1 Introduction

Stem cells (SCs) are normal, undifferentiated cells that, if exposed to the proper signal, can multiply, produce, and differentiate into a variety of somatic cells in the laboratory and living organisms. Various SCs inside the body are involved in maturation and repair in adult organisms [1]. The unlimited potential of these cells to produce physiological cells has made them be replaced by recombinant or primer cells [2].

Oral tissues are a rich source of SCs, which have attracted dentists’ attention because of their easy access to other SCs. These cells have unique capabilities making them of great importance in tissue engineering [3, 4], regeneration, or the replacement of damaged or diseased tissues [4, 5]. In dentistry, there are problems such as alveolar bone resorption for patients following tooth extraction or loss due to periodontal disease, dental caries, and tooth fractures caused by trauma. Moreover, in individuals losing their teeth, it leads to bone loss, especially in the lower jaw, thereby making such individuals lose the treatment option of implant placement [6, 7]. Following such problems, stem cell tissue engineering therapies to repair large defects in periodontal tissue and alveolar bone to replace lost teeth seem to be of paramount importance [8,9,10].

Various studies on SC-based tissue engineering and the regeneration of oral and dental tissues and organs have been performed for clinical and dental applications in animal and laboratory models [11,12,13]. However, more invivo studies are recommended to reach further definite results [14, 15]. Given that basic research is required for treatment before evaluating SCs in clinical trials and also given the relatively new role of SCs in dentistry, obtaining ideal SCs, depending on the different locations of the mouth, jaw, and face, is not well described. Dental stem cells (DSCs) are attractive for stem cell transplant therapy approaches because of their simple separation, high flexibility, immunomodulatory properties, and multi-potential capabilities. The use of appropriate scaffolds filled with desirable biomolecules such as growth factors and cytokines can improve the proliferation, differentiation, migration, and functional capacity of DSCs.

Appropriate scaffolds full of desirable biomolecules such as growth factors and cytokines can improve the proliferation, differentiation, migration, and functional capacity of DSCs and optimize cell morphology to construct tissue structures for specific purposes. Since DSCs are a promising cellular source for tissue engineering, especially for repairing teeth, bones, and nerve tissues, the present study aimed to identify more DSCs and their therapeutic applications.

2 Pluripotent stem cells (emryonic stem cells/ induced pluripotent stem cells)

Embryonic cells or induced cells are the main pluripotent stem cells (PSCs), which can produce themselves and a variety of somatic adult cells in vitro and in vivo [16]. Due to their unlimited renewal, these cells are used clinically in evolutionary biology, biological research, regenerative therapies, and pharmaceutical experiments in dentistry [17].

There are two types of SCs: [1]. Embryonic SCs (ESCs) taken from the cells of the inner layer of the embryo before implantation. They were first isolated from mice and then from other species such as rats, humans, and monkeys [13]. Human-derived ESCs are known as pluripotent, meaning that they can form different types of cells in the body [18], and [2]. Induced Pluripotent SCs (iPSCs) are formed by reprogramming adult somatic cells and converting them to SCs. The reprogramming technology on somatic cells was first performed in mice and then in human cells [19]. They are few and are often located deep in the tissue, making them somewhat difficult to identify, isolate, and grow in vitro [20].

Regarding the application of ESCs in dentistry, the controlled differentiation of PSCs to specific ratios of oral tissues and organs such as mucosa, alveolar bone, periodontal tissues, and teeth in vitro and in vivo is not unexpected. However, researchers in this field have faced two obstacles, including ethical issues and technical problems. Because ESCs are allogeneic, they may be immunologically incompatible between donors and recipients [21,22,23,154], stability [155], electrical conductivity [156], porosity [154], connectivity, and the ability to create proper crosstalk between stem cells and adjacent cells [157]. Since DSCs are an acceptable source of cells for regenerating teeth, bones, and nerve tissue, combining DSCs with a suitable scaffold for cell transplantation can provide remarkable results. Two main approaches to this combination are cell-based tissue engineering and the cell-free approach [158].

When a bioactive scaffold with growth and differentiation factors is implanted in relevant tissues, it can induce resident stem cells and their promotion, reproduction, and differentiation [159]. Various morphogens/growth factors, as environmental cues, significantly affect the behavior of DSCs implanted in scaffolds and play a key role in the success of regenerative therapies [160, 161]. If different biomaterials are pretreated with proteins such as BMP, sialoprotein, fibronectin, and osteopontin, it improves the DSCs’ behaviors. They promote the function of DSCs by increasing their adhesion, differentiation, proliferation, and migration and ultimately improve the formation of new tissues [162, 163].

Some sources to produce scaffolds for the teeth repair and reconstruction are natural biomaterials, including collagen [164], gelatin [154], fibrin, and silk [165] with protein structure and alginate [166], hyaluronic acid [167] with polysaccharide structure, and synthetic biomaterials, including polyglycolate/poly-l-lactate [168], polycaprolactone-poly glycolic acid [169], polylactic acid-co-polyglycolic acid [170], polycaprolactone /gelatin/nano-hydroxyapatite [171], nano-hydroxyapatite/collagen/poly-l-lactide [3).

Fig. 3
figure 3

Classification of scaffold fabrication technologies in tissue engineering: conventional and rapid prototy** techniques

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4.3.1 Conventional techniques (Figure 4)

Solvent casting and particle leaching

In this technique, a solvent combined with uniformly-distributed salt particles of a certain size is used to dissolve the polymer solution. Solvent evaporates, leaving a matrix containing salt particles. The matrix is then submerged in water, and the salt leaches away to form a structure with high porosity [192, 193]. Solvent casting with particle leaching only suits thin membranes of thin wall three-dimensional specimens. Scaffolds developed by this method have a porosity of 50–90%. This technique is relatively easy and low-cost [194].

Fig. 4
figure 4

Schematic illustration of conventional techniques in scaffold fabrication

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Freeze-drying

The freeze-drying process is also known as lyophilization and involves the use of a synthetic polymer, first dissolved in an appropriate solvent. After dissolution, the polymer solution is cooled under the freezing point, resulting in a solid solvent evaporated by sublimation to leave a solid scaffold with numerous interconnected pores [195]. In this technique, when the solution is cooled by the freezing point, the solutes can be separated in the ice phase, resulting in a small porous structure characterized by a “fence” of matter surrounding the ice. The scaffolds are achieved after consequent drying; and by simple dissolving and freeze-drying, the macro-porosity corresponds to the empty area initially occupied by ice crystals [196]. The benefit of this technique is the capability of obviating high temperatures, which could decrease the activity of integrated biological factors. Moreover, the pore size is managed by controlled and changing the freezing method

Stereolithography

The stereolithography method is basically used to create solid, three-dimensional objects by consecutively printing a thin layer of ultraviolet (UV) curable material layer-by-layer [197]. A stereolithography system has four main components: a tank with a photosensitive liquid resin, a transferable built platform, a UV laser for radiating resin, and a dynamic mirror system. The process begins with a UV laser by depositing a layer of photosensitive liquid resin on the platform. Following the solidification of the initial layer, the platform is lowered vertically. A second layer is then placed on the first layer; the process is repeated until a 3D scaffold is created. Finally, the uncured resin is cleaned off, and the scaffold is post-cured under UV light [198].

Gas foaming

The gas foaming technique is a technique to cope with using high temperature and organic cytotoxic solvents [199]. This technique uses relatively inert gas foaming agents such as carbon dioxide or nitrogen to pressurize modeled biologically degradable polymers with water or fluoroform until they are saturated or full of gas bubbles. This technique usually produces structures like a sponge with a pore size of 30–700 μm and a porosity up to 85% [200].

Electrospinning technique

The electrospinning technique offers ease and flexibility in controlling scaffold characteristics to suit various tissue engineering applications [201]. Moreover, electrospinning can deliver outstanding control of pore interconnectivity and internal and external scaffold geometry. In the basic principle of electrospinning, the polymer in a liquid phase is pumped via a thin needle of a specific diameter to assemble a conductive object, and when the required high voltage is realized and the applied electric power overpowers the surface tension forces of the used polymer solutions, a jet of polymer fibers is developed [202].

4.3.2 Rapid prototy** technology

Rapid prototy** (RP) technologies, also known as solid freeform fabrication, are widely applied in biomedical and tissue engineering applications. In this technique, the manufacturing method, with the aid of a specifically-designed computer-controlled 3D model, precise 3D scaffold models (based on Cad or CT scan files) are constructed by a layer-by-layer cyclic deposition and dispensation of material [203].

Various RP technologies in the market are as follows: three-dimensional printing (3DP), fused deposition modeling (FDM), stereolithography apparatus (SLA), and selective laser sintering (SLS) [204] (Fig. 5).

Fig. 5
figure 5

Schematic illustration of rapid prototy** techniques in scaffold fabrication

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In the FDM technique, a solid polymer is cast into a hot extrusion nozzle to be melted and extruded on the surface of a 3D object using computer-controlled extrusion and deposition processes. The scaffold is made from multiple layers of adjacent microfilaments [205]. SLS was developed in 1986 by the Texas University of Austin. This technique uses the laser as the power source to sinter powdered material defined by a 3D model in thin layers. Due to using a laser, this technique has been utilized to make various materials such as polymers, metals, or ceramics [206].

Self-assembling technology

The current treatment practice mainly relies on inert biomaterials as substitutes for the decay of soft and mineralized tissues. However, lately, a tissue engineering method using a hydrogel scaffold seeded with two dental stem cell lines together with peptide-amphiphile (PA) was used to establish novel regenerative processes and regenerate dental tissues [207].

Three-dimensional printing (3DP)

Three-Dimensional Printing (3DP) is a process of creating tools, and functional prototype features directly from the computer models. It is a new fabrication method for tissue engineering, which can be utilized to control scaffold structure at the micron level precisely [208].

Three-dimensional printing for regeneration of the tooth and tooth-supporting tissues

Three-Dimensional bioprinting is a novel technology fundamentally derived from printing technology, which can print living cells directly into 3D structures [209]. The 3D printing technology is driving major innovations in regenerative dentistry [210]. The rise of 3D printing in dentistry has been parallel with CAD advancements and enhanced imaging techniques such as cone beam computed tomography (CBCT) and magnetic resonance imaging (MRI) to plan and print dental and maxillofacial prostheses to restore and replace lost structures [211]. The reconstruction of the complex system of the tooth and its supporting apparatus (like the ligament, alveolar bone, and cement) has been improved by 3D-printed bioengineered scaffolds [212]. 3D bioprinting boosts regenerative medicine and is being applied to address the need for tissues and organs suitable for transplantation. A wide range of biomaterials and printing strategies are used for 3D printing such as hydrogels, metals, ceramics, resins, and thermoplastics. Table 4 summarizes the material and techniques used in regenerative dentistry using the 3D printing technology.

Table 4 3D printing technology, materials and techniques in regenerative dentistry
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5 Growth factors

Polypeptides that can stimulate cell proliferation and act as the major growth-regulatory molecules for cells in culture and in vivo are known as growth factors (GFs). Gfs, along with other morphogens comprise one of the three vital components in tissue engineering, which are combined with scaffolds and progenitor or stem cell population [213]. Various investigations have studied the use of recombinant growth factors separately or in combination with other growth factors or biomaterials for the regeneration of different oral tissues, including mandibular or maxillary bone [214], salivary glands [215], nerve regeneration [216], dentin–pulp complexes [217, 218], and periodontal tissued [219].

5.1 Regenerative endodontics (Dentin–pulp complexes)

Adding signaling molecules and various growth factors to natural and artificial scaffolds can increase the regeneration of pulp-like tissues inside the canal by promoting dentin formation, mineralization, neovascularization, and innervation [220]. For example, DSCs linked to growth factor stromal cell-derived factor-1 (SDF-1) or granulocyte colony-stimulating factor (G-CSF) on a collagen scaffold, have promoted pulp regeneration in the animal pulpitis model [164, 221]. DSCs loaded on peptide hydrogels along with growth factors such as vascular endothelial growth factor (VEGF), TGF-β1, and FGF-1 can differentiate into odontoblast-like cells and vascularized dental pulp-like tissue within the dentin cylinder [222]. DSCs isolated from adult human dental pulp implanted on the surfaces of three-dimensional collagen gel cylinders show significant cellular uptake when combined with BMP-7, SDF-1α, and bFGF [223]. Furthermore, SDF, FGF, TGF-β1, VEGF, and BMP as growth factors, when loaded on scaffolds such as peptide hydrogels, collagen, gelatin hydrogels, and alginate hydrogels, enhance the endodontic regeneration of DSCs [224]. The combination of SDF-1 with biomaterials to use different endogenous stem cells is highly effective. A study revealed SDF-1 embedded in a silk fibroin scaffold resulted in pulp regeneration through DPSC induction in a pulpectomized mature canine preclinical model [225]. Further, SDF-1 induces and regenerates the structure of pulpdentin by absorbing and transferring SCAP from the apex to the root canal space [226]. The implantation of DSCs with poly-ε-caprolactone and hydroxyapatite along with SDF-1 and BMP-7 results in tooth-like structures in the mandibular incisor extraction socket [227]. The two growth factors, G-CSF and FGF-2, have the greatest impact on the migration of SCAPs. previous studies have revealed that combining G-CSF with TGF-β1 leads to the migration and high biomineralization of endogenous SCAPs in root canal repair methods. G-CSF also has stimulatory effects on the movement of DPSCs from adult teeth. These mobilized DPSCs have higher vascularity and pulp regeneration ability than colony-derived DPSCs [228]. DSCs implanted in a collagen/chitosan scaffold containing a non-cellular ECM result in the expression of dentin sialoprotein in nude mice, which ultimately produce the pulp-like tissue in the tooth [229]. Observations have indicated that the co-culture of DSCs with other stem cells improves neovascularization, and the co-culture of DSCs and human umbilical vein endothelial cells with gelatin methacrylate xenogeneic hydrogel leads to the formation of new vascular pulp in rat teeth [230]. Dissanayaka et al. found that the transplantation of DSCs and human umbilical vein endothelial cells into PuraMatrix containing VEGF increased the vascularization and mineralization of mouse vascularized pulp-like tissue and osteodentin [231]. Woloszyk et al. also reported that the use of silk fibroin scaffolds increased the ability of human DSCs to attract vessels, thereby improving and regenerating damaged tissues [232]. Yang et al.‘s study showed that the transplantation of DSCs with a piece of silk fibrin tooth/scaffold loaded with SDF-1 resulted in the formation of pulplike tissues with vascularity, the formation of an organized fibrous matrix, and the formation of dentin in the nude mice [225].

5.2 Periodontal and alveolar bone regeneration

Many studies have addressed bone regeneration [233, 234], according to which the ossification capacity of DSCs varies depending on their origin (i.e., dental pulp, tooth follicle, gingival tissue, and periodontal ligament), which can change the ossification ability of DSCs depending on the selected biomaterial scaffolds [235, 236]. For example, the ability to repair bone defects is greater in the DSCs of periodontal ligament origin encapsulated in an arginine glycine-aspartic acid tripeptide scaffold [237]. DSCs derived from dental pulp have a high potential for neovascularization, and, due to their ability to differentiate into osteoblasts, they can enhance bone repair [238]. One of the most common known scaffolds in bone tissue engineering to seed DSCs with human dental pulp or exfoliated deciduous teeth origin is included collagen sponge membranes (to repair defects in the human mandible bone) and hydroxyapatite/tri-calcium phosphate ceramic granules [14]. Hernández-Monjaraz et al.‘s study showed that in patients with periodontal problems, DSCs implanted on collagen-polyvinylpyrrolidone sponge scaffold increased bone density and decreased tooth mobility and periodontal pocket depth in the bone defect area [239]. Tanikawa also managed to reconstruct bone and fill alveolar defects in cleft lip and palate patients through DSCs with a hydroxyapatite-collagen sponge scaffold [240]. In a study, Chamieh et al. found that DSCs implanted in dense collagen gel scaffolds had a greater effect on the healing process of the skull and face than cell-less scaffolds [241]. Ferrarotti et al. used DSCs implanted in collagen sponges to treat patients with chronic periodontitis with deep intraosseous defects, which significantly improved periodontal regeneration [242]. The important point in a successful cell transplant is the optimal number of DSCs. Moreover, the composition of the scaffold and its surface properties play a critical role in the bone differentiation of DSCs and the process of bone tissue regeneration [238, 243]. For example, DSCs implanted in a type I collagen matrix, fibrin, hyaluronic acid, and polyesteramide type-C play a vital role in mineralization [244]. Due to ceramic scaffolds’ chemical and structural similarity to native bone, they are commonly used to enhance bone regeneration and repair DSCs [245]. Strong bone formation in the femoral bone defect area of rats was observed after applying DSCs implanted in bioactive glass nanoparticles/chitosan-gelatin bionocomposite compared to mesoporous bioactive glass nanospheres [246]. Some biomaterial scaffolds facilitate biomolecule-induced tissue formation. Fu showed that the 3D matrix scaffold enriched with DSCs in nude mice increased BMP-9-induced osteogenesis and mineralization in ectopic bones [247].

5.3 Nerve regeneration

In addition to the abovementioned points, DSCs can differentiate into neuron-like cells, Schwann, glia, and oligodendrocytes [248]. Various studies have indicated that the implantation of DCSs in different scaffolds increases the lifespan of cells and their differentiation into neuronal-like cells [249, 250]. The use of combined DSCs with different scaffolds, including chitosan, heparinpoloxamer, silicone tubes, and poly-ε-caprolactone/ poly-lactide-co-glycolic acid, improves the function of damaged nerve tissues and reduces inflammatory responses [251]. For example, in experimental models of spinal cord injury, the transplantation of DSCs with chitosan scaffolds enhanced motor function and suppressed inflammatory responses, in which glial cell-derived neurotrophic factors and brain-derived neurotrophic factors seem to play a vital role. Combining DSCs with scaffolds also reduces caspase activity, thereby preventing cell damage and death [252]. Human DSCs isolated from periodontal ligament gingival tissues and enclosed in three-dimensional alginate and hyaluronic acid scaffolds in the presence of nerve growth factor (NGF) differentiate DSCs to neural tissues [253]. Human DSCs with the expression strength of STRO-1, c-Kit, and CD34 markers, when implanted on collagen scaffolds, could have axonal regeneration from proximal to distal stumps in mice with sciatic nerve defects [63].

Figure 6 demonstrates the application of dental tissue-derived stem cells combined with growth factors and scaffolds in oral regenerations.

Fig. 6
figure 6

Application of dental tissue-derived stem cells combined with growth factors and scaffolds in dentistry

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6 Discussion

In recent years, there have been many studies on stem cell therapy. This field is revitalizing in various fields of medicine such as dentistry and medical diseases. Because the oral and maxillofacial areas are a promising source of SCs, physicians and dentists need to have adequate and up-to-date information on SCs recognition and access during patients’ treatment. According to such studies, different types of DSCs have been introduced [101, 254], all of which are well suited for early research in resuscitation medicine. Preclinical studies and some clinical trials have yielded successful results regarding the use of DSCs. It has been observed that tooth SCs are safe and supportive for regenerating lost or damaged tooth tissues [255,256,257].

Most SCs used in dentistry come from dental structures such as dental/apical papilla, PDLs, and even decayed deciduous teeth. These dental cells have features such as a high proliferation rate, wide differentiation potential in different mesenchymal cell lineages, and weak immunogenic effects, making them special in regenerative medicine and dentistry [254].

The results of various studies show the strong potential of DSCs in the production of dental components such as dentin, pulp, cement, and periodontal ligament associated with the presence of odontoblasts and cementoblasts. For example, some DSCs can form chondrocytes, osteocytes, neurons, and adipocytes in vitro. According to the research findings, DSCs such as DPSCs can regenerate dentin/pulp [221, 258, 259], SHEDs and DFPCs can strengthen bone [83, 256, 260], and PDLSCs play a role in periodontal regeneration [101, 257, 261].

Before using DSCs for tissue regeneration, the key point is to find reliable ways to control previous inflammatory environments. Further studies are needed to elucidate the underlying mechanisms of lost tissue regeneration and the immune system modifying features of the DSCs, followed by human clinical trials [78].

7 Conclusion

Recently, stem cell-mediated therapeutic interventions have received much attention and have made significant advancement in treating diseases, especially those not cured by conventional methods. Although many studies have addressed the use of biomolecules with appropriate scaffolds to treat effective cell transplantation with DSCs and have yielded significant results, there is still a long way to identify these molecules for better therapeutic outcomes and their interaction with ECMs and DSCs. Importantly, the focus is on the innovative combinations of biomaterials and biomolecules to enhance the ability of DSCs to provide new therapeutic approaches. Stem cell transplantation is a promising option; however, at the moment, it cannot be considered a therapeutic miracle. In general, although the SCs of dental origin have many applications they also have certain limitations. One of its main limitations is the difficulty in identifying, isolating, purifying, and growing these cells continuously in laboratories. Rejection by the immune system is another problem requiring further thorough investigation. However, autologous cells can help solve this problem. SCs research in dentistry has its own challenges and risks, and this necessitates further research.