Prospects For Tooth Regeneration

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Prospects For Tooth Regeneration

  1. 1. Periodontology 2000, Vol. 41, 2006, 177–187 Copyright Ó Blackwell Munksgaard 2006 Printed in Singapore. All rights reserved PERIODONTOLOGY 2000 Prospects for tooth regeneration S I L V I O E. D U A I L I B I , M O N I C A T. D U A I L I B I , J O S E P H P. V A C A N T I & P A M E L A C. Y E L I C K Regenerative dental medicine uses an integrated bud, cap, bell, crown, and root (70, 71) (Fig. 2). The sciences approach, involving developmental and coordinated development of tooth supporting struc- molecular/cellular biology, molecular genetics and tures, including periodontal ligament and alveolar chemical engineering (5, 25, 28, 30, 41, 62, 65, 75). bone, begins around the bell stage (68). Recent advances in the fields of dental tissue The tooth germ is first identifiable as a localized engineering, materials science and stem cell biology, thickening and proliferation of the oral epithelium suggest that tooth regeneration will be possible (Fig. 2A). The dental epithelium forms a bud that in the foreseeable future. Published reports have extends into the underlying dental mesenchyme, demonstrated that dental tissue progenitor cells marking the first stage of tooth development. The present in the pulp tissue of deciduous and adult dental epithelium subsequently undergoes signifi- teeth can be used to generate dentin and alveolar cant proliferative activity (Fig. 2B), extending around bone (59, 84), while those present in immature tooth the periphery to form a cap-like structure (Fig. 2C). buds can be used to bioengineer small, anatomically During this process, the nonproliferating enamel correct, whole tooth crowns consisting of enamel, knot signaling center (72) becomes identifiable, as dentin and pulp tissue (13, 81–83). These promising epithelial cells organize themselves into three results, along with many other studies reporting distinct regions, namely the outer epithelium, the dental tissue regeneration, suggest a means for the inner epithelium, and central cell layers called eventual regeneration of replacement teeth in the stratum intermedium and stellate reticulum humans. However, before this can occur, certain (Fig. 2C,D). The ectomesenchymal cells of the dental obstacles must first be overcome. Below, we will papilla condense beneath the invaginating dental review recent progress in tooth-regeneration efforts epithelium, eventually giving rise to dentin and pulp to date, and identify challenges that must be met in tissues. The dental follicle forms around the enamel order for this approach to reach clinical relevance in organ and dental papilla, eventually forming the humans. periodontal tissues (Fig. 2C,D). The bell stage is characterized by continued proliferation and histodifferentiation of the dental epithelium (Fig. 2D). Inner dental epithelial cells Tooth development as a model for assume a cuboidal shape and produce high levels tooth regeneration of glycogen, adjacent stratum intermedium pro- duces high levels of alkaline phosphatase, and the The interactions of the dental epithelium and stellate reticulum assumes a distinctive star shape, mesenchyme during natural tooth development surrounded by the outer epithelial cell layer provide insight into how bioengineered tooth for- (Fig. 2D). mation may be facilitated. Although human incisor, As tooth development proceeds through differen- canine, premolar and molar teeth exhibit distinct tiation stages, dental mesenchyme-derived odonto- morphologies, (Fig. 1), they develop in basically the blasts differentiate and elaborate the dentin matrix, same manner. Tooth development, the result of and epithelial cell-derived ameloblasts cells secrete reciprocal interactions between the dental epithe- the enamel matrix for enamel production (Fig. 2E). lium and the neural crest cell-derived ectomesen- After the tooth crown has formed, tooth root struc- chyme, is initiated by the dental epithelium and tures develop from the rudimentary Hertwig’s epi- proceeds through five distinct morphological stages: thelial root sheath, forming dentin, cementum, 177
  2. 2. Duailibi et al. Deciduous Teeth Permanent Teeth 11 years 7 years 12 years 8 years 15 years 9 years 21 years 10 years Fig. 1. Human deciduous and adult tooth from 7 to 21 years of age. Deciduous and permanent human teeth form and ´ ˜ erupt as shown. (Adapted from: Picosse M. Anatomia dentaria, 2nd edition. Sao Paulo: Sarvier, 1977: 66–67.) periodontal ligament and alveolar bone (Fig. 2F). Human replacement teeth form as a localized The co-ordinated processes of tooth root maturation proliferation of the dental lamina of a pre-existing and tooth eruption proceed in an interdependent deciduous tooth (68). Humans form only one set of manner that is not well understood at the present replacement teeth, which do not exhibit subsequent time (68). regenerative capabilities. The molecular signaling 178
  3. 3. Prospects for tooth regeneration Thus, natural primary and replacement tooth formation provides a foundation upon which tooth- regeneration strategies can be based. It is anticipa- ted that tooth-regeneration strategies need not necessarily be as complex as natural tooth devel- opment, but rather can mimic certain aspects of natural tooth formation to facilitate tooth-regener- ation therapies. Regenerative capabilities of naturally formed dental tissues Human deciduous and adult tooth tissues exhibit a limited degree of regenerative capacity. Each type of mineralized dental tissue – enamel, dentin, cemen- tum and alveolar bone – exhibits distinct properties. Enamel is formed by a process called amelogenesis, dentin formation is termed dentinogenesis, cemen- tum formation is termed cementogenesis, and alve- olar bone forms by osteogenesis (26). Enamel, the most highly mineralized dental tissue, is composed of crystalline calcium phosphate, and is approximately 96% mineral with the remaining 4% consisting of organic components and water. Its hardness, when allowed to dry, is comparable with brittle steel. The basic structural unit of enamel is an enamel rod, which is tightly packed and mechanically adherent to other rods, providing high resistance to stress fractures (68). The interwoven architecture of Fig. 2. Human tooth developmental stages. (A) Tooth enamel crystals provides both strength and protec- bud forms as a proliferation of the dental epithelium (de) tion for the tooth (43). The enamel-forming progen- into the underlying dental mesenchyme (dm). (B) The itor cells – ameloblasts – undergo apoptosis as they dental epithelium continues to proliferate, and the underlying dental mesenchyme undergoes condensation. elaborate the enamel matrix, such that by the time EK, enamel knot signaling center. (C) The cap stage tooth the enamel is fully formed, no ameloblasts remain exhibits distinct outer epithelium (oe), and inner epi- (60). Enamel regeneration therefore is not possible in thelium (ie) layers, surrounding the condensed dental erupted teeth because the progenitor cells are no mesenchyme (dm). (D) The bell-stage tooth exhibits dif- longer present. ferentiated enamel organ, consisting of distinct inner enamel (ie), stratum intermedium (si) and stellate reti- Dentin, the mineralized tissue underlying enamel, culum (sr) cell layers, which surround the dental papilla is characterized by distinctive fluid-filled dentin tu- (dp). (E) Differentiation-stage teeth exhibit polarized bules (7, 68), and is approximately 74% mineral, with odontoblasts (od) and ameloblasts (am), which elaborate the organic phase consisting mostly of type-1 colla- dentin and enamel, respectively. (F) Late differentiation- gen, with small amounts of dentin proteins and water stage teeth display rudimentary root structures, called Hertwig’s epithelial root sheath (hers), periodontal liga- (68). Primary dentin consists of two distinct miner- ment tissues (pdl), polarized odontoblasts (od), and alized types – circumpulpal dentin, which surrounds mineralized dentin (d) and enamel (e) layers. (Courtesy the pulp chamber, and mantle dentin, which is of Katchiburian E, Arana VE. Histologia e Embriologia located at the dentin–enamel junction (44). Dentin is ´ Oral, 2nd edition. Guanabara – Koogan: Medica Pan- composed of millions of tubules (approximately americana S.A.C.F., 2004. Reproduced with permission from the editors.) 60 000 tubules/mm2), which extend through a colla- gen- and calcium-rich zone of intertubular dentin, cascades regulating the fascinating process of from the pulpal wall to the dentin–enamel junction replacement tooth formation remain largely unchar- (66). The tubule diameter at the dentin–enamel acterized as a result of the lack of a suitable animal junction is 0.06 lm, and widens to approximately model. 3.0 lm at the pulpal wall. Dentin tubules are fluid 179
  4. 4. Duailibi et al. filled and may contain an odontoblast process, col- supply, localized inflammation or unfavorable pros- lagen and nonmyelinated pulpal nerves (68). The thesis pressure (1, 14). distinct architecture of the fluid-filled dentin tubules In summary, in naturally formed teeth, enamel – is thought to provide a means of communication the only mineralized tooth tissue derived from the from the enamel layer to the tooth pulp. Tertiary dental epithelium – exhibits no regenerative proper- dentin consists of reactionary dentin (formed by pre- ties, while the remaining mineralized periodontal existing odontoblasts) or reparative dentin (formed and dental tissues, including dentin, pulp, cemen- from newly differentiated odontoblasts) (64). Once tum, periodontal ligament and alveolar bone, all of erupted, teeth maintain a limited capacity to form which are formed from neural crest-derived dental reparative (or tertiary) dentin, when progenitor cells ectomesenchyme, each exhibit a certain degree of are recruited from the pulp to elaborate localized regenerative capability. Therefore, while the devel- dentin matrix at the site of injury (68). opment of clinically relevant regeneration strategies Cementum, the thin layer of mineralized tissue for all types of dental tissues remains a challenging that covers the dentin of tooth roots, is also highly task, that of enamel is the greatest, owing to the mineralized, but softer than dentin, consisting of absence of dental epithelial progenitor cells in approximately 45% inorganic material, 33% organic erupted teeth. material and 22% water. Cementum, formed by cementoblasts, is sandwiched between the inner Postnatal dental stem cells and dental dentin surface and the outer periodontal ligament tissue regeneration surface, and serves to secure the tooth, via the per- iodontal ligament, to the alveolar bone. Cementum Progenitor stem cells have been identified in hun- consists of thin, plate-like hydroxyapatite crystals, dreds of human postnatal tissues (10–12, 52, 54, 55, approximately 55-nm wide and 8-nm thick, and is 85). Stem cells are defined as quiescent cell popula- similar in chemical composition and physical prop- tions present in low numbers in normal tissue, which erties to bone. Three types of cementum are present exhibit the distinct characteristic of asymmetric cell on the tooth: acellular cementum, which covers one- division, resulting in the formation of two distinct third to one-half of the tooth root adjacent to the daughter cells – a new progenitor/stem cell, and an- cemento-enamel junction (the area where cemen- other daughter cell capable of forming differentiated tum and enamel meet); afibrillar cementum, which tissue (18, 32). In this way, stem cells are able to self- is present at the cemento-enamel junction; and renew and maintain themselves in an undifferenti- cellular cementum, which typically covers the apical ated state, while also giving rise to differentiating one-half to two-thirds of the tooth root. Cementum daughter cells. Progenitor cells differ from stem cells naturally regenerates, slowly forming throughout in that they exhibit a finite life span rather than the life of the tooth, allowing for continual re- existing throughout the life of the organism, and attachment of the periodontal ligament fibers, exhibit limited differentiative potential, with the although the identification of the cementoblast capacity to form only limited tissue types. progenitor cells remains somewhat controversial at The characterization of dental progenitor/stem the present time (6). cells has increased significantly over the past 5– Alveolar bone, formed from neural crest cell- 10 years. Dental mesenchymal progenitor cells have derived dental mesenchymal cells, functions as the been characterized using transgenic mouse reporter primary support for teeth, and is composed of models (3, 38), and mesenchymal stem/progenitor bundles of bone that are built up in layers in a par- cells have been identified and characterized in dental allel orientation to the coronal–apical direction of the pulp obtained from both deciduous and adult human tooth. Alveolar bone exhibits rapid turnover in teeth (16, 42, 59). Evidence for both common and response to mechanical stimulation (79). This char- distinct progenitor cells for periodontal tissues has acteristic plasticity of alveolar bone distinguishes it been reported (4, 6, 56, 67), as described in more from other types of bone and allows for constant detail in other chapters of this volume. Preliminary minor accommodation of tooth movements during characterization of postnatal epithelial and mesen- mastication (68). Alveolar bone resorption following chymal dental stem/progenitor cells present in tooth loss can be significant – estimated to be immature tooth buds demonstrated the ability to 40–60% during the first 3 years, with an additional generate bioengineered, anatomically correct tooth 0.25–0.5% loss every year thereafter (1, 2) – presum- crowns containing enamel, dentin, pulp and alveolar ably as a result of disuse atrophy, decreased blood bone, as described below. 180
  5. 5. Prospects for tooth regeneration cells isolated from both rat and pig tooth buds, which Regenerative therapies for dental were cultured in vitro for 6 days (Fig. 3), seeded onto tissues biodegradable scaffolds, and implanted and grown in the omenta of adult rat hosts. Cultured rat tooth bud Tissue-engineering approaches have proven to be cells formed distinctly mineralized tissues in 12 weeks, useful for dental tissue- and whole-tooth-regener- while pig tooth bud cells formed tooth crowns in ation strategies (33). Based on preclinical cell- and approximately 20–30 weeks (Fig. 4). Histologic and gene-therapy strategies used for soft tissue organs, immunohistochemical analyses revealed that bio- reports of the emerging use of tissue-engineering engineered rat and pig dental tissues exhibited many strategies for dentin, pulp and cementum, as an characteristics of naturally formed dental tissues (13, alternative to commonly used root canal and crown 81–83). The fact that rat tooth bud cells were cultured therapies, are becoming more numerous. Advances for up 6 days before being used to generate bioengi- in vital pulp therapies to regenerate the dentin–pulp neered dental tissues suggests that progenitor dental complex without the removal of the whole pulp, in- stem cells can be maintained in culture for at least clude application of exogenous growth factors and/or this long. stem/progenitor cells (46, 68). The formation of multiple small tooth crowns in We have previously shown that tooth bud cells can our bioengineered tooth constructs, rather than one be used to bioengineer anatomically correct tooth large tooth, reveals a number of challenges that crowns (13, 19, 81–83). Our approach used tooth bud need to be addressed before this approach can Fig. 3. Cultured tooth bud cells. (A) Rat tooth bud cells cells. (B) After 5 days in culture, small epithelial colonies cultured for 1 day contain fibroblastic dental mesenchy- are evident (de), surrounded by confluent dental mesen- mal (dm) cells and small, rounded, dental epithelial (de) chymal (dm) cells. Fig. 4. Bioengineered mammalian tooth crowns. (A) Bio- ameloblasts (am) and stellate reticulum (sr). (B) Bioengi- engineered pig tooth exhibits distinct cell layers and tis- neered rat tooth crowns exhibits distinct pulp (pu), pre- sues observed in naturally formed teeth, including: dentin (pd), dentin (d), and enamel (e) layers. odontoblasts (od), predentin (pd), dentin (d), enamel (e), 181
  6. 6. Duailibi et al. reliably be used for tooth-regeneration applications. are all highly significant properties. A variety of First, as bioengineered tooth crown formation re- hydrophilic polymers have been synthesized that quires the interactions of both dental epithelial cell provide cell support and guidance. Importantly, progenitors and mesenchymal cell progenitors (as scaffold materials provide a three-dimensional in natural tooth formation), the ability to bioengi- macromolecular structure to guide the final shape of neer a tooth of specified size and shape will depend bioengineered tissues. Poly-L-lactic acid and poly on the ability to first identify, and then guide, the lactic co-glycolic acid co-polymers have been used interactions of both types of cells. As the cultured to generate composite scaffolds that degrade within tooth bud cell populations used in our studies were a period of a few weeks up to 1 year (37). Poly-L- quite heterogeneous, methods to generate purified lactic acid sponges can support the growth of dental stem cell populations must be developed. chondrocytes in a uniform cellular distribution, their Next, methods to guide the interactions of epithelial utility has been demonstrated in cartilage tissue and mesenchymal postnatal dental stem cells to regeneration (36), and polyglycolic acid and poly- form dentin and enamel layers characteristic of lactic acid have been shown to support the growth of natural teeth, using modified scaffold materials and biopsied neonatal intestine cells into functional, designs, for example, must be developed. Finally, as small intestinal tissue (9). our bioengineered teeth consisted of fairly well- Optimized polymer fabrication techniques have developed tooth crowns, while the tooth roots were been used to generate three-dimensional structures relatively undeveloped, we also need to devise composed of an intercommunicating network of methods to improve bioengineered tooth root for- pores, where the resulting morphology and mech- mation. Progress in each of these areas is discussed anical properties of the scaffold walls were found to below. influence tissue engineering applications (29, 34). Others authors (20) investigated the use of macro- molecular materials of natural origin (i.e. collagen, Generation of enriched epithelial alginate, and agarose), derived from hyaluronic acid and mesenchymal dental stem cell and fibrin glue, to identify polymers that provide the populations best guidance for cellular differentiation and prolif- eration (with the objective of replacing tissues by We are currently using two methods to generate en- cellular transplantation). Natural silk proteins have riched postnatal dental stem cell populations. The been successfully used to generate scaffolds suitable first method uses the stem cell antibody, STRO-1 for bone tissue engineering (35). Each type of scaffold (61), to generate enriched, STRO-1-positive stem cell has unique features that provide flexibility for a populations from cultured tooth bud cell prepara- variety of tissue-engineering applications. We are tions. The second method uses side population currently testing a variety of scaffold materials and profiling to generate enriched dental stem cell designs for guided dentin–enamel junction forma- populations, based on the demonstrated ability for tion, and optimized whole tooth tissue-engineering stem cells to efflux Hoechst dye, while nonstem cell applications, based on morphologic cell movements populations cannot (15). Fluorescent-activated cell of natural tooth development. sorting allows the separation of Hoechst-negative stem cells from dye retaining nonstem cell popula- tions. Clonal cell lines are being established from Functional bioengineered tooth cells sorted by both methods for future testing in root formation dental tissue engineering applications. Our current bioengineered tooth model exhibits crowns that are much more developed than the tooth Scaffold materials and design for roots. To improve bioengineered tooth root forma- whole tooth tissue regeneration tion, we designed hybrid tooth/bone constructs to test whether the co-ordinated alveolar bone forma- The importance of scaffold materials and design for tion could improve bioengineered tooth root devel- tissue engineering has long been recognized. Scaf- opment (82). These studies demonstrated that fold porosity, biocompatibility and biodegradability, bioengineered tooth roots generated from the hybrid the ability to support cell growth, and use as a tooth/bone constructs were, in fact, more developed controlled gene- and protein-delivery vehicle (45) than those formed by tooth constructs alone, indi- 182
  7. 7. Prospects for tooth regeneration cating that this approach is promising. Further in zebrafish is morphologically very similar to adult modifications of our whole tooth tissue-engineering tooth formation in humans, where adult teeth form methods are likely to reveal distinct properties of as a dental laminar proliferation from a deciduous epithelial and mesenchymal dental progenitor cells, tooth (Fig. 5). Although humans generate only two which can be manipulated to improve current tooth- sets of teeth, zebrafish continuously regenerate teeth regeneration efforts. throughout their lives. Replacement teeth form approximately every 7–14 days in juvenile zebrafish, and more slowly in adults (77), providing the means Animal models for tooth to study molecular signaling pathways that are per- regeneration missive for continuous tooth regeneration. The significant nucleotide and amino acid sequence Analyses of tooth development in a variety of animal identity, and genomic synteny shared between ze- models have provided significant insight into tooth- brafish and humans, make the zebrafish a pertinent regeneration strategies. Characterizations of epithe- model for elucidation of molecular strategies for lial dental stem cell populations have largely been human replacement tooth formation. These charac- limited to rodent models exhibiting continuous teristics, together with the molecular/genetic mani- incisor and molar eruption, including the rat, mouse, pulations possible in zebrafish, make them a superb rabbit and vole (17, 27, 73). These recent reports model for in vivo analyses of replacement tooth demonstrate that the epithelial dental stem cell niche formation. is a specialized epithelial structure located at the apical end of the tooth, termed the apical bud, and provide models for identification of genes that Autologous tissues for dental maintain the epithelial dental stem cell niche (17). tissue-regeneration applications The vole, which exhibits continuously erupting mo- lars as well as incisors, has proven useful for com- A major challenge for autologous dental tissue- paring molecular signaling cascades regulating tooth engineering applications in humans is the identifi- crown versus tooth root cell fates (73). These studies cation of suitable cell populations to use for these revealed that the notch signaling pathway, in partic- applications. Reports documenting the successful ular, is important for maintaining dental stem cell bioengineering of dental tissues include the following populations, and that the absence of notch signaling provocative findings. Human dental pulp tissues, results in the terminal differentiation of dental stem isolated from both adult and juvenile teeth, exhibit cells to tooth root tissue fates. the capacity to differentiate, in vitro and in subcu- The zebrafish has also proven useful as a model for taneous implants, into dental mesenchyme-derived tooth regeneration (21, 22, 31, 50, 51, 76, 78, 80). Al- tissues, including dentin, cementum and bone, as though the zebrafish teeth are pharyngeal, located in well as nerve and vascular endothelium (8, 16, 39, 57, the pharynx rather than the jaw, their development is 58). Mesenchymal stem cells have been identified in remarkably similar to that of mammals (51). In a variety of adult tissues as a population of pluripo- addition, zebrafish continuously regenerate teeth tential self-renewing cells isolated from bone mar- throughout their lives. Replacement tooth formation row, which exhibit the capacity to differentiate into Successional Tooth Germ Dental Lamina Fig. 5. Human and zebrafish replacement tooth formation. (A) dl Human successional tooth germs Dental Organ form as a localized proliferation of the dental lamina (arrow). (B) Simi- pu larly, zebrafish replacement teeth Dental Papilla form as a localized proliferation of d the dental lamina (dl, arrow) of an Dental Follicle existing functional tooth. Functional zebrafish teeth exhibit distinct pulp A B (pu) and dentin (d). 183
  8. 8. Duailibi et al. bone, cartilage, muscle, tendon and adipose tissue. the highly mineralized tooth organ, the product of Recent studies have identified and characterized reciprocal signaling events between the dental epi- stem cell populations for cementum, dentine and thelium and dental mesenchyme (23, 69). The need periodontal ligament (53). As a result of the suc- for alternative dental tissue-replacement therapies cessful generation of bioengineered reparative den- is evident in recent reports (48, 74), which reveal tin, it is anticipated that this technique may become startling statistics regarding the high incidence of clinically relevant in the near future (47). Reports tooth decay and tooth loss in the USA. Recent demonstrating the use of embryonic stem cells for advances in the identification and characterization of dental tissue formation reveal the potential utility of dental stem cells, and in dental tissue-engineering embryonic stem cells for tooth tissue-engineering strategies, suggest that within the next decade, bio- applications (40, 49). Whole tooth tissue-engineering engineering approaches may successfully be used to studies revealed that dissociated mammalian tooth regenerate dental tissues (25) and whole teeth (13, bud cells retain a cell-autonomous developmental 81–83). program, even when dissociated into single-cell Interest in dental tissue-regeneration applications suspensions and grown in culture (13, 19, 81, 83). continues to increase as clinically relevant methods These studies suggest the potential for human tooth for the generation of bioengineered dental tissues, buds for dental tissue-engineering applications. The and whole teeth, continue to improve. Although recent identification and characterization of dental obvious practical obstacles remain to be overcome stem cells suggests the potential use of dental stem before routine clinical treatments become com- cells for tooth tissue-replacement therapies. Gene- monly available, dental tissue regeneration research delivery techniques have demonstrated the potential efforts provide an example of how advances in basic for periodontal ligament regeneration, as well as research can be translated into clinically relevant for dentin and alveolar bone (24, 47). Methods to dental therapies (63). Tissue engineering offers generate enriched dental stem cell populations are exciting opportunities for innovative collaborative suggested by studies of transgenic mice expressing research efforts, integrating the fields of medicine, green fluorescent protein under the direction of the developmental biology, and physical sciences. collagen type I gene promoter, which exhibit a Clearly, the future application of regenerative and population of green fluorescent protein-expressing tissue-engineering techniques to dentistry is one of dental pulp cells that exhibit stem cell-like charac- immense potential, capable of meeting a variety of teristics (8, 38). patient needs. High-quality basic dental research is In contrast to dental mesenchymal dental stem paramount to ensuring that the development of cells, the identification and characterization of epi- novel clinical treatments is supported by robust thelial dental stem cells has proven more elusive, mechanistic data and that such approaches are partly owing to the fact that human enamel does not effective. These efforts reveal how successful inno- exhibit regenerative capabilities in vivo, reflecting the vations in the field of dentistry can be guided by fact that epithelial dental stem cells are no longer advances in basic research, highlighting the need for present in functional human teeth. The recent suc- close partnerships between basic research and clin- cessful bioengineering of whole tooth crowns con- ical scientists. taining pulp, dentin and enamel, from immature pig and rat tooth buds, provides the first evidence that postnatal epithelial and mesenchymal dental stem Acknowledgments cells may exhibit utility for whole tooth tissue engineering (13, 82, 83). This work was supported by the Center for the In- tegration of Medicine and Innovative Technology (CIMIT), NIH grants R41DE015445, R21DE16370, and The future of regenerative dentistry R01DE016132-01A1, CAPES grant SAUX-PE 1295/ 2005, and FAPESP grant 04/08924-08. Recent advances in the fields of tissue engineering and surgery have merged to create a promising new era in which it is possible that bioengineered tissues References and organs may routinely be used to replace con- genitally missing tissues and organs, and/or those 1. Ashman A, Lopinto J. Placement of implants into ridges lost to injury or disease. These advances also apply to grafted with bioplant HTR synthetic bone: histological 184
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