Patent Publication Number: US-8979534-B2

Title: Tooth scaffolds

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present application claims the benefit of PCT International Application No. PCT/US10/39035, filed 17 Jun. 2010; which claims the benefit of U.S. Provisional Application Ser. No. 61/187,875, filed 17 Jun. 2009, and U.S. Provisional Application Ser. No. 61/354,164, filed 11 Jun. 2010; all of which are incorporated herein by reference in their entireties. 
    
    
     BACKGROUND 
     The present invention relates to tissue engineering scaffolds. 
     Biomedical Engineering and Tooth Regeneration. 
     Tooth loss often results from a variety of oral diseases and physiological causes, including dental caries, periodontal disease, trauma, genetic disorders and aging (Amar 2003, Philstrom 2005, Kim 2006). Tooth loss can lead to physical and mental suffering that can lower an individual&#39;s self-esteem and quality of life (Amar 2003, Philstrom 2005, Kim 2006). Many forms of dental disease and some medical conditions like uncontrolled diabetes increase the risk of tooth loss. For the treatment of edentulism, the current options have been limited to the use of dental implants and/or conventional fixed or removable prostheses. 
     Recently, the emergence and development of biomedical engineering tools have led to a new scope of patient care in the field of medicine. For example, preliminary human clinical trials have reported of improved levels of bone formation in children with osteogenesis imperfecta, following systemic infusions of bone marrow stromal stem cells (BMSSC) or bone marrow cells (Horwitz 2001, Horwitz 2002). Recent advances in the fields of dental tissue engineering, materials science and stem cell biology suggest that tooth regeneration will be possible (Duailibi 2006). Additionally, the recent identification of different mesenchymal stem cells (MSCs) residing in dental or craniofacial tissues expands the scope of potential clinical benefits in helping to regenerate the dental tissues such as dentin, cementum and periodontal ligament (PDL) (Shi 2005). Dental tissue progenitor cells present in the pulp tissue of deciduous and adult teeth can be used to regenerate dentin and alveolar bone (Shi 2005; Zhang 2005). Additionally, cells isolated from both rat and pig tooth buds can be used to bioengineer anatomically correct tooth crowns but with limited predictability (Duailibi 2004, Honda 2005, Young 2002, Young 2005). 
     The tooth/periodontal complexes are often referred to as an individual organ. Although this organ is considered relatively small, its structural and developmental complexity is well recognized. The tooth structure consists of three calcified tissue types—enamel, dentin and cementum, and dental pulp. Dentin occupies the bulk of the tooth, while enamel and cementum cover the coronal and apical portions, respectively. The periodontium has a supportive role to the teeth and consists of cementum, periodontal ligaments, alveolar bone and gingiva. Periodontal ligaments are connective tissues that attach the cementum to the alveolar bone via the Sharpey&#39;s fibers. Periodontal ligaments enable sensory perception and cushion mechanical forces during mastication. 
     Despite the tooth&#39;s structural complexity, the advancement of biomedical engineering techniques has given rise to two currently employed approaches for tooth regeneration. The first is based on tissue engineering, aiming to regenerate teeth by seeding stem cells in scaffolding biomaterials (Young 2002, Duailibi 2004, Honda 2005). This technique has shown promising results in regeneration of the periodontium (Nakahara 2006). The second approach attempts to reproduce or mimic the developmental processes of embryonic tooth formation (Nakahara 2006). This approach uses embryonic tissues (dental epithelium and dental mesenchyme) harvested from a mouse fetus and requires an understanding of the principles that regulate early tooth development in the embryo (Ohazama 2004, Hu 2006, Nakao 2007). Following these approaches, in many studies, biologically engineered tooth germs are transplanted into the bodies of animal hosts, usually rodents, where there is sufficient blood flow to provide the necessary nutrients and oxygen to optimize tissue formation (Nakahara 2006). 
     Use of Stem Cells in Tissue Regeneration and Challenges Encountered. 
     Stem cells are quiescent cell populations present in normal tissue, which exhibit the distinct characteristic of asymmetric cell division, the formation two daughter cells—a new progenitor/stem cell, and another daughter cell capable of forming differentiated tissue (Hawkins 1998, Lin 1998). Dental mesenchymal progenitor cells have been identified and characterized in the dental pulp of both deciduous and adult human teeth (Gronthos 2000, Mooney 1996, Shi 2005). As previously mentioned, these postnatal epithelial and mesenchymal dental stem/progenitor cells present in immature tooth buds have demonstrated the ability to generate bioengineered and anatomically correct, but miniature-sized tooth crowns containing enamel, dentin, pulp, and alveolar bone (Shi 2005; Zhang 2005). 
     Periodontal ligament cells are known for their regenerative potential to give rise to the formation of lamina propria, cementum, bone, and periodontal ligament (Melcher 1985, McCulloh 1985). The capacity of periodontal ligament stem cells to form mineralized deposits in vitro has been demonstrated for a subpopulation of cells derived from primary explants of periodontal ligament (Arceo 1991, Cho 1992). It is believed that periodontal ligament stem cells require a suitable scaffold to induce the formation of bone, dentin and cementum in vivo (Gronthos 2000, Krebsbach 1998). When periodontal ligament stem cells were incorporated into a hydroxyapatite/tricalcium phosphate scaffolds and ectopically implanted in the subcutaneous regions of the mouse dorsum, a typical cementum/periodontal ligament-like structure formed (Seo 2004). Moreover, a type I collagen-positive periodontal ligament-like tissue within the transplants connecting with the newly formed cementum that is morphologically similar to Sharpey&#39;s fibers has been demonstrated (Seo 2004). 
     Recent advances in dental stem cell biotechnology and cell-mediated murine tooth regeneration have encouraged researchers to explore the potential for regenerating living teeth with appropriate functional properties (Duailibi 2004, Ohazama 2004, Shi 2005). Murine teeth can be regenerated using many different stem cells to collaboratively form dental structures in vivo (Duailibi 2004, Ohazama 2004, Young 2005). In addition, dentin/pulp tissue and cementum/periodontal complex have been regenerated by human dental pulp stem cells (DPSCs) and periodontal ligament stem cells (PDLSCs) respectively, when transplanted into immune-compromised mice (Gronthos 2000, Seo 2004). However, owing to the complexity of human tooth growth and development, the regeneration of a whole tooth structure including enamel, dentin/pulp complex, and periodontal tissues as a functional entity in humans is a challenge with the currently available regenerative biotechnologies (Sonomaya 2006). 
     The challenges with the use of stem cells in regeneration of dental tissues have been reported in previous studies (Duailibi 2004, Young 2002, Young 2005). It is acknowledged that, while formation of multiple miniature tooth crowns in the bioengineered tooth constructs is possible, real-size whole-tooth regeneration encounters a number of challenges. These challenges are attributed, again, to the complex nature of tooth development (Duailibi 2006, Tummers 2003). 
     Concepts of Cell Homing. 
     As put forward, conventional approaches of stem cell-seeding within a scaffold aim to mimic cellular structure and recreate a functional tissue equivalent in vitro or in vivo. The cells are derived from end organs or from more undifferentiated sources such as the bone marrow (Schantz 2007). These approaches are limited by issues such as donor site morbidity from harvesting of cells and tissue formation of heterogeneous quality at the site of implantation of the cell-scaffold construct (Schantz 2007). Hence, the concept of cell homing is recently attracting more attention. Cell homing aims to induce the homing of desired cells to cytokine-impregnated scaffolds at specific anatomical sites (Schantz 2007). This approach attempts in vivo tissue regeneration without cell-seeding. Therefore, cell homing could provide enhancements in cellular methodology for tissue engineering and a novel, minimally invasive option for tissue regeneration (Schantz 2007). 
     Based on the above discussion, further development of tooth scaffolds is needed. The present invention address that need. 
     SUMMARY 
     In some embodiments, an acellular mammalian tooth-shaped scaffold is provided. The scaffold comprises a compound that is chemotactic, osteogenic, dentinogenic, amelogenic, or cementogenic. 
     In other embodiments, a method of replacing a tooth in the mouth of a mammal is provided. In these embodiments, the tooth is absent and a tooth socket is present in the mouth at the position of the absent tooth. The method comprises implanting an acellular scaffold having the shape of the missing tooth into the tooth socket. 
     Additionally, methods of making a tooth scaffold is provided. The methods comprise synthesizing an acellular scaffold in the shape of a mammalian tooth and adding at least one compound that is chemotactic, osteogenic, dentinogenic, amelogenic, or cementogenic. 
     Other objects and features will be in part apparent and in part pointed out hereinafter. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Those of skill in the art will understand that the drawings, described below, are for illustrative purposes only. The drawings are not intended to limit the scope of the present teachings in any way. 
         FIG. 1  is a flow chart showing the design of the study described in the Example. 
         FIG. 2  is photographs showing scaffold fabrication using a 3D printing system (Bioplotter™). Panel A shows the Bioplotter™ used to create the scaffolds; Panel B shows a fabricated rat mandibular central incisor root (left) and human molar-shaped PCL-HA scaffold (right); Panel C shows the ethylene oxide sterilizer used to sterilize the scaffolds; Panel D shows scaffolds being treated with growth factors (SDF1 and BMP-7); Panel E shows the scaffold incubated for collagen cross-linking in the scaffolds prior to implantation; Panel F shows scaffolds being loaded by the growth factors and collagen gel; Panel G show scaffolds implanted in rats at the extraction socket and dorsum sites. 
         FIG. 3  is photographs of fabricated human mandibular molar-shaped scaffold for implantation in a rat dorsum. The crown and root were fabricated separately and fused later. The microchannels are evident. 
         FIG. 4  is photographs of the extraction of a mandibular central incisor followed by implantation of a root-form scaffold in the socket. Panel A shows the retraction of the lower lip; Panel B shows incision and reflection of the gingival flap; Panel C shows atraumatic extraction of the left mandibular central incisor showing preserved bony walls of the socket; Panel D shows the extracted incisor and root-form scaffold in comparison; Panel E shows the scaffold implanted in the extraction socket; Panel F shows the gingival flap sutured and primarily closed. 
         FIG. 5  is photographs of the subcutaneous implantation of a human mandibular molar-shaped scaffold in a rat dorsum site. Panel A shows a 2-cm incision being made; Panel B shows the creation of a subcutaneous pouch; Panel C shows the implantation of a scaffold into the pouch; Panel D shows the primary closure. 
         FIG. 6  is photographs showing en bloc harvest of the mandibular central incisor scaffold. Panel A shows complete wound healing which was evident before the harvest; Panel B shows an incision made to access the scaffold; Panel C shows the en bloc harvest of the scaffold (right) and the adjacent incisor (left). 
         FIG. 7  is photographs showing the procedure for implanted scaffold harvest at the dorsum site. Panel A shows complete wound healing prior to the harvest; Panel B shows the incision made to access the scaffold; Panel C shows fascial encapsulation of the scaffold; Panel D shows retrieved scaffolds. 
         FIG. 8  is micrographs of stained scaffold sections showing the regions selected for histological analyses. Panel A shows three regions selected in the mandibular central incisor scaffold; Panel B shows four regions selected in the human mandibular molar scaffold. 
         FIG. 9  is micrographs of stained scaffold sections showing the tissue-scaffold interface of an incisor root-form scaffold within the extraction socket (test and control groups). Panel A shows a von Kossa (VK)-stained slide depicting bony ingrowth at the interface; Panel B shows a hematoxylin and eosin (H&amp;E)-stained section showing close adaptation and integration of the scaffold to the socket wall; Panel C shows a higher magnification view demonstrating evident angiogenesis and soft tissue ingrowth between the PCL-HA strands. 
         FIG. 10  is micrographs of stained scaffold sections showing the tissue-scaffold interface of a human mandibular molar-shaped scaffold from a dorsum site (test and control groups). Panel A shows angiogenesis and ingrowth of tissues around and between the strands; Panel B shows a higher magnification view showing integration between the scaffold and the encapsulating tissues. 
         FIG. 11  is micrographs of stained scaffold sections showing representative views of scaffolds from extraction sockets showing differences in cellular density of a test scaffold (Panel A) vs. a control scaffold without growth factors (Panel B). 
         FIG. 12  is micrographs of stained scaffold sections showing representative views of scaffolds from dorsum sites showing differences in cellular density of a test scaffold (Panel A) vs. a control scaffold without growth factors (Panel B). 
         FIG. 13  is a graph showing differences in cellular density between experimental groups and implantation sites. GF+: test; GF−: control. “*”: p&lt;0.05. 
         FIG. 14  is micrographs of stained scaffold sections showing representative views of scaffolds from extraction sockets showing differences in vessel density of a test scaffold (Panel A) vs. a control scaffold without growth factors (Panel B). 
         FIG. 15  is micrographs of stained scaffold sections showing representative views of scaffolds from dorsum sites showing differences in cellular density of a test scaffold (Panel A) vs. a control scaffold without growth factors (Panel B). 
         FIG. 16  is a graph showing differences in vessel density between experimental groups and implantation sites. GF+: test; GF−: control. “*”: p&lt;0.05. 
         FIG. 17  is micrographs of stained scaffold sections showing representative views showing differences in vessel diameter of scaffolds from an extraction socket (Panel A) vs. a dorsum site (Panel B). 
         FIG. 18  is a graph showing differences in vessel diameter between experimental groups and implantation sites. GF+: test; GF−: control. “*”: p&lt;0.05. 
         FIG. 19  is micrographs of VK-stained scaffold sections showing representative views of test group scaffolds from the extraction sockets showing mineralization. 
         FIG. 20  is micrographs of VK-stained scaffold sections showing representative views of test group scaffolds from dorsum sites showing mineralization. 
     
    
    
     DETAILED DESCRIPTION 
     The present invention is based in part on the surprising discovery that a tooth-shaped scaffold, when implanted into a tooth socket, will attract cells that colonize the scaffold to provide a living tooth, even without exogenously providing cells with the scaffold. 
     Thus, in some embodiments, an acellular mammalian tooth-shaped scaffold is provided. The scaffold comprises a compound that is chemotactic, osteogenic, dentinogenic, amelogenic, or cementogenic. These scaffolds are thus implanted without exogenously applied cells. As established in the Example, colonization of the implanted scaffolds proceeds adequately by native cells that migrate into the scaffold. The colonization is further encouraged by the chemotactic, osteogenic, dentinogenic, amelogenic, or cementogenic compound incorporated into the scaffold. 
     The compound can be of any structure including but not limited to a protein, oligopeptide, small organic molecule (i.e., less than about 2000 mw, or about 1000 mw or about 500 mw), metal ion-containing molecule, carbohydrate, or lipid. As used herein, a chemotactic compound is a compound that attracts cells. An osteogenic compound is a compound that encourages new bone synthesis. A dentinogenic compound is a compound that encourages new dentin synthesis. An amelogenic compound is a compound that encourages tooth enamel synthesis. A cementogenic compound is a compound that encourages cementum synthesis. 
     As used herein, a “scaffold” is a structure that provides a matrix for the growth of cells and/or the formation of tissue. Useful properties of a scaffold are porosity, biocompatibility and biodegradability, the ability to support cell growth, and its use as a controlled gene- and protein-delivery vehicle (Murphy 1999). The three-dimensional macromolecular structure provided by the scaffold is to guide the final shape of bioengineered tissues (Murphy 1999). 
     The scaffolds of these embodiments can have the shape of any mammalian tooth. In some of these embodiments, the scaffold has the shape of a human incisor, a human cuspid, a human bicuspid or a human molar. 
     The compound in these embodiments can be any compound that is chemotactic, osteogenic, dentinogenic, amelogenic, or cementogenic. Nonlimiting examples include platelet-derived growth factor (PDGF), endothelial cell growth factor (ECGF), transforming growth factor-β1 (TGF-β1), epidermal growth factor (EGF), hepatocyte growth factor (HGF), stromal cell-derived factor-1 (SDF1), a bone morphogenetic protein (BMP), a TGF-β, a growth and differentiation factor (GDF), insulin-like growth factor-1 (IGF1), vascular endothelial growth factor (VEGF), fibroblast growth factor (FGF), a dentin matrix protein, a dentin sialoprotein, a bone sialoprotein, amelogenin, or an integrin. 
     In some embodiments, the compound is SDF1, which has chemotactic properties. SDF1 is a chemokine that is believed to be essential in stem and progenitor cell recruitment for tissue repair after injury (Kollet 2003). SDF1 can also induce migration of hematopoietic progenitor cells within a chemotaxis chamber (Kim 1998). Additionally, SDF1 is important for the migration of marrow stromal cells to bone marrow, as shown by the dose-dependent migration of mesenchymal stem cells (MSCs) in response to stimulation with SDF1 (Win 2004). SDF1 also has anti-apoptotic properties, protecting hematopoietic stem cells directly from the apoptotic effects of γ-irradiation in the presence of other growth factors (Herodin 2003). Further, mesenchymal stem cells (MSCs) derived from bone marrow can be directed to migrate toward SDF1 (Schantz 2007). 
     In other embodiments, the compound is a BMP. BMP2, 6, and 9 are apparently the most potent agents for osteogenic differentiation of MSCs, while the rest of the BMPs are more effective in promoting the terminal differentiation of committed osteoblastic precursors and osteoblasts (Cheng 2003). 
     In some aspects of these embodiments, the BMP is BMP-7. BMP-7 plays a key role in the transformation of mesenchymal cells into bone and cartilage. BMP-7 treatment is sufficient to induce all of the genetic markers of osteoblast differentiation in various cell types (Chen 2004). It is noted that BMP-7 has received Food and Drug Administration (FDA) approval for human clinical uses. 
     Many studies have investigated the role and action of exogenous growth factors in a carrier to deliver the growth factor to an implantation site. Although the carrier may not contribute any additional factors necessary for tissue formation, it can still be an important component of the growth process (Wozney 1990). One of the carrier functions is to maintain the factor at the site of implantation and thus enhance its local concentration. The carrier also serves as an environment in which tissue can form and therefore helps to define the region in which new tissue can be formed (Whang 1998). Collagenous or synthetic carriers have been used as delivery vehicles, and their physicochemical properties, together with the microenvironment they create, play a role in the inductive outcome. Carriers can be solid xenogenic (e.g., hydroxyapatite) (Kuboki 1995, Murata 1998), solid alloplastic (polyethylene polymers) materials (Saito 1998, Isobe 1999), or gels of autogenous (Sweeney 1995, Schwartz 1998), allogenic (Bax 1999, Viljanen 1997), or alloplastic origin (Santos 1998), and combinations of the above (Alpaslan 1996). 
     One of the carrier functions is to maintain the factor at the site of implantation and thus enhance its local concentration. A collagen matrix retains up to 65% of BMPs during initial impregnation and releases it in two phases: an initial phase within hours of implantation and a second phase that depends on the nature of the carrier and its geometrical characteristics (Uludag 1999). It is believed that BMPs do not bind to the carrier (Uludag 1999), but rather become physically entrapped in its structure which makes certain designs more favorable for osteoinduction over others (Tsuruga 1997). In the case of collagen sponge carriers, mass, collagen cross-linking and sterilization methods affect BMP precipitation and subsequent resistance of sponge degradation by collagenase (Friess 1999). A collagen carrier can also result in increased bone density of the regenerate compared to the polymeric matrix (Cochran 1997). 
     BMP-7 plays a key role in the transformation of mesenchymal cells into bone and cartilage. BMP-7 treatment is sufficient to induce all of the genetic markers of osteoblast differentiation in various cell types (Chen 2004). It is also worthwhile to note that BMP-7 has received the Food and Drug Administration (FDA) approval for human clinical uses. 
     In some of these scaffolds, a chemotactic growth factor as well as a growth factor that is osteogenic, dentinogenic, amelogenic, or cementogenic is present. In particular embodiments, the chemotactic growth factor is SDF1 and the osteogenic, dentinogenic, amelogenic, or cementogenic growth factor is BMP-7. 
     The scaffolds of these embodiments can further comprise any other bioactive molecule, for example an antibiotic or an additional chemotactic growth factor or another osteogenic, dentinogenic, amelogenic, or cementogenic growth factor. In some embodiments, the scaffold is strengthened, through the addition of, e.g., human serum albumin (HSA), hydroxyethyl starch, dextran, or combinations thereof. Suitable concentrations of these compounds for use in the compositions of the application are known to those of skill in the art, or can be readily ascertained without undue experimentation. 
     The concentration of compound in the scaffold will vary with the nature of the compound, its physiological role, and desired therapeutic or diagnostic effect. A therapeutically effective amount is generally a sufficient concentration of therapeutic agent to display the desired effect without undue toxicity. In some embodiments, The scaffold comprises BMP-7 in the scaffold at about 10 ng/g to 1000 μg/g scaffold and SDF1 in the scaffold at about 10 ng/g to 1000 μg/g scaffold. In more specific embodiments, the BMP-7 is in the scaffold at about 100 μg/g scaffold and the SDF1 is in the scaffold at about 100 μg/g scaffold. 
     The compound can be incorporated into the scaffold by any known method. In some embodiments, the compound is imbedded in a gel, e.g., a collagen gel incorporated into the pores of the scaffold, as described in the Example. 
     Alternatively, chemical modification methods may be used to covalently link the compound on the surface of the scaffold. The surface functional groups of the scaffold can be coupled with reactive functional groups of the compound to form covalent bonds using coupling agents well known in the art such as aldehyde compounds, carbodiimides, and the like. Additionally, a spacer molecule can be used to gap the surface reactive groups and the reactive groups of the biomolecules to allow more flexibility of such molecules on the surface of the matrix. Other similar methods of attaching biomolecules to the interior or exterior of a matrix will be known to one of skill in the art. 
     The compound can alternatively be introduced into or onto the matrix via a carrier based system, such as an encapsulation vehicle. Such vehicles are useful as slow release compositions. For example, growth factors can be micro-encapsulated to provide for enhanced stability and/or prolonged delivery. Encapsulation vehicles include, but are not limited to, microparticles, liposomes, microspheres, or the like, or a combination of any of the above to provide the desired release profile in varying proportions. Other methods of controlled-release delivery of agents will be known to the skilled artisan. Moreover, these and other systems can be combined and/or modified to optimize the integration/release of agents within the matrix. 
     Polymeric microspheres can be produced using naturally occurring or synthetic polymers and are particulate systems in the size range of 0.1 to 500 μm. Polymeric micelles and polymeromes are polymeric delivery vehicles with similar characteristics to microspheres and can also facilitate encapsulation and matrix integration of the compounds described herein. Fabrication, encapsulation, and stabilization of microspheres for a variety of payloads are within the skill of the art (see e.g., Varde &amp; Pack (2004) Expert Opin. Biol. 4(1) 35-51). The release rate of the microspheres can be tailored by type of polymer, polymer molecular weight, copolymer composition, excipients added to the microsphere formulation, and microsphere size. Polymer materials useful for forming microspheres include PLA, PLGA, PLGA coated with DPPC, DPPC, DSPC, EVAc, gelatin, albumin, chitosan, dextran, DL-PLG, SDLMs, PEG (e.g., ProMaxx), sodium hyaluronate, diketopiperazine derivatives (e.g., Technosphere), calcium phosphate-PEG particles, and/or oligosaccharide derivative DPPG (e.g., Solidose). Encapsulation can be accomplished, for example, using a water/oil single emulsion method, a water-oil-water double emulsion method, or lyophilization. Several commercial encapsulation technologies are available (e.g., ProLease®, Alkerme). 
     Liposomes can also be used to integrate compounds with the scaffolds. The agent carrying capacity and release rate of liposomes can depend on the lipid composition, size, charge, drug/lipid ratio, and method of delivery. Conventional liposomes are composed of neutral or anionic lipids (natural or synthetic). Commonly used lipids are lecithins such as phosphatidylcholines, phosphatidylethanolamines, sphingomyelins, phosphatidylserines, phosphatidylglycerols, and phosphatidylinositols. Liposome encapsulation methods are commonly known in the arts (Galovic et al. (2002) Eur. J. Pharm. Sci. 15, 441-448; Wagner et al. (2002) J. Liposome Res. 12, 259-270). Targeted liposomes and reactive liposomes can also be used in combination with the agents and matrix. Targeted liposomes have targeting ligands, such as monoclonal antibodies or lectins, attached to their surface, allowing interaction with specific receptors and/or cell types. Reactive or polymorphic liposomes include a wide range of liposomes, the common property of which is their tendency to change their phase and structure upon a particular interaction (e.g., pH-sensitive liposomes). See, e.g., Lasic (1997) Liposomes in Gene Delivery, CRC Press, FL). 
     The scaffolds of these embodiments can be fabricated with any material recognized as useful by the skilled artisan. Suitable matrix materials are discussed in, for example, Ma and Elisseeff, ed. (2005) Scaffolding in Tissue Engineering, CRC, ISBN 1574445219; Saltzman (2004) Tissue Engineering: Engineering Principles for the Design of Replacement Organs and Tissues, Oxford ISBN 019514130X. Nonlimiting examples of potentially useful materials for all or part of the scaffold include poly(ethylene) glycol, poly(lactide), poly(glycolic acid), poly(lactide-co-glycolide), poly(caprolactone), polyanhydride, polyglactin, polycarbonates, polyamides, polyanhydrides, polyamino acids, polyortho esters, polyacetals, polycyanoacrylates), polyphosphazene, degradable polyurethanes, polyacrylates, ethylene-vinyl acetate polymers and other acyl substituted cellulose acetates and derivatives thereof, polyurethanes, polystyrenes, polyvinyl chloride, polyvinyl fluoride, polyvinyl pyrrolidone, poly(vinylimidazole), chlorosulphonated polyolifins, polyethylene oxide, polyvinyl alcohol, Teflon®, nylon, agarose, alginate (e.g., calcium alginate gel), fibrin, fibrinogen, fibronectin, collagen (e.g., a collagen gel), gelatin, hyaluronic acid, chitin, and other suitable polymers and biopolymers, or analogs, mixtures, combinations, and derivatives of the above. 
     In some embodiments, the scaffold is fabricated from a composition that comprises an osteoconductive material. A nonlimiting example of an osteoconductive material is hydroxyapatite (HA). HA has been used as a bone substitute for many years because of its excellent biocompatibility and high bioactivity (Liao 2006, Nebahat 2006, Lijun 2006, Wei 2003). 
     Although HA has good bioactivity and osteoconductivity, it is very brittle and has poor inherent tensile properties. Therefore, in some embodiments, the HA is combined with ε-polycaprolactone (PCL). PCL is a good bone scaffold material because it takes several years to degrade in vivo and is biocompatible, relatively inexpensive, and available in large quantities (Rich 2002, Kim 2004). The combination of PCL and HA (PCL-HA) provides a desirable combination of bioactivity, biodegradability, and strength (Patcharaporn 2005, Rezwan 2006, Landis 1995, Ziv 1994). The material of composite PCL-HA has been deemed to possesses the optimal scaffold properties of biocompatibility, cell-adhesion, proliferation, and differentiation (Zhao 2008). In some embodiments, the scaffold comprises a mixture of about 80 wt % polycaprolactone and about 20 wt % hydroxyapatite. In other embodiments, the scaffold comprises anywhere from about 60 wt % polycaprolactone and about 40 wt % hydroxyapatite to about 95 wt % polycaprolactone and about 5 wt % hydroxyapatite. For example, the scaffold can comprise about 70 wt % polycaprolactone and about 30 wt % hydroxyapatite. As another example, the scaffold can comprise about 90 wt % polycaprolactone and about 10 wt % hydroxyapatite. 
     In some embodiments, the scaffold has a high porosity. Such a porous structure provides space for cell migration, adhesion, and the ingrowth of new bone tissue (Gazdag 1995, Rezwan 2006, Mano 2004, Shin 2003, Kim 2001, Leong 2003). 
     Pores and channels of the scaffold can be engineered to be of various diameters. For example, the pores of the scaffold can have a diameter range from micrometers to millimeters. In some embodiments, the pores of the matrix material include microchannels. Microchannels generally have an average diameter of about 0.1 μm to about 1,000 μm, e.g., about 50 μm to about 500 μm (for example about 100 μm, 150 μm, about 200 μm, about 250 μm, about 300 μm, about 350 μm, about 400 μm, about 450 μm, about 500 μm, or about 550 μm). One skilled in the art will understand that the distribution of microchannel diameters can have any distribution including a normal distribution or a non-normal distribution. In some embodiments, microchannels are a naturally occurring feature of the matrix material(s). In other embodiments, microchannels are engineered to occur in the matrix materials. 
     In some embodiments, the compound is imbedded in a gel in the microchannels. Any gel can be used for this purpose. In some embodiments, the gel is a collagen gel. 
     In various embodiments, the scaffold further comprises a nonporous cap. Such a cap provides further strength to the scaffold and prevents infection. The nonporous cap can be simply the same material of the rest of the scaffold except without pores. Alternatively, the nonporous cap can be a different material, e.g., typical dental cap material, such as porcelain or gold. 
     Methods of replacing a tooth in the mouth of a mammal is also provided herein. In these embodiments, the tooth is absent and a tooth socket is present in the mouth at the position of the absent tooth. The methods comprise implanting an acellular scaffold having the shape of the missing tooth into the tooth socket. 
     Several methods are used for fabrication of porous scaffolds, including particulate leaching, gas foaming, electrospinning, freeze drying, foaming of ceramic from slurry, and the formation of polymeric sponge (Mikos 1994, Mooney 1996, Qing 2002, Sylvain 2006). However, scaffolds prepared by using these methods have some shortcomings in controlling the structure and interconnectivity of pores, which may limit their application in terms of cell penetration in tissue engineering (Yeong 2004, Tan 2003). 
     In some embodiments, the methods further comprise making a model of the absent tooth by computer aided design (CAD) and synthesizing the scaffold with a bioplotter. Such methods can provide scaffolds with high porosity and good interconnectivity. As described in the Example, three-dimensional (3D) scaffolds with controllable and reproducible porosity and well-defined 3D microstructures can be made. Rapid prototyping (RP) methods such as fused deposition modeling, selective laser sintering, 3D printing, multiphase jet solidification, and 3D plotting have been proposed (Hutmacher 2001, Moroni 2006). 
     A key feature of rapid prototyping is the solid freeform fabrication (SFF) process: 3D computer models are cut into sequences of layers which are used to construct complex objects layer-by-layer. The layers are produced via solidification of melts, layer photopolymerization or bonding of particles using either laser beam induced sintering (selective laser sintering) or special binders (Landers 2002). Recently, a specialized rapid prototyping system (Bioplotter™, EnvisionTec, Germany) has been introduced, enabling the design and fabrication of anatomically shaped scaffolds with varying internal architectures, thereby allowing precise control over pore size, porosity, permeability, and stiffness (Landers 2002; Landers 2005). The prototyping process using the Bioplotter™ for fabricating a tissue-specific PCL-HA scaffold requires 3D morphological information of the target tissue or tissue defect, which can be obtained by computer tomography (CT) or magnetic resonance imaging (MRI). When an absent tooth has a counterpart on the other side of the mouth, that counterpart can be used as a model to design the scaffold for the missing tooth. 
     Information obtained above is then used to design a functional scaffold by CAD and is transferred to the Bioplotter™ system. In that system, the Bioplotter™ machine melts and dispenses the scaffold material (e.g., PCL-HA) in layer-by-layer on a collecting plate. Pores, for example microchannels, can be created as part of the design. The fabricated 3D scaffolds through the RP system result in significant cell penetration, and thus possess the properties of ideal scaffolds (Heo 2007). These 3D scaffolds may have potential for clinical application by providing patient tissue-specific anatomical shape as well as an optimized internal microstructure for the nutrient transportation and vascularization. Further details of these methods are provided in PCT Publication WO2009006558, incorporated by reference. 
     A method of making a tooth scaffold is additionally provided. The method comprises synthesizing an acellular scaffold in the shape of a mammalian tooth and adding at least one compound that is chemotactic, osteogenic, dentinogenic, amelogenic, or cementogenic. In some embodiments of these methods, the tooth is shaped like a tooth that is absent in a mammal, and the method further comprises making a model of an absent tooth by computer aided design (CAD), and synthesizing the scaffold with a bioplotter. Where the absent tooth has a counterpart in the mouth, e.g., a molar, the method further comprises making a CT scan of the analogous molar, for example on the other side of the mouth, where the CAD utilizes CT scan data of the second molar to design the scaffold. 
     In some embodiments of these methods, the compound is platelet-derived growth factor (PDGF), endothelial cell growth factor (ECGF), transforming growth factor-β1 (TGF-β1), epidermal growth factor (EGF), hepatocyte growth factor (HGF), stromal cell-derived factor-1 (SDF1), a bone morphogenetic protein (BMP), a TGF-β, a growth and differentiation factor (GDF), insulin-like growth factor-1 (IGF1), vascular endothelial growth factor (VEGF), fibroblast growth factor (FGF), a dentin matrix protein, a dentin sialoprotein, a bone sialoprotein, amelogenin, or an integrin. 
     In other embodiments, the scaffold is fabricated from a composition that comprises an osteoconductive material. As discussed above, an example of a useful osteoconductive material is hydroxyapatite. A further example is a mixture of ε-polycaprolactone and hydroxyapatite as discussed above. In various embodiments of these methods, the scaffold comprises microchannels having a diameter of between 50 and 500 μm. In additional embodiments, the scaffold further comprises a nonporous cap. 
     In some embodiments, numbers expressing quantities of ingredients, properties such as molecular weight, reaction conditions, and so forth, used to describe and claim certain embodiments of the invention are to be understood as being modified in some instances by the term “about.” In some embodiments, the term “about” is used to indicate that a value includes the standard deviation of the mean for the device or method being employed to determine the value. In some embodiments, the numerical parameters set forth in the written description and attached claims are approximations that can vary depending upon the desired properties sought to be obtained by a particular embodiment. In some embodiments, the numerical parameters should be construed in light of the number of reported significant digits and by applying ordinary rounding techniques. Notwithstanding that the numerical ranges and parameters setting forth the broad scope of some embodiments of the invention are approximations, the numerical values set forth in the specific examples are reported as precisely as practicable. The numerical values presented in some embodiments of the invention may contain certain errors necessarily resulting from the standard deviation found in their respective testing measurements. The recitation of ranges of values herein is merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each individual value is incorporated into the specification as if it were individually recited herein. 
     In some embodiments, the terms “a” and “an” and “the” and similar references used in the context of describing a particular embodiment (especially in the context of certain of the following claims) can be construed to cover both the singular and the plural, unless specifically noted otherwise. In some embodiments, the term “or” as used herein, including the claims, is used to mean “and/or” unless explicitly indicated to refer to alternatives only or the alternatives are mutually exclusive. 
     The terms “comprise,” “have” and “include” are open-ended linking verbs. Any forms or tenses of one or more of these verbs, such as “comprises,” “comprising,” “has,” “having,” “includes” and “including,” are also open-ended. For example, any method that “comprises,” “has” or “includes” one or more steps is not limited to possessing only those one or more steps and can also cover other unlisted steps. Similarly, any composition or device that “comprises,” “has” or “includes” one or more features is not limited to possessing only those one or more features and can cover other unlisted features. 
     All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g. “such as”) provided with respect to certain embodiments herein is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention otherwise claimed. No language in the specification should be construed as indicating any non-claimed element essential to the practice of the invention. 
     Groupings of alternative elements or embodiments of the invention disclosed herein are not to be construed as limitations. Each group member can be referred to and claimed individually or in any combination with other members of the group or other elements found herein. One or more members of a group can be included in, or deleted from, a group for reasons of convenience or patentability. When any such inclusion or deletion occurs, the specification is herein deemed to contain the group as modified thus fulfilling the written description of all Markush groups used in the appended claims. 
     All publications, patents, patent applications, and other references cited in this application are incorporated herein by reference in their entirety for all purposes to the same extent as if each individual publication, patent, patent application or other reference was specifically and individually indicated to be incorporated by reference in its entirety for all purposes. Citation of a reference herein shall not be construed as an admission that such is prior art to the present invention. 
     Having described the invention in detail, it will be apparent that modifications, variations, and equivalent embodiments are possible without departing the scope of the invention defined in the appended claims. Furthermore, it should be appreciated that all examples in the present disclosure are provided as non-limiting examples. 
     EXAMPLES 
     The following non-limiting examples are provided to further illustrate the present invention. It should be appreciated by those of skill in the art that the techniques disclosed in the examples that follow represent approaches the inventors have found function well in the practice of the invention, and thus can be considered to constitute examples of modes for its practice. However, those of skill in the art should, in light of the present disclosure, appreciate that many changes can be made in the specific embodiments that are disclosed and still obtain a like or similar result without departing from the spirit and scope of the invention. 
     Example 1 
     Regeneration of Anatomically Correct Tooth by Cell Homing 
     One of the fundamental missions of dentistry is the restoration of diseased, missing and lost dental structures. Currently, conventional treatment of tooth loss includes prosthodontic management with or without surgically placed dental implants. Dental implants, despite their reported high success rates, are not without complications such as loosening, infection, and bone loss. Recently, there has been a common aspiration among practitioners and scientists that separate tooth structures or an entire tooth can be regenerated using biomedical engineering cues. This emerging area, however, has encountered several obstacles, not the least of which is the complexity of regeneration and the morphology of tooth structures. The present study proposes to establish the regeneration of an entire tooth organ with bioengineered tooth scaffolds and by delivery of growth factors known to be important in tooth development. Full size human tooth scaffolds were first fabricated by rapid prototyping with layer deposition of hybrid of ε-polycaprolactone and hydroxyapatite (PCL-HA) using the Bioplotter™ machine. In parallel, scaffolds in the shape of mandibular central incisor root of Sprague Dawley rats were also constructed in 3D. The scaffolds were then infused with stromal cell derived factor-1 (SDF1) and bone morphogenetic protein 7 (BMP-7). There were 22 Sprague Dawley rats—11 each in the test and control groups. In each of the rats, a human mandibular molar shaped scaffold was subcutaneously implanted in the dorsum site, and a rat mandibular incisor root shaped scaffold was implanted into the extraction socket after surgical extraction of one of the two mandibular central incisors. The test group scaffolds were impregnated with SDF1 and BMP-7 while the control group scaffolds contained no growth factors. All implanted scaffolds were harvested at 9-week post-implantation and histologically evaluated for tissue ingrowth, cellular penetration, angiogenesis and mineralization. 
     Materials and Methods. 
     Approval from Columbia University Institutional Animal Care and Use Committee (IACUC) was obtained prior to the commencement of this study. This study used the mandibular central incisor extraction sockets and subcutaneous dorsum sites of twelve-week old male Sprague Dawley rats (Charles River, N.Y.). All sites received PCL-HA scaffolds. There were a total of twenty-two SD rats randomly divided into two groups of eleven rats in each of two groups: a test group and a control group. As shown in  FIG. 1 , each rat was given an identification number—#1 to #11, and #12 to #22—test and control groups, respectively. The rats were maintained in pairs during the one-week acclimation period after purchase, in a twelve-hour light/dark cycle and were fed rat chow (Rodent Laboratory Chow 5001, Ormond Veterinary Supply, Ontario, Canada) and water ad labium before the surgical procedures for scaffold implantation. Table 1 summarizes the study groups and number of scaffolds implanted. 
     
       
         
           
               
             
               
                 TABLE 1 
               
             
            
               
                   
               
               
                 Study groups and number of scaffolds implanted. 
               
            
           
           
               
               
               
            
               
                   
                 Test 
                 Control 
               
               
                   
                   
               
            
           
           
               
               
               
            
               
                 Total Subject Number 
                 11 
                 11 
               
            
           
           
               
               
               
               
               
            
               
                 Implantation Sites 
                 E 
                 D 
                 E 
                 D 
               
               
                   
                 n = 11 
                 n = 11 
                 n = 11 
                 n = 11 
               
               
                   
               
               
                 E = extraction socket site implantation; 
               
               
                 D = subcutaneous dorsum implantation. 
               
            
           
         
       
     
     Each experimental site per rat—mandibular central incisor extraction socket and subcutaneous dorsum—surgically received one PCL-HA scaffold. The extraction socket site received a scaffold that resembled the root of the mandibular central incisor. The dorsum site received a scaffold that was in the shape of the human mandibular molar. The scaffolds implanted in the test group rats were impregnated with SDF1 and BMP-7, while the control group received the scaffolds with collagen gel only. All rats were kept for nine weeks post-surgery prior to the harvest of scaffolds for laboratory analyses and quantification. 
     Scaffold Preparation. 
     The rat mandibular central incisor root shaped and human mandibular molar shaped PCL-HA scaffolds were fabricated by layer-by-layer deposition using a 3D printing system (Bioplotter™, Envision TEC, Gladbeck, Germany) ( FIG. 2A , B). The composite consisted of 80 wt % polycaprolactone (PCL) (Mw ˜65,000, Sigma, St. Louis, Mo.) and 20 wt % of hydroxyapatite (HA) (Sigma, St. Louis, Mo.). PCL-HA was molten in the chamber at 120° C. and dispensed through a 27-gauge metal needle (DL technology, Haverhill, Mass.) to create interlaid strands and interconnected microchannels ( FIG. 3 ). The crown and root structures of the human mandibular molar-shaped scaffolds were produced individually and later fused to form the whole tooth due to difficulties encountered in its continuous fabrication as a one piece ( FIG. 3 ). All scaffolds included microchannels of 200 μm in diameter created by the interlocking structure of the PCL-HA strands of 200 μm in diameter ( FIG. 3 ). The macromolecular 3D structure of the scaffold was meant to contribute to formation of the outer morphology of the final outcome while the internal architecture with microchannels aimed to provide room for cellular occupation and tissue ingrowth. All fabricated scaffolds were sterilized in an ethylene oxide sterilizer for twenty-four hours prior to treatment with collagen gel containing SDF1 and BMP-7 (test group), and collagen gel without a growth factor (control group) ( FIG. 2C ). The treatment of the scaffolds was performed in a laminar flow hood using sterile laboratory techniques. 
     For the test group, 100 ng/ml stromal cell-derived factor-1 (SDF1) (R&amp;D systems, MN) and 100 ng/ml bone morphogenic protein-7 (BMP-7) (R&amp;D systems, MN) were combined in 2 mg/ml neutralized bovine type I collagen (Cultrex®, R&amp;D Systems, Minneapolis, Minn.). The growth factor-collagen solution was then infused into microchannels in the PCL-HA scaffold and cross-linked for 1 hr in a humidified incubator at 37° C. Collagen gel loaded with SDF1 and BMP-7 was prepared in the mixture of PBS, 10×PBS, NaOH as summarized in Table 2. The dose of SDF1 and BMP-7 were selected based on previous works. Chemotaxis assays have shown that mesenchymal stem cells grow toward an SDF1 stimulus with maximal chemotaxis at a concentration of 100 ng/ml (Schantz 2007). The BMP-7 concentration of 100 ng/ml has been shown to be effective in promoting osteoblast growth on the collagen carrier (Laflamme 2008). The control group scaffolds were not loaded with any growth factor after sterilization. Rather, empty collagen gel was infused into the microchannels in the same manner as described for the test group prior to the surgery. 
     
       
         
           
               
             
               
                 TABLE 2 
               
             
            
               
                   
               
               
                 Details of the growth factor solution delivered into 
               
               
                 the test group scaffolds. 
               
            
           
           
               
               
               
            
               
                   
                 Contents 
                 Amount (ml) 
               
               
                   
                   
               
            
           
           
               
               
               
               
            
               
                   
                 10X PBS 
                 1 
                   
               
               
                   
                 1N NaOH 
                 0.14 
               
               
                   
                 PBS 
                 4.86 
               
               
                   
                 Collagen I 
                 2 
                 (5 mg/ml) 
               
               
                   
                 BMP7 
                 1 
                 (100 ng/ml) 
               
               
                   
                 SDF1 
                 1 
                 (100 ng/ml) 
               
               
                   
                 Total Volume 
                 10 
               
               
                   
                   
               
            
           
         
       
     
     Surgical Procedures. 
     Twelve-week old Sprague Dawley rats (Charles River, N.Y.) were purchased and were allowed to acclimate for approximately one week. All rats were anesthetized with the cocktail of ketamine (80 mg/kg, IP) and xylazine (5 mg/kg, IP). Depth of anesthesia was monitored during the procedure by toe-pinch; and when required, ketamine (⅓ of full dose: 25 mg/kg, IP) alone was given to maintain anesthesia depth as necessary. A pulse-oximeter device was used during the surgery to monitor the pulse rate and the oxygen saturation level. 
     The surgical techniques were identical between the two groups ( FIGS. 4 ,  5 ). Careful, atraumatic surgical extraction of a mandibular central incisor was performed followed by immediate implantation of the root-form scaffold into the extraction socket ( FIG. 4C , D, E). The extraction procedure was done as atraumatically as possible using periotomes, taking care to preserve the bony walls of the socket ( FIG. 4C ). During the implantation of the construct, care was taken to preserve the labial walls by passive fitting of the construct ( FIG. 4E ). After implantation, the flap was advanced for primary closure and each socket was closed with one or two single-interrupted sutures, using the polyglylactin suture material (Vicryl 5-0, Johnson and Johnson, NJ) ( FIG. 4F ). 
     At the dorsum site of the same rat, subcutaneous implantation of the prepared human mandibular molar-shaped scaffold was performed ( FIG. 5 ). A 2-cm long horizontal incision was placed and the subcutaneous area was relieved and pouched using a sharp surgical scissor ( FIG. 5B ). A human mandibular molar-shaped scaffold was implanted in the pouch created subcutaneously ( FIG. 5C ). The site was closed with polyglylactin suture material (Vicryl 5-0, Johnson and Johnson, NJ) making sure that there was no entrapped air bubble before closure ( FIG. 5D ). Multiple single-interrupted sutures were placed for primary closure. 
     Upon completion of the implantation procedures, buprenorphine (0.05 mg/kg, IP) was given for analgesia prior to relocating to the animal intensive care unit. The rats were monitored closely by veterinary technicians during the recovery period of three to five days and kept in single occupancy cages in a twelve-hour light/dark cycle and fed rat chow (Rodent Laboratory Chow 5001, Ormond Veterinary Supply, Ontario, Canada) and water ad libitum for nine weeks before being euthanized. During the nine-week period, the rats were closely monitored on a regular basis for any sign of infection or illness. Proper management was carried out when such a sign was observed. The remaining incisors were monitored for their continual growth to avoid malocclusion and resultant malnutrition. When indicated, the teeth were clipped for ease of mastication. At the ninth week post-surgery, each rat was humanely euthanized immediately before the harvest using overdose of pentobarbital injection IP. 
     Harvest and Laboratory Procedures. 
     Prior to the harvest, it was evident that the gingival tissues over the mandibular central incisor extraction socket had been maintained without exposure of the scaffolds ( FIG. 6A ). The dorsum sites also showed its optimal wound healing ( FIG. 7A ). 
     The scaffolds in the mandible were harvested en bloc including the remaining adjacent central incisor and alveolar bone ( FIG. 6 ). The dorsum scaffolds were retrieved with the surrounding fascia encapsulating the scaffolds ( FIG. 7 ). All harvested constructs were stored in 10% formalin prior to transportation to the histology lab for 5 μm-thick slide preparations and hematoxylin and eosin (H&amp;E) and von Kossa (VK) staining of each specimen. 
     Quantification of Cell Homing, Vascularization and Mineralization. 
     Quantification of cell homing, vascularization and mineralization was based on any observed differences in cellular density, angiogenesis (blood vessel number and diameter), and presence of mineralization between the study groups and implantation sites. 
     The quantification procedures were performed by a blinded examiner who was not aware which rat belonged to which group. Prior to examining the slides of each scaffold, a decision was made as to which areas would contribute to the histological data analysis. It was agreed to look at the mid-regions of coronal third, middle third, and apical third of the scaffolds prepared on the slides as shown in  FIG. 8 . Hence, the scaffold harvested from the extraction socket had three regions evaluated ( FIG. 8A ), while the scaffold from the dorsum site had four regions evaluated due to the presence of the two roots ( FIG. 8B ). Each slide was examined thoroughly using a digital research microscope (Leica DM6000, Leica, Switzerland) at 100× magnification. The slide photos were taken digitally. The software program provided with the microscope, Leica Application Suite (LAS), was used to carry out quantification of cells and blood vessels within the agreed regions. The counts were later converted into number/mm 2 . The blood vessel diameter was measured using a computer software program (Photoshop CS) and converted into millimeters (mm). Presence or absence of mineralization was evaluated also. 
     Statistical Analyses. 
     All statistical analyses were carried out using a computer program (Microsoft Office Excel 2007). For each variable previously mentioned, mean average and standard deviation values were calculated. Student t-tests were carried out to determine the level of significance between the two experimental groups and between the implantation sites. A p-value &lt;0.05 was considered significant. 
     Results 
     Tissue Integration with the PCL-HA Scaffolds. 
     As evident in  FIGS. 9 and 10 , all scaffolds from both implantation sites of both groups showed that the tissue-scaffold interface regions demonstrated comparably tight tissue-to-scaffold adaptation. It did not appear that there was any noticeable difference between the two groups regardless of the implantation sites. However, the microscopic characteristics of integration appeared to be different between the two implantation sites. With the scaffolds within the extraction sockets, the interface was characterized by noticeable bone-to-scaffold adaptation, possibly with fibrous lining. Some interfaces exhibited bony ingrowth between the strands of the scaffold ( FIG. 9A , B). Also, there was a definite evidence of angiogenesis and soft tissue ingrowth at the interface of the scaffold and the socket wall ( FIG. 9C ). It appeared that the tissue grew around and between the PCL-HA strands ( FIG. 9C ). Tissue was also along the bony socket walls ( FIG. 9C ). The dorsum sites showed that the interface had soft tissue ingrowth well into the internal areas of the scaffolds ( FIG. 10A , B). 
     Cell Penetration and Density. 
     Previous studies utilizing PCL-HA scaffolds have demonstrated cellular penetration and proliferation around strands comprising the PCL-HA scaffold (Heo 2007). As shown in  FIGS. 11 and 12 , there was a noticeable difference in cell density (cells/mm 2 ) levels between the test and control groups of each implantation site. FIG.  11  depicts representative regions of the scaffolds retrieved from the extraction sockets while  FIG. 12  represents the regions from the dorsum site scaffolds, test and control groups, respectively. The cellular density observed from the test group scaffolds was far greater than the one from the control group—p=0.049 and p=0.001, extraction socket site and dorsum site, respectively. The scaffolds retrieved from the extraction sockets had denser cellular populations than the ones from the dorsum sites—p=0.016 and p=0.002, test and control groups, respectively ( FIG. 13 ). 
     Angiogenesis. 
     Angiogenesis was evident within all of the harvested scaffolds from the both experimental groups ( FIG. 14 ,  15 ). In general, there was a greater extent of angiogenesis observed (vessels/mm 2 ) in the test group scaffolds than in the control, regardless of the implantation sites—p=0.011 and p=0.002, extraction socket and dorsum site, respectively ( FIG. 16 ). However, the density observed in the scaffolds from the extraction sockets was seemingly greater overall despite of its statistical insignificance between the groups—p=0.222 and p=0.095, test and control groups, respectively ( FIG. 16 ). 
     The blood vessel diameter (μm) in the dorsum implantation site was greater than in the extraction socket—p=0.028 and p=0.022, test and control groups, respectively ( FIG. 18 ). However, there was no statistical difference between the experimental groups—p=0.426 and p=0.732, extraction socket and dorsum site, respectively ( FIG. 18 ). The representative photos of the slides showed that the vessel diameter appeared much greater in the scaffolds harvested from the dorsum sites than from the extraction sockets ( FIG. 17  A, B). As shown  FIG. 18 , within the dorsum scaffolds there was an apparent greater mean value of vessel diameter within the control groups, although the difference did not reach statistical significance (p=0.732). 
     Mineralization. 
     Mineralization regions were observed in the test group scaffolds only ( FIGS. 19 ,  20 ). Scaffolds from test group extraction socket and test group dorsum implantation site showed regions of mineralization in the von Kossa (VK) slides. At both implantation sites, mineralization was seen well into the scaffolds, and not at the tissue-to-scaffold interface areas. 
     Discussion 
     Cell transplantation is the default strategy of cell based therapies which includes transplantation or injection of culture-expanded or modified tissue progenitors, or fully formed tissue (Mooney 1996). However, therapeutic transplantation of culture-expanded adult cells has several critical barriers, including limited life-span, slow proliferation, and loss of cell phenotype during elongated culture period (Muschler et al. 2004). Accordingly, technological and economic viability of cell delivery approaches, especially for those require substantial cell manipulation ex vivo, has been questioned (Muschler et al. 2004). Recently, there has been a growing interest to regenerate tissues by cell homing, often followed by orchestrating the morphogenesis of the innate cells (Stosich et al. 2007). Cell homing is defined as active recruitment of endogenous cells into predetermined anatomic compartment, and represents an under-studied approach in tissue regeneration (Quesenberry 1998). Schantz, similarly, termed this strategy “cell homing”, and defined it as “site-directed homing of native stem cells to induce tissue formation within cytokine-loaded scaffolds” (Schantz 2007). In many adult tissues, stem cells homing and migration are critical for the ongoing replacement of mature cells and regeneration of damaged cells (Laird et al. 2008). 
     The instant results suggest the internal architecture of interconnected microchannels (200 μm dia.) created by the layer-by-layer deposition of PCL-HA (200 μm dia.) contributed to the intimate tissue-to-scaffold adaptation at the interface areas, followed by the successful homing of host cellular and vascular ingrowth into the large scale (˜17 mm) scaffolds. The scaffolds with SDF1 and BMP-7 promoted host cell penetration. 
     Stromal cell-derived factor-1 (SDF-1), which is secreted by stromal cells in the bone marrow microenvironment, plays an essential role in promoting cell homing by recruitment of progenitor cells that express its cognate receptor, CXC chemokine receptor 4 (CXCR4) (Vandervelde 2005). CXCR4+ positive cells include CD34+ hematopoietic stem cells (HSCs) and mesenchymal stem cells (MSCs) in bone marrow (Brenner et al. 2004; Wynn et al. 2004; Honczarenko et al. 2006; Cheng et al. 2008). Since these cells are essential for vascularization and bone regeneration, we speculate that SDF-1 incorporated in the 3D scaffolds recruited not only local cells but also hematopoetic stem cells and MSCs. 
     In addition to SDF1, BMP-7 was delivered with the scaffold. Since BMP-7 plays a central role in the transformation of mesenchymal cells into osteoblasts, it is speculated that the ectopic or orthotopic mineralization observed in our cell-free scaffolds was achieved by BMP-7-mediated osteogenic differentiation of stem/progenitor cells, which were recruited by SDF1. However, the results show that there has been little and inconsistent mineralization evident in scaffolds with SDF1 and BMP-7, in both implantation sites. Suboptimal osteogenesis may be because of rapid release of BMP-7, for collagen degrades in vivo quickly. 
     Interestingly, less mineralized area was observed in the extraction socket implantation site compared to the dorsum, despite of abundance of the blood and the bone marrow cells. This may be because the healing in the extraction socket after removal of a mandibular central incisor possibly was delayed due to the implantation of the scaffold. Extended surgical duration may have also led the paper-thin labial wall more prone to post-operative resorption. The atraumatic extraction procedure of the rat mandibular incisor is known to be extremely technically demanding as the tooth and the surrounding tissues are both extremely fragile. The handling of small rat mandible further complicates the procedure as well. 
     The histological evaluation confirms in this study that many of the sockets have lost their labial bony plates. This may be due to the extreme thinness of the remaining labial walls. During this process of labial resorption and simultaneous socket remodeling, we raise the possibility that there could have been increased osteoclastic activities. It has been established previously that the healing process of the rat molar extraction sockets is divided into three phases: an early phase (1-5 days) during which organization of the blood clot is completed and the socket is partially covered by epithelium; a bone formation phase (5-20 days); and a bone remodeling phase (20-60 days) when the young bone matures and the alveolar ridge is remodeled (Pietrokovski 1967). Histomorphometric analysis has shown that the edentulous mandible undergoes a significant reduction in size as a result of reduction in both height and width up to 112 days post-extraction (Elsubeihi 2004). Considering the fact that the harvesting procedures took place at the 9 th  week post-implantation, it might be possible that the remodeling and shrinkage had been actively taking place at the time of the harvest. 
     In summary, the present findings demonstrated in the in vivo rat model that innate cells could be induced to migrate into the PCL-HA scaffold with simultaneous angiogenesis and vascularization. This study underscored the exaggerated cellular penetration and angiogenesis in the PCL-HA scaffolds impregnated with SDF1 and BMP-7 than in the scaffolds of the control group. The greater extent of cellular ingrowth and angiogenesis was demonstrated in the extraction socket sites compared to the subcutaneous dorsum sites. These findings confirmed the hypothesis that the scaffolds impregnated with SDF1 and BMP-7 in the extraction socket sites would exhibit the greatest proliferative potential. Thus, by demonstrating the rich cellular and vascular density observed in the extraction socket in presence of SDF1 and BMP-7, the potential for the orthotopic regeneration of a tooth using the cell homing techniques has been shown. 
     Example 2 
     Regeneration by Chemotaxis; PDGF Induced Recruitment of Alveolar Stem/Progenitor Cells 
     Stem/progenitor cells have been isolated from numerous tissues. Bone marrow is known as one of rich sources of stem/progenitor cells including both hematopoietic stem cells (HSCs) and mesenchymal stem/stromal cells (MSCs). Whereas fibroblast-like MSCs were first discovered in the marrow of long bones in 1970s, marrow of alveolar bone of the face was later found to contain cells analogous to long-bone MSCs, but perhaps with greater potency towards at least osteogenic differentiation. Since alveolar MSCs derive from neural crest/mesenchymal cells, different in embryonic origin from mesoderm-derived appendicular MSCs, the present example explored a novel model for tissue regeneration by chemotaxis of MSCs. Dental pulp is the only soft tissue of a tooth and maintains homeostasis of tooth as an organ. Root canal therapy is one of the most common dental treatments in which viable dental pulp tissue is extirpated and replaced with a bioinert thermoplastic material. Post-root canal teeth are deprived of biological viability and therefore susceptible to re-infection, fracture and trauma. Since dental pulp connects to alveolar bone marrow, it was thought by the inventors that alveolar MSCs can be recruited to regenerate dental pulp tissue. 
     Small alveolar bone samples were obtained from multiple healthy patients who underwent medically necessary tooth extraction. Mononucleated and adherent cells were slightly culture-expanded. Early-passage MSCs (p3) were first screened and found to express Stro-1, CD105, CD73, CD44 and CD90, but negative to CD34 by immunocytochemistry and flow cytometry. Alveolar MSCs differentiated into osteogenic, adipogenic, chondrogenic and myogenic cells in respective, chemically defined media. Migration of alveolar MSCs was studied in transwell insert system under the influence of multiple cytokines and growth factors. 
     PDGFββ at 50 ng/ml was most significant in elaborating cell migration at multiple time points. Receptor expression was confirmed. 
     Together, these findings demonstrate inducing the recruitment of endogenous and/or transplanted stem/progenitor cells towards tissue regeneration. 
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     In view of the above, it will be seen that the several advantages of the invention are achieved and other advantages attained.