Regeneration of Nondental Tissues

The first tissue-based therapies for skin grafting were developed in India around 3000 bce , but the synthesis of substitute materials for skin and various grafting techniques (eg, autologous and allografts) were not developed until the eighteenth century. The first engineered skin tissues were generated by Howard Green and colleagues in 1975. This product, which contained only a few layers of cells and did not contain dermis, led to the development of the first commercial skin product, named Epicel (Genzyme, Cambridge, MA, USA), which contains sheets of autologous keratinocytes. Another engineered product for skin was generated using bovine type I collagen and shark chondroitin 6-sulfate. These compounds were crosslinked and packed into a porous matrix with a silicone sheet attached onto one side as a temporary epidermis-like barrier. A composite product of reconstituted dermis and epidermis has led to the development of a commercial skin graft product called Apligraf (Organogenesis, Canton, MA, USA). The strategy of combining cells and extracellular matrix in skin-graft products was also used to successfully produce cartilage-graft materials. Cell-based cartilage repair techniques were first described in 1994. This technology led to the development of the first commercial product for cartilage grafts, called Carticel (Genzyme). Since 2008, significant advances in tissue engineering have been made for other tissues such as bone, kidney, bladder, blood vessels, and liver.

Unlike other tissues, the skin and cartilage do not require an extensive vascular supply. An important challenge in organ regeneration is the acquisition of a functional vascular supply for the engineered organ. Endothelial cells and the paracrine factors that regulate them, such as vascular endothelial growth factor, were shown to induce angiogenesis and facilitate the integration of transplanted tissues/organs into the host. This finding led to a new treatment strategy in regenerative medicine by using peripheral blood-derived or bone marrow–derived endothelial progenitor cells to induce de novo vessel formation in regenerated organs. Vascular endothelial cells can also be generated from human embryonic stem (ES) cells. These cells can integrate into the host and form chimeric vasculature. Vascular endothelial cells can facilitate the differentiation of ES cells into various cell types such as pancreatic insulin-producing cells, cardiomyocytes, neurons, and glial cells. 3D cardiac tissues with endothelial cell networks have been created and implanted onto infarcted rat hearts, which regain function after the surgery. The improvement of cardiac function was dependent on the endothelial cell densities within the engineered cardiac tissues. The number of capillaries in the transplanted tissues with the endothelial cell network is also greater than those without the endothelial cells.

Regeneration of Dental Tissues and Supporting Structures

The regeneration of periodontium was the first tissue-engineering technology in dentistry, and was invented by Nyman and colleagues in 1982. This procedure, termed guided tissue regeneration (GTR), involves inserting a barrier membrane under the periodontal tissue flap to prevent the ingrowth of gingival epithelium and connective tissue, while creating a space on the root surface for progenitor cells from the periodontal ligament including cementoblasts, fibroblasts, and osteoblasts to migrate in and form new periodontal structures including cementum, periodontal ligament, and alveolar bone. Various types of bone-graft materials such as autogenous grafts, allografts, alloplasts, or xenografts have been placed in the space above root surfaces to facilitate bone formation.

There are 2 main types of barrier membranes, resorbable and nonresorbable. The nonresorbable membranes require a second surgical procedure to remove the membranes at 4 to 6 weeks after the initial surgery. Two types of commonly used nonresorbable GTR barrier membranes include expanded polytetrafluoroethylene (ePTFE), also known as Gore-Tex, and nonexpanded polytetrafluoroethylene (nPTFE). The resorbable barrier materials were more recently developed and are available in 2 formats, synthetic polymers and natural barrier materials. The synthetic polymer GTR materials consist of a lactide/glycolide copolymer or PLA blended with a citric acid ester. The natural barrier membranes include those made from collagen, calcium sulfate, or enamel matrix proteins.

The regeneration of periodontium with these products requires the presence of at least one bony wall at the treatment site, most likely to provide progenitor cells and vascular supply, allowing the repair and regeneration of the periodontal tissues. To improve on the limited level of success, strategies using exogenous growth factors and stem cells have been studied and await translational application to clinical practice. Potential growth factors for periodontal regeneration include bone morphogenetic proteins, platelet-derived growth factor, amelogenin proteins, and fibroblast growth factors.

Current therapeutic approaches involve replacing the missing tooth structure with artificial materials as the capacity of adult human dental tissues to regenerate is virtually nonexistent, particularly for enamel, due to the absence of ameloblasts in formed teeth. The regeneration and repair of inner-tooth dentin can be obtained only if the healthy dental pulp tissue is still present and if bacterial contamination is completely removed. Typically, mechanical removal of decayed enamel and dentin is completed and artificial materials are used to fill in the prepared cavity, to prevent bacterial contamination and induce the formation of reparative dentin onto the dentinal floor of the cavity.

The regeneration of dentin is usually not possible in necrotic teeth. However, in children with incompletely formed teeth with wide-open root apices, pulp tissue can be regenerated through the opened root apices. Findings from prior revascularization studies of traumatized teeth showed that the success of pulp-tissue regeneration in replanted avulsed teeth depends on the diameter of the opening of root apices. A diameter of 1 mm (1000 μm) of the opening of root apices has been suggested as a minimum requirement to allow new tissues with neural and vascular structures to regrow into the tooth. Because diameters of the neural, vascular, and cellular structures are less than 100 μm (ie, 10–30-μm diameters for eukaryotic animal and human cells; 0.2–20-μm diameters for nerve fibers; and <100-μm diameters for most arteries in the dental pulp), theoretically the regeneration of pulp tissues may not need as much as a 1000-μm–diameter opening. However, the positive correlation of clinical success in revascularization of the replanted teeth and a 1-mm minimum apical opening requirement may be due to the existence of stem cells or progenitor cells in the apical area. Further studies are needed to test this notion.

Several case series showing clinical success of pulp-tissue regeneration in immature necrotic teeth led to the growing recognition of the regenerative potential of tissues at the apical end of these immature teeth. The recent identification of adult mesenchymal stem cells in these tissues also suggests that this cell population regrows into the tooth and regenerates the dentin-pulp complex of such immature necrotic teeth. However, the exact mechanisms by which such precursor cells contribute to clinical outcomes remain unknown.