Dental tissue includes enamel, dentin, pulp and cementum. Pulp, a critical structure rich in cells, blood vessels and nerves, is protected by mineralized enamel and dentin. If caries is not treated properly and timely, bacteria will easily enter the pulp system and affect pulp tissue, leading to irreversible pulpitis or pulp necrosis that requires root canal treatment. However, after the treatment, teeth lose their nutrient supply and are prone to discoloration or even fracture, which is not conducive to maintaining oral cavity health. To achieve pulp tissue regeneration, regenerative endodontic procedures have been proposed as an alternative to conventional pulp treatment.

Periodontitis is considered as a global epidemic. Long-standing pathological factors may destroy the supporting periodontal tissue and result in tooth loss. Conventional periodontal treatment can successfully reduce the number of pathogens in periodontal defects. However, no effective methods for regenerating and reconstructing lost tissues have been developed. Since the inflammatory microenvironment caused by periodontitis is the main cause for the abnormal function and regenerative defects of local stem cells, the progression of periodontitis and disease severity is closely related to the host’s immune response. Therefore, many researchers have begun to promote periodontal tissue regeneration by reducing inflammation and mediating immunoregulation.

The temporomandibular joint comprises the mandibular condyle, articular facet, articular disc, articular capsule and articular ligaments. The cartilage on the joint surface has a strong protective effect on the bone. Due to the limited self-healing capacity of joint cartilage, temporomandibular joint osteoarthritis (TMJOA) often occurs after excessive wear (Zhao and Xie, 2021). The main treatment strategy is to inhibit the inflammatory response, prevent the gradual destruction of cartilage and subchondral bone, induce cartilage regeneration, and restore function (Wang et al., 2015).

The key to repairing cartilage defects is regulating the inflammatory response to prevent cartilage damage, promote the proliferation and migration of cartilage cells, and increase cartilage matrix synthesis. MiR-100, the product of EVs derived from SHEDs, inhibited the expression of proinflammatory factors (such as IL-6 and IL-8) and matrix metalloproteinases (such as MMP1 and MMP-9), thereby reducing inflammation in the temporomandibular joint and preventing further cartilage damage (Luo et al., 2019). Researchers established a rabbit TMJOA model and found that human umbilical cord blood mesenchymal stem cells (UCB-MSCs) demonstrated robust potential for cartilage protection and regeneration. MSC-EVs also promoted the recovery of TMJOA by regulating inflammation (Kim et al., 2019). In addition, MSC-EVs were proved to enhance the repair of critical-sized osteochondral defects in an adult immunocompetent rat model, and the new hyaline cartilage and underlying subchondral bone closely resembled normal tissue (Zhang et al., 2016). Researchers have suggested that this outcome could be attributed to exosomal CD73-mediated adenosine activation of AKT, ERK, and AMPK signaling, which increases chondrocyte activity and induces cell proliferation and tissue regeneration through kinase phosphorylation (Zhang et al., 2018).

In recent years, soft tissue replacement has been increasingly demanded for treating congenital malformation, traumatic repair and postoperative rehabilitation of maxillofacial tumors. Researchers are seeking a stem cell-based therapy or cell-free approach to producing engineered adipose tissue to develop an ideal soft tissue substitute. Exosome-like vesicles derived from adipose tissue induced adipogenesis in ADSCs and promoted the proliferation, migration, and angiogenic potential of aortic endothelial cells (Dai et al., 2017). MiR-450a-5p carried by EVs derived from adipose tissue triggered adipogenic signals in ADSCs by inhibiting the expression of WISP2 and promoting the differentiation of adipose cells (Zhang et al., 2017). In recent years, EVs have also been widely studied in the context of skin damage repair, but there have been few reports on the repair of maxillofacial skin. The main mechanism by which skin defects are repaired has been verified in different phases. For example, ADSC-EVs exerted anti-inflammatory effects by eliciting macrophage switching from an M1 to an M2 phenotype during the inflammatory phase (Lo Sicco et al., 2017), while EVs derived from UCB-MSCs stimulated wound healing by reducing scar formation in the remodeling stage (Zhang et al., 2021). In the proliferative phase, critical steps involve promoting the proliferation and migration of human dermal fibroblasts and human keratinocytes, as well as inducing angiogenic activity in endothelial cells (Zeng and Liu, 2021). Exosomes from human endometrial MSCs promoted the expression of angiogenic markers and the proliferation and migration of human umbilical vein endothelial cells (Ha et al., 2020).

Peripheral nerve injuries typically lead to motion and sensory dysfunction in different parts of the body. For example, the trigeminal and facial nerves are easily affected by maxillofacial tumors or severe trauma, causing damage to facial sensation and even leading to facial paralysis. Nerve regeneration after injury is a complex process involving the differentiation of SCs, the recruitment of macrophages, the growth of blood vessels and the regeneration of axons (Hercher et al., 2022). Interestingly, dental MSCs, which derived from the neural crest during embryonic development, show stronger nerve regenerative and neuroprotective effects than MSCs derived from other sources (Ibarretxe et al., 2012; Mayo et al., 2014). Furthermore, a study compared three types of dental MSCs (follicle, papilla, and pulp) from a single donor and found that MSCs derived from pulp showed higher neurogenic potential than those from the follicle and papilla (Ullah et al., 2016). Application of conditioned media (CM) or concentrated EVs secreted by dental stem cells have also been found to facilitate nerve repair and regeneration. With a model of polyneuropathy in diabetic rats, DPSC-conditioned media (DPSC-CM) were found to promote neurite outgrowth of dorsal root ganglion neurons and increase the viability and myelin-related protein expression of SCs (Omi et al., 2017). The gene expression of transient receptor potential vanilloid channel-1 (TRPV1), a pain receptor thought to be related to the regeneration of peripheral nerves, can be upregulated after nerve injury, and a TRPV1 antagonist can inhibit the injury process and promote axon regeneration (Ren et al., 2015). However, researchers found that DPSC-CM upregulated neuron-related markers and the gene expression of TRPV1, and promoted the survival and regeneration of isolated primary trigeminal ganglion neuronal cells (TGNCs), thereby repairing damaged trigeminal nerves (Sultan et al., 2020). GMSC-EVs effectively promoted the proliferation and migration of SCs, upregulated the expression of genes associated with the SC repair phenotype, and activated this phenotype to promote the regeneration of peripheral nerves (Mao et al., 2019). In addition, the nerve repairing effect of EVs from other source of cells has also been reported, such as SCs (Ching et al., 2018) or macrophages (Qing et al., 2018) derived EVs. Mechanical stimulation affected intercellular communication between neurons and SCs by changing the miRNA composition of SC-EVs. EVs produced by mechanical stimulation of SCs transferred miR-23b-3p from SCs to neurons, downregulated the expression of neuropilin 1, and promote the repair and regeneration of damaged peripheral nerves to some extent (Xia et al., 2020).

Morbidity associated with oral cancers is high worldwide, and the tongue is the most vulnerable tissue. Currently, the standard method for treating oral cancer is surgical resection of the malignant tumor and involved tissue and reconstruction of the defective tissue with a free flap. Subsequently, postoperative chemotherapy is usually performed. However, using tissue flaps to rebuild the tongue has difficulty in functional restoration, especially taste bud regeneration in the reconstruction area. Thus, this treatment seriously affects patients’ prognosis and quality of life (Mannelli et al., 2018). Therefore, regenerating the lingual papilla and taste buds is the key to successfully rebuilding the tongue. By establishing a critical-sized tongue defect model in rats and combining the transplantation of small intestinal submucosa–extracellular matrix (SIS-ECM) with GMSC-EVs, researchers found that local implantation of exosome/SIS-ECM significantly increased the expression of CK14 and CK8 compared to the SIS-ECM control group, thus enhancing the regeneration of epithelial progenitor cells and facilitating taste bud and lingual papilla restoration (Zhang et al., 2019). Although studies of EV repair of the tongue are rare and have certain limitations, the results suggest that EVs are promising for clinical tongue reconstruction and taste bud regeneration after tongue carcinoma resection (The effects of EVs in dental and maxillofacial tissue repair and regeneration are listed in Table 1).

MSCs regulate the biological properties of local tissue cell by releasing EVs, thereby maintaining tissue homeostasis and regeneration. Regarding wound healing, EVs enhanced the proliferation and migration of keratinocytes, fibroblasts, and endothelial cells (Tutuianu et al., 2021). This effect is often accompanied by upregulation of the PI3K-AKT-mTOR-HIF-1α signaling pathway and blocked by PI3K-AKT inhibitor both in vitro and in vivo (Liu et al., 2021). Another study showed that exosomes mediated the crosstalk between keratinocytes and macrophages in cutaneous wound healing as impairment in miRNA packaging of keratinocyte exosomes inhibited inflammation resolution and compromised functional wound closure (Zhou et al., 2020).

EV-mediated cellular communication between osteoblasts and osteoclasts is a newly discovered mechanism that regulates bone reconstruction. Osteoblasts release EVs containing RANKL and transfer them to the precursor cells of osteoclasts, which then differentiate into osteoclasts by stimulating the OPG/RANKL/RANK signaling pathway, which is critical in bone metabolism (Huynh et al., 2016). Exosomes secreted by SHEDs (SHED-Exos) effectively promoted human umbilical vein endothelial cells (HUVECs) blood vessel formation and osteogenic differentiation in BMMSCs. These effects can be suppressed by an AMPK inhibitor, suggesting that SHED-Exos contribute to bone regeneration by promoting neovascularization and new bone formation through the AMPK signaling pathway (Wu et al., 2019; Liu et al., 2020). In addition, increasing evidence suggests that apoptosis is a crucial link in tissue repair and regeneration, but the exact functions of apoptotic events remain largely unclear (Li et al., 2020). Apoptosis is closely related to cell proliferation (Pellettieri et al., 2010), differentiation (Hristov et al., 2004), and elimination of senescent cells (Jeon et al., 2017), and is essential for the maintenance of normal tissue homeostasis (Gaipl et al., 2005). During apoptosis, BMMSCs produce large amounts of apoptotic cell-derived extracellular vesicles. These EVs activate JNK signal transduction by increasing intracellular reactive oxygen species levels, effectively promoting the proliferation, migration, and osteogenic differentiation of BMMSCs and accelerating new bone formation (Li et al., 2022).

Despite promoting cell proliferation, migration and differentiation, EVs also facilitate the survival of recipient cells. Under pathological conditions such as hypoxia and inflammation, MSC-EVs promote tissue regeneration by maintaining tissue cell survival and preventing apoptosis. For example, EVs suppressed the inflammatory immune microenvironment, which is often enriched by proinflammatory immune cells and cytokines that reduce chondrocytes apoptosis (Yin et al., 2022). Another study revealed that hucMSC-EVs inhibited chondrocyte apoptosis by lowering the m6A level of NLRP3 mRNA with miR-1208 targeting combined with METTL3 (Zhou et al., 2022).

Neurons are a special kind of tissue cells and EVs can also promote nerve regeneration by inhibiting neuronal apoptosis. Researchers have found that BMMSC-EVs prevented hippocampal neuron apoptosis induced by oxygen and glucose deprivation by regulating the JMJD3/p53/KLF2 axis, which caused miR-93 to be transferred into hippocampal neurons (Luo et al., 2022). MSC-EVs reduced apoptosis and inflammation, and promoted angiogenesis after spinal cord injury, and miR-21-5p was one of the most highly expressed miRNAs in these EVs. Moreover, inhibition of miR-21-5p in MSC-EVs significantly reversed the beneficial effects of EVs on motor function and apoptosis, an effect that was associated with modulating FasL expression (Zhou et al., 2019).

Mineralization is one of the most important steps in repairing and regenerating teeth, alveolar bone and the jawbone. Exogenous administration of osteogenic exosomes increased BMMSC mineralization in a dose-dependent manner, and the increase in mineralization was accompanied by upregulation of the expression of osteogenic markers including osteocalcin, bone sialoprotein and RUNX family transcription factor 2 (RUNX2), and increased protein secretion of the same gene products (Swanson et al., 2020). A study showed that mineralizing preosteoblast MC3T3-E1 exosomes had a significant promotion effect on bone marrow stromal cell osteogenic differentiation, as manifested by upregulated expression of the osteogenic marker genes RUNX2 and ALP, as well as enhanced matrix mineralization (Cui et al., 2016). In addition, during dentin formation, calcium mobilization and Ca²⁺ entry across the plasma membrane by odontoblasts are key physiological events. STIM1 is a Ca²⁺ concentration sensor located in the endoplasmic reticulum. When STIM1 expression is upregulated, Ca²⁺ is transferred to the extracellular matrix (ECM), and DPSCs release more EVs carrying minerals and organic matrix. These EVs are capable of promoting the odontogenic differentiation of DPSCs and improving hydroxyapatite nucleation, thus playing a key role in the differentiation of pulp stem cells and the formation and mineralization of dentin matrix (Chen et al., 2021).

Blood vessels transmit mineral substances, growth factors and various progenitor cells to corresponding regions to participate in regenerative activity and help maintain homeostasis. Rapid vascular regeneration and reconstruction provide sufficient oxygen and nutrients for tissue and cells (Grosso et al., 2017). Angiogenesis is also a dynamic process based on vascular endothelial cells (ECs) and their surrounding environment. EC proliferation, migration, and tube formation are the foundations of vascularization (Lv et al., 2020). Liu et al. found that EVs derived from human induced pluripotent stem cells promoted the proliferation, migration and angiogenesis of vascular endothelial cells by activating the PI3K/Akt pathway (Liu et al., 2017). MiR-126, miR-130a, and miR-132 in EVs also play important roles in regulating these processes (Zhu et al., 2018). Huang et al. found that LPS-pretreated exosomes from DPSCs changed the expression profiles of miRNAs and activated MAPK signaling by increasing the expression of Kinase insertion domain receptor (KDR/VEGFR2), and also raised the levels of vascular endothelial growth factor (VEGF) expression, which eventually promoted the proliferation, migration and tube formation capacity of HUVECs in vitro (Huang et al., 2021). In addition, vascular damage through different mechanisms can also be inhibited or even reversed by MSC-derived exosomes. Exosomes play a critical role in repairing DNA double-strand breaks and alleviating oxidative damage. Exposure to MSC exosomes reduced apoptosis caused by radiation-induced DNA damage in vascular endothelial cells (Wen et al., 2016).

EVs have strong anti-inflammatory and immunoregulatory effects. Researchers have shown that MSC-Exos combined with functionalized scaffolds induced innate and adaptive immunomodulatory responses to guide the biological communication network (Su et al., 2021). The number, size and biologically active material of EVs are altered in numerous inflammatory conditions. Moreover, EVs affected the cellular functions of neutrophils, monocytes, macrophages and their precursor hematopoietic stem and progenitor cells (Akbar et al., 2021). TNF-α stimulation increases the number of exosomes secreted from GMSCs and enhances exosomal expression of CD73, thereby inducing anti-inflammatory M2 macrophage polarization (Nakao et al., 2021). DPSC-Exos inhibit CD4⁺ T-cell differentiation into T helper cells and induce the transformation of these cells into regulatory T cells, thus elevating the levels of anti-inflammatory factors and playing an immunomodulatory role. In addition, scholars constructed a rat model of bisphosphonate-related osteonecrosis of the jaw and demonstrated that MSC-EVs prevented aging in stem cells, osteoblasts and fibroblasts induced by zoledronic acid and reduced the generation of inflammatory cytokines, thereby preventing bisphosphonate-related jaw necrosis (Watanabe et al., 2020). Moreover, because of their powerful anti-inflammatory properties, EVs have the ability to promote angiogenesis in an inflammatory environment. Studies have shown that inflammation may increase the number of exosomes secreted by PDLSCs. Exosomes released from inflamed PDLSCs promote angiogenesis in HUVECs by upregulating the expression of the vascular-specific marker CD31 and vascular endothelial growth factor (Zhang et al., 2020).