Estrogen secretion increases during the follicular phase of the normal menstrual cycle and binds to estrogen receptors to promote proliferation and regeneration of endometrial epithelial cells (EECs). Estrogen affects embryo implantation via endometrial estrogen receptor α and cytokines such as leukocyte inhibitory factor (LIF) and IL-6 (19–22); thus, it can also positively affect endometrial tolerance.

The concentration, duration of action, and route of supplementation of estrogen may also be related to endometrial tolerance. A retrospective study by Amir et al. found that EMT correlates with serum estradiol levels up to 1000 pg/ml, and that the correlation was not significant >1000 pg/ml (23). It has also been reported that endometrial growth correlates with duration of estrogen action, while there is no significant correlation with estrogen levels (24).

Estrogen supplementation can be administered in several ways, among which the oral route is more convenient and transdermal ingestion has the most pronounced effects. A prospective randomized controlled study by Yi et al. found similar endometrial growth in patients with uterine adhesions after one cycle of oral estradiol valerate tablets versus transdermal gel treatment after hysteroscopy (25). The highest serum drug concentrations and endometrial proliferation rates are obtained with transvaginal estrogen administration, which is recommended after other modes have failed (26). Regarding dosage, patients with TE are not sensitive enough to respond to normal physiologic doses of estrogen because of their relatively low number, and dysfunction, of estrogen receptors. However, when using estrogen to treat TE, the risk of endometrial pathology remains an important consideration (e.g., endometrial cancer, thrombosis). These may result from prolonged, high-dose estrogen use, and the endometrium should also be protected by regular progestin use, except for contraindications. In conclusion, the role of estrogen in promoting endothelial growth remains controversial, and the administration mode, usage, and dosage require further standardization.

GH is a peptide hormone secreted by the pituitary gland, the receptors for which are found on the human endometrium. GH improves EMT, and possibly endometrial tolerance, in women with infertility, possibly by mechanisms such as increased endometrial blood perfusion, increased expression of genes and proteins associated with endometrial tolerance, and induction of insulin-like growth factor 1 (IGF-1) production; however, the exact mechanisms remain to be elucidated (27, 28).

The effectiveness of GH treatment on endometrial quality and function in women undergoing IVF was evaluated in a systematic review and meta-analysis (29). Adjuvant application of GH was found to not only increase the LBR, CPR, and IR, it also improved EMT. Based on subgroup analysis, a dose- and time-dependent relation between GH combination therapy and CPR was also found. Thus, GH may promote fertility by improving endometrial quality.

HCG is a glycoprotein hormone synthesized by gestational trophoblasts and is commonly used as a trigger drug in ovulation promotion. HCG receptor expression is also present on the endometrium during the endometrial secretory phase. Beyond a certain concentration, HCG instilled in the uterine cavity before embryo transfer can bind to receptors to upregulate expression factors including VEGF, LIF, matrix metalloproteinase-9 (MMP-9) (30); these cytokines improve maternal–fetal immune tolerance, upregulating endometrial thickness, and improving pregnancy outcomes.

GnRH-a drugs are the first-line of clinical use in assisted reproduction and endometriosis. After the first application of GnRH-a, it binds to the GnRH receptor in the pituitary gland and instantly stimulates the release of gonadotropins (follicle stimulating hormone and luteinizing hormone) from pituitary cells, resulting in a ‘flare-up’ effect, which raises the serum estrogen level and stimulates the body to produce the cytokines necessary for implantation, thus improving the EMT and increasing endometrial tolerance (31). GnRH-a in combination with human menopausal gonadotropin also enhances the stimulation of follicular development and improves EMT in women undergoing FET after intensified estrogen administration (32). A retrospective study also found that clinical pregnancy and LBRs were significantly higher in the GnRH agonist group than in the GnRH antagonist group (33).

TMX is a selective estrogen receptor modulator. As an antitumor drug, it has both anti-estrogenic and estrogen-like effects, and plays different roles in different target organs, inhibiting breast tumors while promoting endometrial hyperplasia. Combination TMX+hormone replacement therapy (HRT) in patients with infertility with TE (EMT <8mm) increased EMT more than HRT alone; however, differences in clinical pregnancy outcomes (CPR, embryo IR, early miscarriage rate, LBR) did not differ significant between these groups (34).

Clinical studies have found that TMX somewhat improves EMT and LBR and reduces the likelihood of multiple pregnancies in patients with TE who are treated with clomiphene; however, in patients with polycystic ovary syndrome, the ovulation induction by TMX may be inadequate (35). It is also clear that current use of TMX in reproduction is beyond its scope of application, and its safety remains unclear.

Aspirin, a cyclooxygenase inhibitor, does not directly increase EMT, but may improve pregnancy outcomes by improving endometrial morphology, perfusion, and tolerance (36). In a prospective randomized study of aspirin in patients with TE (≤8mm) undergoing intrauterine insemination, aspirin administration improved pregnancy rates and endometrial state; however, it did not significantly increase EMT or improve resistance to uterine and ovarian blood flow (37). Another study showed that supplemental low-dose aspirin after adhesive separation of the uterine cavity for TE may promote endometrial growth and repair, and the combination of low-dose aspirin in patients treated with high-dose estrogens may also help reduce the risk of blood clots (38).

Sildenafil citrate is a specific phosphodiesterase type 5 inhibitor that enhances the vasodilatory effect of nitric oxide on vascular smooth muscle by preventing cGMP degradation. It has been suggested that endometrial thinning may be caused by impedance to high blood flow in uterine arteries, and that vaginal administration of sildenafil citrate may be effective for increasing EMT and CPRs, and improving biochemical pregnancy rates in women with TE (39). A randomized, placebo-controlled trial examining the effect of vaginally administered sildenafil in patients with at least two failed IVF/intracytoplasmic monospermous injection rounds showed that women receiving sildenafil (alone or in combination with placebo) showed a twofold increase in chemical pregnancies compared with the placebo group. This increase was clinically significant, although not statistically significant based on sample size (40).

BoTA is widely used in plastic, reconstructive, and aesthetic surgeries (41, 42). It has also been reported to enhance re-epithelialization of human keratinocytes and angiogenesis of endothelial cells (43). Animal experiments by Koo et al. demonstrated that intrauterine BoTA administration significantly induces endometrial angiogenesis and can enhance endometrial tolerance (44). Further studies found that intrauterine BoTA injection improved the endometrial environment in mice with TE by increasing endometrial tolerance and angiogenesis via the regulatory effect of IGFBP3 on hydrolytic cleavage of OPN proteins; these findings suggest BoTA as a potential therapeutic strategy for patients with TE. However, there is currently insufficient basis for clinical trials, and clinical validation is needed (45).

Vitamin E is a fat-soluble vitamin, the hydrolysis product of which is tocopherol, which increases estrogen levels and improves fertility. Tocopherol is an antioxidant that acts to limit reactive oxygen species (ROS) during oxidative stress. Vitamin E also reduces free radical-induced chromosome damage by inhibiting ROS formation and nucleic acid endonuclease activation. Pentoxifylline (PTX) is a methylxanthine derivative used in the treatment of vascular diseases; it is an antioxidant and promotes vasodilation. In a retrospective study of 143 patients with EMT <7mm and repeated implantation failures, combination tocopherol and PTX treatment significantly improved endometrial volume (EV) thresholds and EMT (46).

Many previous studies have confirmed the positive effects of BMDMSCs on endometrial diseases (47, 48). Cervello et al. found that BMDMSCs accumulated around small blood vessels in the injured endometrium and, through a paracrine manner, up-regulated platelet-responsive protein 1 and downregulated IGF-1 through paracrine means, ultimately leading to proliferation of epithelial glandular cells and thus endometrial thickening (49). By transplanting rat BMDMSCs, Wang et al. were able to perform endometrial repair on model heat-damaged endometrial injury, showing that BMDMSCs helped restore normal endometrial tissue structure, improved fertility, reduced the area of endometrial fibrosis, and improved endometrial tolerance (50).

Patel et al. (48) reported the case of a patient with TE who, despite several attempts at HRT treatment, had suboptimal EMT and failed subsequent IVF. This patient subsequently received BMDMSC therapy via uterine artery injections with interventional radiology, followed by HRT for 3 months, to improve the endometrium. This was followed by donor egg IVF, with EMT 7.1mm achieved before embryo transfer. Treatment outcome was successful conception and delivery of a 3.1 kg male infant without prenatal, intrapartum, or postpartum complications. In a prospective study of 36 patients, Natalya et al. showed significant EMT improvements in patients with TE who were treated with autologous BMDMSCs, demonstrating a good safety profile (51). Though current research favors BMDMSCs as a novel option for patients with Asherman syndrome and recalcitrant TE, it is worth noting that BMDMSC therapy requires bone marrow biopsy and subsequent cell sorting, as well as interventional radiation to gain small uterine artery access; these and other factors make this treatment expensive and potentially harmful. The differentiation ability of BMDMSCs is also related to patient age; thus, more research is needed to guide clinical practice and potential applications of this therapy.

MenSCs have multi-directional differentiation abilities. Due to the noninvasive process of obtaining them, the use of MenSCs to treat endometrial pathology has gradually attracted attention in recent years. One researcher collected menstruation tissue from patients with severe uterine cavity adhesions, cultured them in vitro to isolate these stem cells, and then reintroduced them into the uterine cavity. Seven cases were successfully isolated and reintroduced, and endometrial thickening to 7mm was observed in five of them; among these, one patient was naturally impregnated and four underwent subsequent frozen embryo transplantation, two of whom were successfully impregnated (52). This study suggests that MenSCs are helpful in the treatment of TE.

Further studies have investigated the mechanism of MenSCs in the treatment of TE and found that these cells stimulate the invasion and proliferation of EECs and regulate cytokines (53). Another animal experiment confirmed that human MenSCs differentiated into endometrial cells in vitro can rebuild endometrial tissue after induction with estrogen and progesterone in mice (54). Recently, researchers have converted MenSCs into decidualized stromal cells (DSCs), which are more secretory, and have demonstrated that DSCs promote the restoration of EMT and contribute to the production of glands and blood vessels (55). Currently, MenSC research focuses on animal experiments, but since MenSCs are derived from an individual’s own menstruation tissue, and their production is closely related to the endometrium, clinical application scenarios are worthy of further study. The limitation of this approach is mainly related to difficulty with implementation among amenorrheic women.

ADSCs can be obtained and isolated from subcutaneous adipose tissue (e.g., from human thighs, abdominal wall). ADSCs have the ability of multidirectional differentiation, are relatively available and easy to obtain, have high proliferative activity in in vitro cultures, and have been applied to wound repair and skin tissue regeneration. Adipose-derived stromal vascular fraction (AD-SVF) was obtained by enzyme preparation and centrifugation, which contains adipose-derived MSCs, adipose stromal vasculature, and other components. Lee et al. extracted adipose tissue from one patient with severe Asherman syndrome, then injected their AD-SVF through the cervix into the patients’ uterus with subsequent hormone supplementation therapy, eventually observing a significant increase in EMT (56).

ADRCs were obtained by subcutaneous fat isolation and characterized by flow cytometry. Yotsumoto et al. found that ADRCs improved EMT, endometrial fibrosis, and embryo implantation, and increased VEGF expression in the mouse uterus (57). ADRCs may thus become an effective therapeutic strategy to improve fertility in women with TE.

Despite the additional cosmetic effects of liposuction, the overall cost of this treatment remains high, and its clinical efficacy has yet to be validated in trials with larger samples.

UC-MSCs are derived from discarded neonatal umbilical cord tissue, which are easy to culture in vitro, have the ability of multidirectional differentiation, and have been widely used in stem cell transplantation research because of their low immunogenicity, short proliferation cycle, and long survival time. Zhang et al. treated endometrial injury with single and multiple transplants of human UC-MSCs, finding that human UC-MSCs repaired damaged rat endometrium and improved fertility (58). Furthermore, UC-MSCs have been shown to restore EMT, attenuate endometrial injury-induced hyper fibrosis, promote blood vessel growth and endothelial cell proliferation, downregulate some pro-inflammatory factors, and upregulate some anti-inflammatory factors, which may help to restore uterine structure and function. Despite the advantages of this therapy, its clinical application remains controversial due to its allogenic origin. In a study of patients with uterine adhesions who underwent implantation of UC-MSCs and subsequent estrogen-progestin replacement therapy, endometrial thickening and improvement in uterine adhesions were found after three months, and DNA short tandem repeat analysis showed that no xenogenic nucleic acids were detected in the regenerated endometrial tissue (59).

Endometrial MCSs are thought to exhibit similar properties to BMDMSCs and are multipotent in their differentiation, with the ability to differentiate into fat, bone, cartilage, skeletal muscle, and smooth muscle (60, 61). Sapozhak et al. recently reported a case of autologous endometrial MSCs in a patient with TE and a history of previous miscarriages, which resulted in a live birth (62). After endometrial MSCs and subsequent hormonal supplementation, the patient’s EMT grew to 6.9mm; at the patient’s insistence, an embryo transfer was performed, and two weeks after the transfer her HCG was 4,795 IU/L. Four weeks after the transfer, an ultrasound confirmed a bi-chorionic twin pregnancy, which eventually resulted in the birth of a female infant. This study demonstrates that autologous endometrial MSCs can be effective for assisting in promoting EMT growth, and enabling live birth. However, as this is an isolated case, more clinical validation is needed.