The homing capacity of MSCs is an important determinant of effective MSC-based therapy (Li et al., 2017; Lin et al., 2017). Homing refers to the process by which MSCs migrate to damaged tissues and promote their recovery. Therefore, enhancing the homing efficiency of MSCs is essential for optimizing the therapeutic outcome of POF. Noory et al. reported the application of MenSC transplantation as a treatment modality in a rat model of POF and observed that MSCs can survive in ovarian stroma at 2 months after MSC transplantation and directly differentiated into GCs (Noory et al., 2019). Experiments from Liu et al., Jalalie, et al., Lai et al., Song et al. and Park et al. also demonstrated that after transplantation, MSCs home to damaged tissue and reach the site of injured ovaries (Liu et al., 2014; Lai et al., 2015; Jalalie et al., 2019; Park et al., 2021). However, studies have also shown that although MSCs have a homing effect, they cannot directly differentiate into oocytes but do localize in the ovarian matrix, secrete various cytokines and improve ovarian reserve function through the paracrine pathway (Takehara et al., 2013; Gabr et al., 2016; Li et al., 2017). A study by Taheri et al. demonstrated that MSC isolated from follicular fluid cultured in human recombinant BMP15 medium may differentiate into oocyte-like cells in vitro, but they did not investigate whether such MSCs can differentiate into oocytes in vivo (Taheri et al., 2021). Therefore, whether MSCs can directly differentiate into oocytes remains unclear, and more in-depth laboratory experiments are still necessary to solve this scientific problem.

Folliculogenesis is an important part of ovarian function, as it provides oocytes for reproduction (Hua et al., 2015). A large number of genes/proteins have been identified to be associated with follicular development, growth, ovulation and atresia processes. It has been reported that PMSC transplantation can increase the secretion of growth factors, angiogenic factors, pleiotropic cytokines, chemotactic cytokines and extracellular matrix proteins, which are all essential for folliculogenesis (Kupcova Skalnikova, 2013). In POF treatment, the widely discussed follicular development related genes are Nanos3, Nobox and Lhx8. Lai et al. proved that SMSC transplantation could reactivate injured mouse ovaries, with increased expression of the folliculogenesis marker genes Nobox, Nanos3, and Lhx8 in the ovaries of SMSC-treated mice (Lai et al., 2014). Kim et al. showed that three-dimensional cultured PDMSC spheres could upregulate the expression level of Nanos3, Nobox and Lhx8, and resume ovulation through regulation of the follicular microenvironment and stimulation of follicular development (Kim et al., 2018). Peng et al. also showed that the mRNA levels of these three genes in POF mice treated with BMSCs were significantly higher than those in the untreated group (Peng et al., 2018). Other follicular development-related genes include Foxo3a and Foxo1. Ding et al. found that UCMSCs on a collagen scaffold can activate primordial follicles in vitro via phosphorylation of FOXO3a and FOXO1, and transplantation of collagen/UCMSCs to the ovaries of POF patients can elevate estradiol concentrations, improve follicular development and increase the number of antral follicles (Ding et al., 2018).

Cytokines are critical regulators of folliculogenesis and ovulation. They contribute to creating an environment supporting follicle selection and growth, regulating cellular proliferation/differentiation, follicular survival/atresia and oocyte maturation (Field et al., 2014). The most important cytokines in POF treatments are TGF-β and IFN-γ. TGF-β superfamily members, including TGF-βs, AMH, activins, inhibins, BMPs and GDFs, impact several stages of follicular development (Trombly et al., 2009; Sanfins et al., 2018). According to Knight et al., the positive TGF-β regulators of preantral follicle growth, include GDF-9 and BMP-15 of oocyte origin, activins of granulosal origin, BMP-4 and BMP-7 of thecal origin and TGF-β from theca and GCs; in contrast, AMH plays a negative role in preantral follicle development (Knight and Glister, 2006). However, the existing research conclusions are not consistent with each other. El-Derany et al. transplanted BMSCs to a γ-ray induced POF rats model and reported that BMSCs recovered the folliculogenesis process, upregulating Foxo1, Gdf-9 and Fst gene expression accompanied by downregulating TGF-β (El-Derany et al., 2021), whereas Song et al. and Yin et al. found that MSC transplantation could increase the level of TGF-β and decrease the level of IFN-γ in POF models (Yin et al., 2018a; Song et al., 2018). Additionally, Ling et al. reported that amnion-derived mesenchymal stem cell transplantation can inhibit granulosa cell apoptosis and that the expression levels of AMH were significantly increased in the treatment group compared to the POF group (Ling et al., 2017). Zhang et al. and Mohamed et al. also found that after MSC transplantation, AMH expression in ovarian tissue was significantly higher than that in the POF group (Mohamed et al., 2018; Zhang et al., 2018).

Although the mechanism of MSCs on follicular development is not completely clear, most research agrees that MSC transplantation can promote the development and formation of primordial follicles, eggs and reduce the apoptosis of GCs. All of the involved genes and their correlated mechanisms are listed in Table 2.

Multiple studies have shown that MSCs can be induced and differentiate into PGCs. Fang et al. and Li et al. proved that CD61 could promote the differentiation of ADMSC into PGC-like cells through activation of the TGF-β pathway (Li et al., 2016; Fang et al., 2017). Wei et al. found that AMSC can be induced into PGC-like cells by BMP4 (Wei et al., 2016). Ge et al. found that when hfSDSCs were cultured in porcine follicle fluid, they may differentiate into both male and female germ cell-like cells (Ge et al., 2015). Park et al. proved that female mouse skin-derived stem cells could differentiate into ovarian-cell-like cells that are consistent with female germ, and ovarian follicle somatic cells. When ovarian cell-like cells are transplanted into ovariectomized mice, they restore the estrus cycle and serum estradiol levels (Park et al., 2014). Unfortunately, no in vivo research has reported whether MSC-differentiated germ cells can be fertilized and form embryos, and studies in this area are still lacking.

OGCs are the most important stromal cells in the ovary, providing necessary nutrition for oocyte development and follicle maturation, participating in the regulation of gonadotropins that modulate oocyte development and maintaining the microenvironment of oocyte maturation through autocrine and paracrine mechanisms. GCs play an important role in all developmental stages of follicles. GCs abnormalities can lead to abnormal hormone secretion, follicular development disorders and even follicular atresia (Lai et al., 2014). Chemotherapy induces GC apoptosis by damaging DNA and activating apoptosis pathways, thus leading POF. Therefore, enhancing GC function and inhibiting GC apoptosis may effectively prevent POF (Bedoschi et al., 2016). Studies have shown that GCs and MSCs express some similar surface markers (Dzafic et al., 2014; Maleki et al., 2014). Transplanted MSCs are mainly located in the GC layer around follicles, suggesting that MSCs have a significant effect on follicle formation and ovulation (Manshadi et al., 2019).

MSCs can inhibit GC apoptosis and promote GC proliferation by releasing cytokines and hormones, upregulating proliferation-related genes and inhibiting apoptosis-related genes (He et al., 2018; Wang et al., 2020). Zhang et al. showed that PMSC transplantation could upregulate the expression of AMH and FSHR in GCs of POF mice, inhibit GC apoptosis and follicular atresia, and thus restore ovarian function (Zhang et al., 2018). Fu et al. also found that BMSC transplantation may reduce GC apoptosis and improve ovarian function by releasing VEGF, HGF, and IGF-1 and upregulating Bcl-2 expression (Fu et al., 2008). Ding et al. showed that coculturing of AMSCs and GCs might inhibit the apoptosis of GCs, and transplantation of AMSCs may improve ovarian function during natural aging by secreting HGF and EGF (Ding et al., 2018).

The underlying mechanism of MSC treatment of POF may be related to exosome-mediated microRNA modulation. Multiple studies have highlighted the potential therapeutic advantages of using exosomal miRNAs from MSCs for the treatment of various diseases and injuries, including POF. Yang et al. demonstrated that BMSC-derived exosomes prevent ovarian follicular atresia in POF rats via the delivery of miR-144-5p, which can decrease GC apoptosis by targeting the PTEN pathway (Yang et al., 2020). Xiao et al. found that miR-146a and miR-10a are rich in exosomes secreted by AFSCs. miR-146a can restore ovarian function by downregulating IRAK1 and TRAF632 expression and miR-10a can inhibit GC apoptosis and prevent follicular atresia by suppressing Bim and caspase-9 expression (Xiao et al., 2016). Sun et al. found that exosomes derived from UCMSCs may prevent and treat chemotherapy-induced OGC apoptosis in vitro by upregulating the expression level of Bcl-2 and downregulating the expression levels of caspase-3, Bax, cleaved caspase-3 and cleaved PARP (Sun et al., 2017). miR-21 is related to apoptosis. Studies have shown that MSC treatment suppresses the expression of PTEN and PDCD4 through upregulation of miR-21 and inhibiting the apoptosis of GCs (Fu et al., 2017). Sun et al. reported that miR-644-5p carried by MSC exosomes could regulate p53 signaling and inhibit GC apoptosis (Sun et al., 2019).

The establishment and remodeling of the ovarian vascular system is the basis of ovarian development and functional recovery. The follicles and corpus luteum can obtain nutritional support through ovarian blood vessels and transport hormones to target organs. Some researchers observed the distribution of BMSCs in ovaries by labeling specific markers of BMSCs and found that BMSCs were mainly distributed in the blood vessels of damaged ovaries (Liu et al., 2014), implying that BMSCs may play a role in ovarian blood vessels construction. Angiogenesis-related factors secreted by MSCs, such as VEGF, HGF, IGF and FGF, are increased in MSC-transplanted POF ovaries. VEGF and HGF have a synergistic effect and synergistically promote angiogenesis (Golocheikine et al., 2010). The combination of VEGF and HGF leads to an increased vascular diameter (Beilmann et al., 2004); VEGF promotes the length, area and branch point number of the induced vessels, while HGF contributes to vascular area growth (He et al., 2018). Wang et al. showed that MSCs could promote ovarian angiogenesis and reduce interstitial fibrosis by secreting VEGF, IGF-1, GCSF and HGF (Wang et al., 2017). Xia et al. demonstrated that MSC transplantation could enhance the expression levels of VEGF, FGF2 and angiogenin, significantly stimulate neovascularization and increase blood perfusion of the grafts in ovarian tissue (Xia et al., 2015). Zhang et al., Cho et al. and Park et al., also proved that MSC transplantation could repair damaged POF ovaries and promote ovarian development and function through angiogenesis (Zhang et al., 2017; Park et al., 2019; Cho et al., 2021).

Microvesicles are cell-derived membrane and cytoplasmic components. There are three subtypes of EVs: exosomes, microvesicles and apoptotic bodies. Exosomes and microvesicles can transfer mRNA, protein and lipids to target cells through surface-expressed ligands and surface receptors, thus affecting the phenotype and function of the target cells (Bidarimath et al., 2017). EVs have a therapeutic effect on female reproductive disorders, such as repairing injured endometrium, suppressing fibrosis of the endometrium, regulating immunity and anti-inflammation, and repressing the apoptosis of GCs in ovaries (Liao et al., 2021). Several studies have shown that MSC-derived microvesicles contain multiple pro-angiogenic proteins, such as VEGF and HGF (Merino-Gonzalez et al., 2016; Pakravan et al., 2017; Han et al., 2019; Shi et al., 2019). Yang et al. showed that UCMSC microvesicles transplantation in POI mice could induce angiogenesis by activating the PI3K/Akt signaling pathway and improve ovarian function (Yang Z. et al., 2019). Sun et al. found that miR-644-5p carried by BMSC-derived exosomes inhibited the apoptosis of ovarian GCs by targeting the p53 pathway (Sun et al., 2019); Zhang et al. also found that UCMSC-derived microvesicles can inhibit the apoptosis of GSs by downregulating the expression level of caspase-3 and upregulating the ratio of Bcl-2/Bax (Zhang J. et al., 2020).

POF is an autoimmune disease. Autoimmune dysfunction is one of the most important pathogeneses of POF, causing inflammatory reactions of the ovary, destroying the ultrastructure of follicular cells (such as zona pellucida damage, gap link rupture and mitochondrial swelling), causing apoptosis of ovarian cells, affecting the maturation and atresia of follicles and inducing a decline in ovarian function (Nelson, 2001; Luo et al., 2017). It has been reported that certain types of immune cells will expand in ovaries with POF and infiltrate into the ovarian tissue, indicating that they are involved in the inflammation associated with POF (van Kasteren et al., 2000; Chernyshov et al., 2001; La Marca et al., 2010; Wang et al., 2018).

The anti-inflammatory effect is a critical mechanism by which MSCs restore ovarian function. MSCs may inhibit the activation and proliferation of lymphocytes, inhibit the secretion of proinflammatory cytokines, inhibit the function of antigen-presenting cells, and convey regulatory messages to immune cells (Zhou et al., 2019). In contrast, since the ovaries of most POF patients are in inflammatory conditions, the presence of inflammatory cytokines is also crucial for the regulation of MSC immunological and regenerative functions. Beldi et al. proved that the tumor TNF-α-TNFR2 axis is necessary for MSCs to produce anti-inflammatory mediators (such as IL-10, TGFβ and NO) and sustain regenerative functions such as wound healing, complex tube formation and endothelial pro-angiogenic support (Beldi et al., 2020a; Beldi et al., 2020b). IFN-γ and MSCs have a synergistic effect on immunosuppression. They upregulate PGE2, HGF, IL-6 and TGF-1 in MSCs and induce MSCs to express IDO, promoting GC proliferation and increasing the number of follicles (Najar et al., 2016; Liang et al., 2018). Yin et al. showed that the level of proinflammatory IFN-γ increased and the level of anti-inflammatory TGF-β decreased in POF mice, whereas PMSC transplantation reversed this situation and improved ovarian function (Yin et al., 2018a). A study also showed that PMSCs increase the secretion of IL-10 by inhibiting NF-κB-mediated pro-inflammatory reactions and thus promote tissue repair (Wang et al., 2016).

Immune cells (Treg cells, NK cells, Th cells, etc.) are important pathogenic factors in several models of autoimmune diseases (Alvarez Arias et al., 2014; Gianchecchi et al., 2018; Zhang X.-M. et al., 2020; Sakaguchi et al., 2020). These results indicate that the interaction of MSCs and immune cells plays a critical role in regulating the inflammatory microenvironment of POF. Yin et al. showed that PMSC transplantation might restore the ovarian function of POF mice by balancing the ratios of Th17/Tc17 and Th17/Treg cells (Yin et al., 2018b). Lu et al. reported that the serum levels of IL-2 and IFN-γ secreted by Th1 cells increased, while IL-4 secreted by Th2 cells decreased in POF mice; however, after UMSC transplantation, the amounts of these cytokines were reversed (Lu et al., 2019). Yin et al. showed that UCMSC transplantation into POF mice upregulates the ratio of CD8⁺ Treg cells, which have a typical immunosuppressive function and can reduce immune rejection (Su et al., 2014; Yin et al., 2020).

Oxidative stress is a phenomenon of imbalance between the oxidative system and the antioxidant system caused by excessive ROS produced in cells. Reduction of ROS can protect the structure and function of ovarian mitochondria, increase the levels of antioxidant and antiapoptotic enzymes, and reduce apoptosis and oxidative damage of the ovary (He et al., 2018). Abumaree et al. indicated that cocultured PMSCs could reverse the destructive effect of OS on H₂O₂-treated endothelial cells and increase cell proliferation and migration (Abumaree et al., 2017). One study showed that ROS inhibit the expression and activity of TERT and induce POF (Jiang et al., 2018). MSCs can increase the production of antioxidant enzymes and inhibit ROS production through secretion of HGF, IL-6, IL-8, VEGF, BDNF and LIF and activation of the FOXO, NOQ1/MAPK, PI3K/Akt and Nrf2-ARE pathways (Amoroso et al., 2017). One study indicated that fMSCs upregulate MT1, JNK1, PCNA and AMPK levels and enhance antioxidant effects (Huang et al., 2019). Recently, it has been found that PMSC transplantation can reduce the levels of UCP-2, SOD1, reactive oxygen species and 8-hydroxydeoxyguanosine in POF rats, improving mitochondrial function in vivo, inhibiting oxidative stress and improving ovarian function (Zhang et al., 2016).

Using MSCs to treat POF is a sophisticated project. To better understand the mechanism by which MSCs improves ovarian functions, we summarized the cytokines and regulatory factors involved in the homing effect, follicular development, cell proliferation/apoptosis, angiogenesis, immunomodulation and oxidative stress processes, as shown in Table 3.