplantation of demineralized bone powder, only 39% of defects had more than 50% bone fill (Lorente et al., 1992).
Combinatorial therapies appear to improve bone regeneration in irradiated sites. When a pedicle muscle flap was combined with BMP-3 treatment, complete healing of craniofacial defects occurred after a single pre-operative 15-Gy radiation dose (Khouri et al., 1996). Treatment with no implant, with a microvascular muscle flap alone, or with BMP-3 alone did not heal the defect, suggesting the need for a well-vascularized recipient bed and an adequate population of responsive osteogenic cells, in addition to the delivering of an osteoinductive protein (Khouri et al., 1996). The most effective bone regeneration for irradiated cranial sites appears to be a combination of cell transplantation and gene therapy approaches (Nussenbaum et al., 2003, 2005). For example, it has been demonstrated that bone formation by transplanted bone marrow stromal cells could be greatly enhanced by the concurrent administration of anabolic doses of parathyroid hormone (Schneider et al., 2003). However, it has yet to be established if such combinations of therapies would be effective in pre-operatively irradiated sites.
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(6) IMPACT OF CRANIOFACIAL TISSUE ENGINEERING ON CLINICAL PRACTICE |
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Decades of contemporary biomedical research have focused primarily on the understanding of the mechanisms of biological functions in health and disease. For example, our understanding of the mechanisms of dental caries has advanced tremendously. In contrast, the end goal of tissue engineering is to develop products capable of healing diseased or lost tissues and organs, thus representing a departure from conventional biomedical research, whose primary focus is an understanding of mechanisms. This does not imply that the understanding of mechanisms is unimportant in tissue engineering. Instead, an understanding of the mechanisms of interactions among cells, growth factors, and biomaterials undoubtedly will advance the end goal of developing cell-based therapies and off-the-shelf tissue-engineering products. However, an understanding of mechanisms is the means for tissue engineering, but not the end. A cadre of craniofacial tissue engineers with interdisciplinary skills in stem cell biology, molecular biology and genetics, polymer and materials science, and mechanical engineering, and a clinical knowledge of dental, oral, and craniofacial disorders is needed to advance the field of craniofacial tissue engineering.
Craniofacial tissue engineering is an opportunity that dentistry cannot afford to miss. This notion is based on both biological and strategic reasons. Biologically, mesenchymal cells are primarily responsible for the formation of virtually all dental, oral, and craniofacial structures. Mesenchymal stem cells, the reservoir of mesenchymal cells in the adult, have been demonstrated, in tissue engineering, to generate key dental, oral, and craniofacial structures. Many dental and craniofacial structures are readily accessible, thus presenting a convenient platform for biologists, bioengineers, and clinicians to test tissue-engineered prototypes (Hollinger and Winn, 1999; Alsberg et al., 2001). Strategically, tissue-engineering technologies pioneered outside the dental community may have profound implications on dental practice.
The impact of craniofacial tissue engineering extends beyond dental practice. Several craniofacial structures engineered thus far serve as prototypes for the tissue engineering of non-craniofacial structures. Craniofacial-derived stem cells have potential implications in the tissue engineering of not only craniofacial structures, but also non-craniofacial tissues (Shi et al., 2005; Sonoyama et al., 2005). Tissue-engineered bone with customized shape and dimensions has the potential for the biological replacement of not only craniofacial bones, but also of segmental defects in the appendicular bones (Feinberg et al., 2001; Chu et al., 2002; Moore et al., 2004). Calvarial bone defects are the most frequently used models for bone tissue engineering (Cowan et al., 2004; Meinel et al., 2005). The tissue-engineered mandibular condyle has served as a prototype for the engineering of other joints, such as the knee and hip (Alhadlaq and Mao, 2003, 2005; Alhadlaq et al., 2004; Hollister, 2005; Hollister et al., 2005; Mao, 2005a; Marion et al., 2005). Tissue-engineered cranial sutures and periodontium offer clues for the fabrication of composite tissue constructs with multiple cell types and encapsulated in multiple materials (Giannobile and Somerman, 2003; Hong and Mao, 2004; Nakahara et al., 2004; Rahaman and Mao, 2005; Moioli et al., 2006). Several meritorious studies have led to the in vitro fabrication of a TMJ disc from cell- and growth-factor-based approaches (Almarza and Athanasiou, 2004; Detamore and Athanasiou, 2005). Adipose tissue grafts engineered from stem cells with sustained shape and dimensions may offer applications in both facial reconstructive surgeries as well as augmentative and/or reconstructive procedures elsewhere (Alhadlaq et al., 2005; Stosich and Mao, 2005, 2007). Conversely, craniofacial tissue engineering could not have advanced to the current stage without the incorporation of interdisciplinary skill sets of stem cell biology, bioengineering, polymer chemistry, mechanical engineering, robotics, etc. Thus, craniofacial tissue engineering and regenerative dental medicine are integral components of regenerative medicine.
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ACKNOWLEDGMENTS |
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We thank our colleagues whose work has been cited in the manuscript for their inspiration. We are grateful to Dr. Anthony Smith, Editor-in-Chief of the Journal of Dental Research, and Dr. Olav Alvares, Editor of Critical Reviews in Oral Biology & Medicine, for providing us the opportunity to write this review. We appreciate the invaluable comments by two anonymous reviewers.
The following research grants from the National Institutes of Health, especially the National Institute of Dental and Craniofacial Research (NIDCR), are gratefully acknowledged: (J.J.M.) DE13964 and DE15391 from NIDCR, and EB02332 from NIBIB; (W.V.G.) DE13397, DE15384, and DE16619 from NIDCR; (J.A.H.) DE012462 from NIDCR; (S.J.H.) DE013608 and DE016129 from NIDCR; (P.H.K.) DE013835, DE016530, and DE007057 from NIDCR; and (M.T.L.) The Oak Foundation and DE13194, DE13028, and DE14526 from NIDCR.
Received September 17, 2005; Accepted April 25, 2006
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REFERENCES |
|---|
|
TOPABSTRACT(1) INTRODUCTION(2) MESENCHYMAL STEM CELLS...(3) TISSUE ENGINEERING OF...(4) PERIODONTAL TISSUE...(5) TISSUE ENGINEERING OF...(6) IMPACT OF CRANIOFACIAL...REFERENCES |
|---|
Akizuki T, Oda S, Komaki M, Tsuchioka H, Kawakatsu N, Kikuchi A, et al. (2005). Application of periodontal ligament cell sheet for periodontal regeneration: a pilot study in beagle dogs. J Periodontal Res 40:245–251.[ISI][Medline]
Alden TD, Beres EJ, Laurent JS, Engh JA, Das S, London SD, et al. (2000). The use of bone morphogenetic protein gene therapy in craniofacial bone repair. J Craniofac Surg 11:24–30.[ISI][Medline]
Alhadlaq A, Mao JJ (2003). Tissue-engineered neogenesis of human-shaped mandibular condyle from rat mesenchymal stem cells. J Dent Res 82:951–956.
Alhadlaq A, Mao JJ (2004). Mesenchymal stem cells: isolation and therapeutics. Stem Cells Dev 13:436–448.[ISI][Medline]
Alhadlaq A, Mao JJ (2005). Tissue engineered osteochondral constructs in the shape of an articular condyle. J Bone Joint Surg Am 87:936–944.
Alhadlaq A, Elisseeff JH, Hong L, Williams CG, Caplan AI, Sharma B, et al. (2004). Adult stem cell driven genesis of human-shaped articular condyle. Ann Biomed Eng 32:911–923.[ISI][Medline]
Alhadlaq A, Tang M, Mao JJ (2005). Engineered adipose tissue from human mesenchymal stem cells maintains predefined shape and dimension: implications in soft tissue augmentation and reconstruction. Tissue Eng 11:556–566.[ISI][Medline]
Allen MR, Hock JM, Burr DB (2004). Periosteum: biology, regulation, and response to osteoporosis therapies. Bone 35:1003–1012.[Medline]
Almarza AJ, Athanasiou KA (2004). Seeding techniques and scaffolding choice for tissue engineering of the temporomandibular joint disk. Tissue Eng 10:1787–1795.[ISI][Medline]
Alsberg E, Hill EE, Mooney DJ (2001). Craniofacial tissue engineering. Crit Rev Oral Biol Med 12:64–75.[Abstract]
Anderson C, Danylchuk KD (1978). Bone-remodeling rates of the beagle: a comparison between different sites on the same rib. Am J Vet Res 39:1763–1765.[ISI][Medline]
Anusaksathien O, Webb SA, Jin QM, Giannobile WV (2003). Platelet-derived growth factor gene delivery stimulates ex vivo gingival repair. Tissue Eng 9:745–756.[ISI][Medline]
Anusaksathien O, Jin Q, Zhao M, Somerman MJ, Giannobile WV (2004). Effect of sustained gene delivery of platelet-derived growth factor or its antagonist (PDGF-1308) on tissue-engineered cementum. J Periodontol 75:429–440.[ISI][Medline]
Arbab AS, Jordan EK, Wilson LB, Yocum GT, Lewis BK, Frank JA (2004). In vivo trafficking and targeted delivery of magnetically labeled stem cells. Hum Gene Ther 15:351–360.[ISI][Medline]
Atala A (2005). Tissue engineering, stem cells and cloning: current concepts and changing trends. Expert Opin Biol Ther 5:879–892.[ISI][Medline]
Baltzer AW, Lattermann C, Whalen JD, Wooley P, Weiss K, Grimm M, et al. (2000). Genetic enhancement of fracture repair: healing of an experimental segmental defect by adenoviral transfer of the BMP-2 gene. Gene Ther 7:734–739.[ISI][Medline]
Barry FP, Murphy JM (2004). Mesenchymal stem cells: clinical applications and biological characterization. Int J Biochem Cell Biol 36:568–584.[ISI][Medline]
Batouli S, Miura M, Brahim J, Tsutsui TW, Fisher LW, Gronthos S, et al. (2003). Comparison of stem-cell-mediated osteogenesis and dentinogenesis. J Dent Res 82:976–981.
Baum BJ, Kok M, Tran SD, Yamano S (2002). The impact of gene therapy on dentistry: a revisiting after six years. J Am Dent Assoc 133:35–44.
Bord S, Horner A, Beavan S, Compston J (2001). Estrogen receptors alpha and beta are differentially expressed in developing human bone. J Clin Endocrinol Metab 86:2309–2314.
Boyne PJ (1996). Animal studies of application of rhBMP-2 in maxillofacial reconstruction. Bone 19(1 Suppl):83S–92S.[Medline]
Boyne PJ, Marx RE, Nevins M, Triplett G, Lazaro E, Lilly LC, et al. (1997). A feasibility study evaluating rhBMP-2/absorbable collagen sponge for maxillary sinus floor augmentation. Int J Periodontics Restorative Dent 17:11–25.[Medline]
Boyne PJ, Nakamura A, Shabahang S (1999). Evaluation of the long-term effect of function on rhBMP-2 regenerated hemimandibulectomy defects. Br J Oral Maxillofac Surg 37:344–352.[ISI][Medline]
Brault V, Moore R, Kutsch S, Ishibashi M, Rowitch DH, McMahon AP, et al. (2001). Inactivation of the beta-catenin gene by Wnt1-Cre-mediated deletion results in dramatic brain malformation and failure of craniofacial development. Development 128:1253–1264.[Abstract]
Brazelton TR, Rossi FM, Keshet GI, Blau HM (2000). From marrow to brain: expression of neuronal phenotypes in adult mice. Science 290:1775–1779.
Brewer S, Feng W, Huang J, Sullivan S, Williams T (2004). Wnt1-Cre-mediated deletion of AP-2alpha causes multiple neural crest-related defects. Dev Biol 267:135–1352.[ISI][Medline]
Bruens ML, Pieterman H, de Wijn JR, Vaandrager JM (2003). Porous polymethylmethacrylate as bone substitute in the craniofacial area. J Craniofac Surg 14:63–68.[ISI][Medline]
Callender DL (1999). Antibiotic prophylaxis in head and neck oncologic surgery: the role of Gram-negative coverage. Int J Antimicrob Agents 12(Suppl 1):S21–S25.
Camelo M, Nevins ML, Schenk RK, Lynch SE, Nevins M (2003). Periodontal regeneration in human Class II furcations using purified recombinant human platelet-derived growth factor-BB (rhPDGF-BB) with bone allograft. Int J Periodontics Restorative Dent 23:213–225.[ISI][Medline]
Caplan AI (1991). Mesenchymal stem cells. J Orthop Res 9:641–650.[ISI][Medline]
Carter DR, Beaupre GS, Giori NJ, Helms JA (1998). Mechanobiology of skeletal regeneration. Clin Orthop Relat Res 355(Suppl):S41–S55.
Chai Y, Jiang X, Ito Y, Bringas P Jr, Han J, Rowitch DH, et al. (2000). Fate of the mammalian cranial neural crest during tooth and mandibular morphogenesis. Development 127:1671–1679.[Abstract]
Chen QP, Giannobile WV (2002). Adenoviral gene transfer of PDGF downregulates gas gene product PDGFalphaR and prolongs ERK and Akt/PKB activation. Am J Physiol Cell Physiol 282:C538–C544.
Christopherson KW 2nd, Hangoc G, Mantel CR, Broxmeyer HE (2004). Modulation of hematopoietic stem cell homing and engraftment by CD26. Science 305:1000–1003.
