kaigler et al jde01

456 Journal of Dental Education ■ Volume 65, No. 5

Tissue Engineering’s Impact on DentistryDarnell Kaigler, B.S.; David Mooney, Ph.D.Abstract: Tissue engineering is a novel and exciting field that aims to re-create functional, healthy tissues and organs in order toreplace diseased, dying, or dead tissues. The field has developed due to the inadequate supply of organs and tissues for patientsrequiring organ and tissue replacement. The following review first describes three major tissue engineering strategies. Althoughsimilar in their objectives, these strategies each maintain a unique component. Next, several examples of preclinical and clinicalprogress engineering oral-maxillofacial tissues are presented. Each of these examples highlights specific tissue engineeringapplications to different tissues of the oral-maxillofacial apparatus. Finally, practical implications are addressed as well aschallenges that must be met in order for tissue engineering to reach its full potential.

Mr. Kaigler is a D.D.S./Ph.D. candidate, Department of Biologic and Materials Sciences, and Dr. Mooney is Associate Professor,Department of Biologic and Materials Sciences and Associate Professor in the Department of Chemical and Biomedical Engi-neering, both at the University of Michigan. Direct correspondence and requests for reprints to Dr. David Mooney, Department ofBiologic & Materials Sciences, Room 5213 Dental School, University of Michigan, 1011 N. University, Ann Arbor, MI 48109-1078; 734-764-2560 phone; 734-647-2110 fax; [email protected] e-mail.

Key words: tissue engineering and regeneration, oral-maxillofacial tissue

Submitted for publication 1/31/01, accepted for publication 4/9/01

Dental and medical treatment for loss of tis-sue or end-stage organ failure is required formillions of Americans each year. Annually,

these individuals account for an estimated total healthcare cost of more than $400 billion, which is approxi-mately one-half of all medical-related costs in theUnited States.1 The field of tissue engineering hasdeveloped over the past decade to re-create func-tional, healthy tissues and organs in order to replacediseased, dying, or dead tissues. As they relate to theoral-maxillofacial apparatus, hard and soft tissuedefects secondary to trauma (e.g, car accidents), con-genital defects (e.g., cleft palate), and acquired dis-eases (e.g., cancer, periodontal disease) are a signifi-cant health problem.

The principal objectives of the current clinicalapproaches to tissue replacement and reconstructionwere to alleviate pain and to restore mechanical sta-bility and function. Current strategies used for treat-ment of lost tissues include the utilization of autog-enous grafts, allografts, and synthetic materials(alloplasts). Although all of these treatment ap-proaches have had successes and have been majoradvances in medicine, each of them has limitations.One of the major shortcomings with autografts, aswell as allografts, is the fact that humans do not havesignificant stores of excess tissue for transplantation.

Other restrictions, particularly related to replacinglost bone, include donor site morbidity, anatomic andstructural problems, and elevated levels of resorp-tion during healing.2 Compounded with this, in thecase of allografts, there always exists the possibilityof eliciting an immunologic response due to geneticdifferences, as well as inducing transmissible dis-eases.3 On the other end of the spectrum lies syn-thetic material replacements (e.g., dental implants).Common with all foreign implanted materials, as partof a natural defense mechanism, the body has a ten-dency to encapsulate foreign materials in a thin, fi-brous membrane. As it relates to the dental implant,the fibrous capsule created by the immune responsecan potentially wall off the implant from its new en-vironment and can prevent the implant from achiev-ing true osseointegration,4 ultimately leading to fail-ure. Furthermore, if implants do achieve initialosseointegration, the changing needs of the bodyoften will lead to failure over time.

It is these critical issues that have led to thequestion: what is the ideal replacement of lost tis-sues? The gold standard to replace an individual’slost or damaged tissue is the same natural healthytissue. This standard has led to the concept of engi-neering or regenerating new tissue from pre-exist-ing tissue. Tissue engineering is a multidisciplinary

Transfer of Advances in Sciences into Dental Education

For a more in-depth review of this topic, see “Craniofacial Tissue Engineering” authored by Alsberg E, Hill EE, and Mooney DJ,which appeared in Critical Reviews in Oral Medicine and Biology, Vol XII, Issue No. 1, January 2001.

May 2001 ■ Journal of Dental Education 457

field. It first employs the knowledge of life sciences(e.g., cell biology, molecular biology, biochemistry)for the growth and development of new tissues. Next,it draws on advances in materials sciences and engi-neering to include current engineering design prin-ciples in the formulation of strategies to engineertruly functional tissues. The field then incorporatesthe therapeutic principles of medical and dental cli-nicians and surgeons5 in order to bring the scientificcomponent to practical application. The advent ofviable tissue engineering will have an effect on thera-peutic options available to oral health specialists. This,in turn, will have implications for curriculum con-tent at the predoctoral and postgraduate levels, aswell as for continuing professional education pro-grams for practicing dentists.

Strategies to Engineer TissueCurrently, strategies employed to engineer tis-

sue can be categorized into three major classes: con-

ductive, inductive, and cell transplantation ap-proaches. These approaches all typically utilize amaterial component, although with different goals(Figure 1). Conductive approaches utilizebiomaterials in a passive manner to facilitate thegrowth or regenerative capacity of existing tissue.An example of this that is very familiar to dentists,and particularly periodontists, is the use of barriermembranes in guided tissue regeneration. Nyman etal. were the first to successfully use osteoconductivemechanisms in providing a means for selective woundhealing by supporting the ingrowth of the periodon-tal supporting cells, while excluding gingival epithe-lial and connective tissue cells from reconstructionsites.6 Techniques and materials are still being opti-mized in guided tissue regeneration. However, theappropriate use of barrier membranes promotes pre-dictable bone repair and histologically verifiable newattachment with new formation of cementum andperiodontal ligament fibers. Treatment options inrestorative and prosthetic dentistry have been revo-lutionized by another relatively widespread applica-

Figure 1. Tissue engineering strategies

Representation of three different tissue engineering approaches: conductive, inductive, and cell transplantation. The conduc-tive approach makes the use of a barrier membrane to exclude connective tissue cells that will interfere with the regenerativeprocess, while enabling the desired host cells to populate the regeneration site. The inductive approach uses a biodegradablepolymer scaffold as a vehicle to deliver growth factors and genes to the host site. The growth factors or genes can be releasedat a controlled rate based on the breakdown of the polymer. The cell transplantation strategy uses a similar vehicle for deliveryin order to transplant cells and partial tissues to the host site. (Figure reprinted with permission of the International/AmericanAssociations for Dental Research from Alsberg E, Hill E, Mooney DJ. Craniofacial tissue engineering. Crit Rev in Oral Biol andMed 2001;12(1):64-75.)


tion of a conductive approach, osseointegration ofthe dental implant. Branemark et al. were the first tosuccessfully achieve this phenomenon, and its ap-plication is relatively simple in that the armamen-tarium does not include living cells or diffusible bio-logical signals.7,8

The second major tissue engineering strategy(induction) involves activating cells in close prox-imity to the defect site with specific biological sig-nals. The origins of this mechanism are rooted in thediscovery of bone morphogenetic proteins (BMPs).Urist first showed that new bone could be formed atnonmineralizing, or ectopic, sites after implantationof powdered bone (bone demineralized and groundinto fine particles).9 Contained within the powderedbone were proteins (BMPs), which turned out to bethe key elements for inducing bone formation. Theseproteins are now available in recombinant forms andproduced on a large scale by biotechnology compa-nies. BMPs have been used in many clinical trialsand are very promising as a means of therapy andsupplementation in the regeneration and repair of

bone in a variety of situations, including nonhealingfractures and periodontal disease.

One limitation of inductive approaches is thatthe inductive factors for a particular tissue may notknown. In this situation the third tissue engineeringapproach, cell transplantation, becomes very attrac-tive. This approach involves direct transplantation ofcells grown in the laboratory.10 The cell transplanta-tion strategy truly reflects the multidisciplinary na-ture of tissue engineering, as it requires the clinicianor surgeon, the bioengineer, and the cell biologist(Figure 2). The clinician is required to biopsy a smallsample of tissue containing the cells of interest. Prin-ciples of cell biology are required to multiply cellsmillion-folds in the laboratory and maintain theirfunction. Meanwhile, the bioengineer manufacturesthe tissue, in bioreactors, and the material onto whichthe cells will be placed for transplantation. Lastly,the clinician is required to transplant the engineeredtissue. After transplantation, the polymer scaffolddegrades and/or is remodeled by host and trans-

Figure 2. Multidisciplinary nature of tissue engineering

The clinician is required in order to sample a small biopsy of tissue. This tissue is then taken to the laboratory and multipliedseveral millionfold. Principles of cell biology must be employed in order to grow these cells and sustain their function.Engineers manufacture the biodegradable polymer matrices and the tissue growth bioreactor in which the tissue will grow.Once the cells have been expanded to an appropriate number, they are placed (seeded) onto the polymer scaffold. The tissueis then allowed further growth in the bioreactor until time of transplantation by the clinician. After transplantation, theengineered tissue may continue to grow until completely developed.


planted cells, resulting in a completely natural tis-sue.

A common feature to all three of the tissueengineering strategies is that they typically employthe use of polymeric materials. In conductive ap-proaches, the polymer is used primarily as a barriermembrane for the exclusion of specific cells that maydisturb the regenerative process. Inductive ap-proaches typically employ a carrier or vehicle forthe delivery of proteins (e.g., BMP) or the actualDNA (gene) that encodes the protein. These mol-ecules then directly (proteins) or indirectly(DNA�mRNA�protein) exert their effects on cellsat the anatomic site by promoting the formation ofthe desired tissue type. Biodegradable polymer car-riers allow a localized and sustained release of theinductive molecules. The rate and dose of moleculedelivery are controlled by features (e.g., degradationrate) of the carrier.11,12 Delivery vehicles are also fre-quently used in cell transplantation approaches. How-ever, in this approach the vehicle serves as a carrierof whole cells and even partial tissues. In addition toserving as simple vehicles for delivery of cells, thevehicles also serve as scaffolds to guide new tissuegrowth in a predictable manner from both the trans-planted cells and interacting host cells. The two ma-jor types of polymeric materials used in all three tis-sue engineering strategies are collagen derived fromanimal sources and synthetic polymers of lactic andglycolic acid (same polymer used in resorbable su-tures). Collagen is degraded by the cells in the tissueas it develops, while the synthetic polymers degradeinto the natural metabolites lactic acid and glycolicacid by the action of water at the implant site. A va-riety of new materials are also being developed forthese applications, and injectable materials that al-low a minimally invasive delivery of inductive mol-ecules or cells are especially attractive.

Tissues of Significance to theOral-Maxillofacial Complex

Two important questions relevant to the dentalpractitioner are “What kind of impact will tissue en-gineering have on dentistry?” and “What oral tissuesdo we have the potential to engineer?” The answer tothe first is still being formulated, but tissue engineer-ing will likely have a revolutionary effect on den-

tistry. The answer to the second question is almostall tissue types. The effect that tissue engineering mayhave in the field of dentistry stems from its wide-spread application to many different types of tissuesrelated to the oral cavity, including bone, cartilage,skin and oral mucosa, dentin and dental pulp, andsalivary glands.

Bone

Tissue engineering will likely have its most sig-nificant impact in dentistry via bone tissue engineer-ing and regeneration. Bony defects secondary to in-jury, disease, and congenital disorders represent amajor health problem. Current strategies aimed atreplacing bony defects include the utilization ofautografts, allografts, and synthetic biomaterials.Despite the fact that these substitutes restore stabil-ity and function to a reasonably sufficient degree,they still contain limitations. This has led to interestin engineering bone, which can be achieved usingall three tissue engineering strategies. Both conduc-tive and inductive approaches can be used to regen-erate small bony defects. Guided tissue regeneration(GTR) after periodontal surgery represents a con-ductive approach to regeneration of bone. BMPs,related proteins, and the genes encoding these pro-teins allow one to engineer bone using inductive ap-proaches in situations where GTR is not sufficient.In contrast, cell transplantation approaches offer thepossibility of pre-forming large bone structures (e.g.,complete mandible) that may not be achievable us-ing the other two strategies. These structures mayeven be completely developed in the lab prior to usein large-scale reconstructive procedures (Figure 313).

Cartilage

As it relates to craniofacial reconstruction, thedesign of polymer scaffolds with defined mechani-cal and degradative properties has opened a new doorto cartilage reconstruction. Cartilage destruction isassociated with trauma and a number of diseases in-cluding degenerative articular cartilage destructionat the temporomandibular joint. The limited capac-ity of cartilaginous tissue to regenerate and the lackof inductive molecules have focused interest amongresearchers and manufacturers in developing celltransplantation approaches to engineer cartilage.


Transplantation of cells without a carrier is now usedclinically to repair small articular cartilaginous de-fects.14 Investigators have also demonstrated in ani-mal models that new cartilaginous tissue with pre-cisely def ined sizes and shapes relevant tomaxillofacial reconstruction (e.g., nasal septum, tem-poromandibular joint) can be engineered using ap-propriate biodegradable scaffolds for transplantingthe cells.15,16

Skin and Oral Mucosa

The most successful application of tissue en-gineering to date is the development of skin equiva-lents. Skin tissue is needed in adjunctive esthetictreatment of individuals who are severely disfiguredfollowing severe burns, in radical resective surgeryto treat invasive cancers, and for major traumawounds (like shotgun wounds and knife lacerations).Skin with both dermal and epidermal components isgrown in the lab using a combination of cells andvarious polymer carriers, and engineered skin prod-ucts were the first tissue-engineered products theFDA approved for clinical use.17,18 A similar approachhas also been developed for the replacement of oralmucosa, although this procedure has not yet beenmarketed.19,20 The engineering and transplantation of

oral mucosa and gingiva could be potentially impor-tant as a new technique in periodontal graft surgeryand in the treatment of gingival recession.

Dentin and Dental Pulp

The production of dentin and dental pulp havealso been achieved in animal and laboratory studiesusing tissue engineering strategies. The greatest po-tential for these engineered tissues is in the treat-ment of tooth decay. Dental caries remains one ofthe most prevalent young adult and childhood dis-eases, while the phrase “root canal” is probably themost dreaded term in dentistry. There are several waysin which one can potentially engineer lost dentin anddental pulp. There is now evidence suggesting thateven if the odontoblasts (cells that produce dentin)are lost due to caries, it may be possible to induceformation of new cells from pulp tissue using cer-tain BMPs.21,22,23 These new odontoblasts can syn-thesize new dentin. Tissue engineering of dental pulpitself may also be possible using cultured fibroblastsand synthetic polymer matrices.24 Further develop-ment and successful application of these strategiesto regenerate dentin and dental pulp could one dayrevolutionize the treatment of our most common oralhealth problem, cavities.

Figure 3. Potential of tissue engineering a mandible

Representation of a potential cell transplantation strategy to engineer half of a mandible required following radical resectivesurgery (e.g., patient with cancer). A polymer scaffold is engineered in the shape of half the mandible. Bone precursor cellsare seeded onto the polymer and allowed to grow in a bioreactor. Over time, the scaffold will undergo degradation whilefacilitating the ingrowth of bone in the shape of a mandible.


Salivary Glands

The most challenging goal of tissue engineer-ing is replacement of complete organs, and signifi-cant progress has been made in efforts to engineersalivary gland function. The loss of salivary glandtissue and/or function, whether it be a sequelae toradiation therapy to treat cancer or part of a diseasesuch Sjogren’s syndrome, is a problem that can sig-nificantly affect quality of life, particularly for medi-cally compromised individuals. One method in treat-ing salivary gland functional deficiencies makes useof an inductive gene therapy approach. The aim inthis approach is to make existing non-secretory duc-tal epithelial cells (following irradiation therapy) intosecretory cells capable of fluid movement. Successin animal models has been demonstrated.25,26 Anothermethod to restore salivary gland function employscell transplantation. Baum et al. have recently initi-ated the development of an artificial salivary glandsubstitute composed of polymer tube lined by epi-thelial cells.27 This relatively simple device couldengraft into the buccal mucosa of patients whosesalivary gland tissue has lost function, or been de-stroyed, and would have the physiological capacityto deliver an aqueous fluid to the mouth via the buc-cal mucosa. These new approaches could be veryeffective for treating conditions associated with lostsalivary gland function, including dysphagia, dys-geusia, rampant caries, and mucosal infections.

Future Directions/Considerations

The promise of tissue engineering in dentistryis great, but there exist major challenges that mustbe met in the next fifteen to twenty years for thisnew field to reach its potential application. Some ofthe main challenges lie not on the scientific side, butin the application of the technology. Once we fullyunderstand how we can re-create functional, viablenew tissues in the laboratory, how will we then beable to translate this knowledge to the patient popu-lation at large? A major issue will be the cost of thesetherapies. Will industry be able to produce tissueproducts in a cost-efficient manner so the patient canafford this type of treatment? Secondly, in order forthe new technology to reach the general masses, therewill need to be health care centers and institutes ca-

pable of applying these engineered products. Indi-viduals sufficiently trained to utilize these therapieswill clearly be required, necessitating new trainingprograms for these scientists, clinicians, and supportteams. Another major challenge lies in the ethicalconcerns regarding engineering tissues. Relevantethical issues include the source of cells (patient’sown vs. donated cells) and type (adult-donor vs. fe-tal cells). In addition, on what basis will it be de-cided who receives these new tissue therapies (ac-cording to need, ability to pay, etc.)? It is also unclearhow third-party groups will react to the new tech-nology and what they will cover. Needless to say,many different perspectives on these questions ex-ist, based on individual, cultural, and scientific prin-ciples.

This is undoubtedly an exciting time in den-tistry and the biomedical community at large. Intwenty to twenty-five years, dentistry as we know ittoday will be remarkably different, as it is now dif-ferent from the way it was twenty-five years ago.Many dental schools and postgraduate programs arecurrently evaluating curriculum content in light ofthe public’s oral health care needs and in light of themany advances in genetics, cell and molecular biol-ogy, and the materials sciences. At the predoctorallevel, tissue engineering provides an ideal opportu-nity to incorporate a multidisciplinary learningexperience into the curriculum which integrates con-cepts in cell biology, molecular biology, bioengineer-ing, and biomaterials with clinical techniques in oralsurgery, periodontics, restorative dentistry, and oralmedicine. Students can see first-hand the interplaybetween the science underlying tissue engineeringand the clinical application to oral disease. Such anexperience would also allow students to see collabo-ration among biomedical scientists, dentists, and phy-sicians, which is extremely rare in most dental schoolprograms. At the postgraduate level, there is need toprovide the community with a cadre of D.D.S./Ph.D.-trained practitioners, researchers, and educators withexpertise in tissue engineering. For the practitioner,continuing education programs can increase aware-ness of tissue engineering as a therapeutic option forvarious oral health problems. These programs canalso help establish linkages between dentists in thecommunity and tissue engineering specialists at aca-demic health centers.

Once the general public are aware of newer andbetter treatments, they will not accept anything less.The well-informed clinician capable of incorporat-


ing this technology into his or her practice will con-tinue to thrive in the future.

AcknowledgmentsThe authors are grateful to the NIDCR for fund-

ing of an individual predoctoral dental scientist fel-lowship (F30 DE05747) to Darnell Kaigler, and forresearch funding to the laboratory of David Mooney(RO1-DE13033; R01-DE13004; R01-DE13349).The authors would like to thank Martin C. Peters inthe Department of Biologic and Materials Sciences,University of Michigan for the creation of Figure 2.The authors also appreciate the graphic support ofChris Jung at the University of Michigan School ofDentistry.

REFERENCES1. Langer R, Vacanti JP. Tissue engineering. Science

1993;260(5110):920-6.2. Damien JC, Parson JR. Bone graft and bone graft substi-

tutes: a review of current technology and applications. JAppl Biomaterials 1991;2:187-208.

3. Buck BE, Malinin TI. Bone transplantation and humanimmunodeficiency syndrome (AIDS). Clin Orthop1989;240:129-36.

4. Branemark P-I. Precision-predictability 1990. Bookletpublished by the Institute for Applied Biotechnology,Gothenburg, Sweden.

5. Lanza RP, Langer R, Chick WL, eds. Principles of tissueengineering. San Diego: Academic Press, Inc., 2000:3-5.

6. Nyman S, Lindhe J, Karring T et al. New attachment fol-lowing surgical treatment of human periodontal disease.J Clin Periodontol 1982;9:290-6.

7. Branemark P, Breinen U, Adell R, et al. Intraosseous an-chorage of dental prostheses: experimental studies. ScandJ Plast Reconstr Surg 1969;3:81-91.

8. Albrektsson T, Hanson HA, Ivarsson B. Interface analy-sis of titanium and zirconium bone implants. Biomaterials1985;6:97-101.

9. Urist MR. Bone: formation by autoinduction. Science1965;150(698):893-9.

10. Krebsbach PH, Kuznetsov SA, Bianco P, Gerhon RobeyP. Bone marrow stromal cells: characterization andclinical application. Crit Rev in Oral Biol Med1999;10(2):165-81.

11. Sheridan ME, Shea LD, Peters MC, Mooney DJ.Bioabsorbable polymer scaffolds for tissue engineering

capable of sustained growth factor delivery. J Control Rel2000;64:91-102.

12. Shea LD, Smiley E, Bonadio J, Mooney DJ. ControllableDNA delivery from polymer matrices. Nat Biotechnol1999;17:551-4.

13. Alsberg E, Hill E, Mooney DJ. Craniofacial tissue engi-neering. Crit Rev in Oral Biol Med 2001;12(1):64-75.

14. Brittberg M, Lindahl A, Nilsson A, Ohlsson C, IsakssonO, Peterson L. Treatment of deep cartilage defects in theknee with autologous chondrocyte transplantation. N EnglJ Med 1994;331(14):889-95.

15. Puelacher WC, Mooney DJ, Langer R, Upton J, VacantiJP, Vacanti CA. Design of nasoseptal cartilage replace-ments synthesized from biodegradable polymers andchondrocytes. Biomaterial 1993;15:774-8.

16. Puelacher WC, Wisser J, Vacanti CA, Ferraro NF, JaramilloD, Vacanti JP. Temporomandibular joint disc replacementmade by tissue-engineered growth of cartilage. J OralMaxillofac Surg 1994;52(11):1172-7.

17. Parenteau N. The organogenesis story. Sci Am1999;280:83-4.

18. Naughton G. The advanced tissue sciences story. Sci Am1999;280:84-5.

19. Mizuno H, Emi N, Abe A, et al. Successful culture andsustainability in vivo of gene-modified human oral mu-cosa epithelium. Hum Gene Ther 1999;10:825-30.

20. Garlick JA, Fenjves ES. Keratinocyte gene transfer andgene therapy. Crit Rev Oral Biol Med 1996;7:204-21.

21. Nakashshima M. The induction of reparative dentin inthe amputated dental pulp of the dog by bone morphoge-netic protein. Arch Oral Biol 1990;35(7):493-7.

22. Lianjia Y, Yuhao G, White F. Bovine bone morphogeneticprotein induced dentinogenesis. Clin Orthop Relat Res1993;295:305-12.

23. Rutherford RB, Wahle J, Tucker M, Rueger D, CharetteM. Induction or reparative formation in monkeys by re-combinant human osteogenic protein-1. Arch Oral Biol1993;38:571-6.

24. Mooney DJ, Powell C, Piana J, Rutherford RB. Engineer-ing dental pulp-like tissue in vitro. Biotech Progress1996;12(6):865-8.

25. Baum BJ, O’Connell BC. In vivo gene transfer to sali-vary glands. Crit Rev Oral Biol Med 1999;10:276-83.

26. Delporte C, O’Connell BC, He X, et al. Increased fluidsecretion following adenovirus mediated transfer of theaquaporin-1 cDNA irradiated rat salivary glands. Proc NatlAcad Sci USA 1997;94:3268-73.

27. Baum BJ, Wang S, Cukierman E, et al. Re-engineeringthe functions of a terminally differentiated epithelial cellin vivo. Ann N Y Acad Sci 1999;875:294-300.

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