Restorative and endodontic procedures have been recently developed in an attempt to preserve the vitality of dental pulp after exposure to external stimuli, such as caries infection or traumatic injury. When damage to dental pulp is reversible, pulp wound healing can proceed, whereas irreversible damage induces pathological changes in dental pulp, eventually requiring its removal. Nonvital teeth lose their defensive abilities and become severely damaged, resulting in extraction. Development of regeneration therapy for the dentin-pulp complex is important to overcome limitations with presently available therapies. Three strategies to regenerate the dentin-pulp complex have been proposed; regeneration of the entire tooth, local regeneration of the dentin-pulp complex from amputated dental pulp, and regeneration of dental pulp from apical dental pulp or periapical tissues. In this paper, we focus on the local regeneration of the dentin-pulp complex by application of exogenous growth factors and scaffolds to amputated dental pulp.
Dental pulp is sometimes affected by external stimuli such as caries infection or traumatic injury. Preservation of dental pulp and maintenance of its viability are essential to avoid tooth loss, and dentists carry out restorative procedures with pulp capping to regulate inflammatory responses of dental pulp, or cement lining on a cavity floor to block external stimuli. Reversible damage induces pulp wound healing, and direct pulp capping and pulpotomy with calcium hydroxide are known to be effective to induce pulp wound healing mechanisms.
After external stimuli such as cavity preparation, apoptosis of pulp cells including odontoblasts is induced [
When the external damage to dental pulp induces irreversible changes of the pulp, dentists carry out pulpectomy. Generally, a root canal after pulpectomy is tightly filled with biomaterials such as gutta-percha to prevent reinfection by bacteria. However, a tooth without vital dental pulp has lost its defensive ability, which is often followed by the severe damage such as the progression of deep radicular caries or tooth facture, resulting in extraction of the tooth. Furthermore, a treated tooth is often reinfected by bacteria because of its complicated anatomical structure or inadequate treatment by a dentist, resulting in formation of a lesion around the root apex with bone resorption. The success rate of the endodontic retreatment is lower than that of pulpectomy [
It is well known that growth factors, such as bone morphogenetic proteins (BMPs) and fibroblast growth factors (FGFs), stem cells, and scaffolds, are essential for tissue engineering to regenerate tissues [
Strategies for regeneration of the dentin-pulp complex with three factors for tissue regeneration; growth factors, scaffolds, and cells (stem cells or progenitor cells). (a) Regeneration of the entire tooth. (b) Local regeneration of the dentin-pulp complex in the dentin defect area from residual dental pulp. (c) Local regeneration of dental pulp from apical dental pulp or periapical tissues.
Regeneration of the entire tooth
Local regeneration of dentin-pulp complex from residual dental pulp
Local regeneration of dentin pulp from apical pulp or periapical tissues
Regeneration of the entire tooth is accepted as a model of organ replacement and regeneration therapy. Recently, it was reported that tooth germs can be bioengineered using a three-dimensional organ-germ culture method, in which dental epithelial and mesenchymal cells isolated from tooth germs were cultured in three-dimensional scaffolds for the replacement therapy. Scaffolds consisted of synthetic polymers such as poly (lactide-co-glycolide) (PLGA) and bioceramics such as hydroxyapatite, tricalcium phosphate and calcium carbonate hydroxyapatite were examined in the three-dimensional organ-germ culture [
Local regeneration of the dentin-pulp complex from residual dental pulp has been mainly delivered by researchers who are engaged in clinical practice. Several studies have reported the use of local applications of bioactive molecules such as BMPs and recombinant fusion ameloblastin to exposed pulp [
Induction of appropriate pulp wound healing and formation of new dentin in dentin defects are essential for the local regeneration of the dentin-pulp complex and vital pulp therapies to form new dentin in defects. Several papers demonstrated the local regeneration of dentin-pulp complex in different methods. It was reported that BMP-4 with dentin powder induced dentinogenesis in dentin cavity with pulp exposure [
FGF-2 is known to play a role in both physiological and pathological conditions [
Controlled release of FGF-2. Gelatin hydrogels has an ability to incorporate growth factors such as FGF-2. After implantation of gelatin hydrogels incorporating FGF-2 with scaffolds, such as collagen sponge, FGF-2 is gradually released from gelatin hydrogels biodegraded by proteinase at tissue defect area. The controlled released FGF-2 can induce tissue regeneration.
Local regeneration of the dentin-pulp complex in dentin defect area by implantation of gelatin hydrogels incorporating FGF-2. (a) Gelatin hydrogels incorporating FGF-2 with collagen sponge are implanted into dentin defect area. Controlled release of FGF-2 from biodegraded gelatin hydrogels can induce pulp stem cells or progenitor cells, as well as vessels, into collagen sponge at defect, resulting in the regeneration of pulp in the defect area and the formation of regenerative dentin on surface of the new pulp. (b) Histological photograph of proliferating pulp and newly regenerated dentin at surface of proliferating pulp. (c) High magnification of the regenerated dentin.
Studies on regeneration of dental pulp from the apical area began from the identification of stem cells in the apical areas of developing teeth in which root formation is incomplete. It is suggested the existence of a new population of mesenchymal stem cells residing in the apical papilla (SCAPs) of incompletely developed teeth, and these cells have the ability to differentiate into odontoblast-like cells [
It is important to select appropriate scaffolds for successful tissue regeneration. It is well known that essential properties of scaffolds are mechanical properties such as porous three-dimension structure, and mechanical strength, as well as biological properties such as biocompatibility and biodegradation [
Recently, we have been focusing on the application of hyaluronic acid for local regeneration of the dentin-pulp complex. Hyaluronic acid is one of the glycosaminoglycans present in the extracellular matrix and plays important roles in maintaining morphologic organization by preserving extracellular spaces, and it has been reported to have excellent potential for tissue engineering [
Dental pulp is a type of connective tissue derived from the dental papilla, and contains large amounts of glycosaminoglycans [
To clarify whether hyaluronic acid sponge (molecular weight 800 kDa) is useful as a scaffold for wound healing and regeneration of dental pulp, we compared
Application of hyaluronic acid sponge for local regeneration of the dentin-pulp complex. (a) KN-3 cells, odontoblastic progenitor cells, adhered to hyaluronic acid sponge, as well as collagen sponge. (b) Histological changes of amputated dental pulp after implantation of hyaluronic acid sponge
Cell Adhesion on Sponges
Pulp proliferation and inflammatory reaction in sponges at defects
It is also important to clarify neuronal differentiation and neurite outgrowth during regeneration of the dentin-pulp complex. We examined the effects of hyaluronic acid gel on neuronal differentiation of PC12 pheochromocytoma cells, which respond to nerve growth factor (NGF) by extending neurites and exhibit a variety of properties of adrenal medullary chromaffin cells. We applied diluted solutions of 800 kDa hyaluronic acid to NGF-exposed PC12 cells, and noted inhibition of NGF-induced neuronal differentiation of PC12 cells via inhibition of ERK and p38 MAPK activation, caused by the interaction of hyaluronic acid to its receptor, RHAMM [
Our results demonstrated that hyaluronic acid sponge is useful for local regeneration of the dentin-pulp complex, whereas hyaluronic acid gel inhibits the differentiation or neurite outgrowth of neurons.
In our strategy, growth factors and scaffolds are exogenously supplied as bioactive materials, while the source of stem cells that are able to differentiate into odontoblast-like cells and pulp cells is dependent on the residual dental pulp. The vitality of the residual dental pulp is a critical point to achieve local regeneration of the dentin-pulp complex. It is generally accepted that the pulp wound healing proceeds well under conditions of low inflammatory responses by the dental pulp. In addition, regulation of dental pulp infection is another critical point regarding the success of such regeneration therapy. The resin bonding system is commonly used as one of materials showing favorable adhesion to enamel and dentin, and composite resin system with antimicrobial ability was reported [