Mesenchymal stem cells for restoring endometrial function: An infertility perspective

Abstract Background Mesenchymal stem cells (MSCs) can be derived from several tissues such as bone marrow, placenta, adipose tissue, or endometrial tissue. MSCs gain a lot of attention for cell‐based therapy due to their characteristics including differentiation ability and immunomodulatory effect. Preclinical and clinical studies demonstrated that MSCs can be applied to treat female infertility by improving of the functions of ovary and uterus. This mini‐ review focuses on the current study of treatment of endometrial infertility by using MSCs. Methods The present study performed a literature review focusing on the effect of MSCs for treatment of women infertility caused by endometrial dysfunction. Results Bone marrow‐, umbilical cord‐, adipose‐, amniotic‐, and menstruation‐derived MSCs enhance endometrial cell proliferation, injury repairs as well as reducing scar formation. The beneficial mechanism probably via immunomodulatory, cell differentiation, stimulates endometrial cell proliferation and down‐regulation of fibrosis genes. The major advantage of using MSCs is to improve endometrial functions resulting in increased implantation and pregnancy. Conclusions MSCs exhibit a potential for endometrial infertility treatment. Adipose‐ and menstruation‐derived stem cells show advantages over other sources because the cells can be derived easily and do not causes graft rejection after autologous transplantation.

derived from cord blood and placenta showed some variation according to the donor. 6 Therefore, according to the minimal criteria proposed by the International Society for Cellular Therapy (ISCT) to define human MSCs are including, (a) under the standard culture condition, MSCs must grow adherently to surface of the culture vessels, (b) MSCs exhibit the phenotype similar to those fibroblast cells, (c) MSCs express at least CD73, CD90, and CD105 while lack expression of CD34, CD45, CD14, CD19, and HLA-DR, 7,8 and (d) MSCs enable to osteogenic-, chondrogenic-, and adipogenic differentiation. 7 Interestingly, MSCs have demonstrated the ability to differentiation into cardiomyocytes, 9 hepatocytes, 10 and pancreatic secreting cells. 11 Despite of the differentiation ability, the immunomodulatory capacity makes MSCs as a candidate cell type for immune therapy. [12][13][14] The immunomodulatory mechanism of MSCs involves with migration of MSCs to the area of injury or inflammation, follows by interaction with immune cells resulting in suppression or stimulation of innate and adaptive immune system of the host. 15 In addition, the ability of MSCs to downregulate of MHC class II upon allogeneic transplantation makes MSCs suitable for cell-based therapy without the risk of immune rejection. 16 With the advantage of MSCs, the first MSC drug was developed for immunomodulatory treatment of graftversus-host disease (GVHD). 17 Recently, the researchers in China have demonstrated the beneficial effect of MSCs after treatment of patients suffering from respiratory complication due to  infection. 18,19 COVID-19 patients received intravenous infusions of MSCs and it improves the patient outcomes, indicates that the MSC treatment is safe and promising.
Therefore, several studies are focusing on using of MSCs as a high potential source of stem cells for regenerative medicine and immunomodulatory therapy in several diseases including infertility.

| Endometrium
Human uterus is anatomically divided into three layers including perimetrium, myometrium, and endometrium. Endometrium locates at the innermost area of uterus which can be divided into two sublayers, the basal layer which connects to myometrium and the functional layer which is facing to the cavity of uterus. 20 Endometrium is dynamically regenerated in the response to reproductive hormones.
The regeneration of endometrium is influenced by the fluctuation of major hormonal levels including estrogen and progesterone, resulting in menstrual cycle. 21 The menstrual cycle begins with the ovaries produces and releases estrogen.
Estrogen stimulates the endometrial cells to proliferate resulting in thicken of the functional layer. During this phase, the vessels and the endometrial glands are increasing their sizes. Later, in the mid of menstrual cycle, the peak of the estrogen level occurs followed by releasing of the oocytes so called ovulation. Following the ovulation, the level of progesterone begins to increase. Increasing of progesterone level activates the function of endometrial layer in order to prepare for implantation of the embryo. However, if the implantation does not occur, the levels of estrogen and progesterone will decline and followed by the shedding of endometrial lining. This shedding of endometrial tissue is recognized as menstruation. The cycle of menstruation is completed by finishing of shedding process and subsequently the new cycle begins. Therefore, regeneration of both functional and basal layers of endometrium importantly involves in menstruation and embryo implantation.
The process that the blastocyst embryo interacts with the functional layer of uterus is known as implantation. The process begins with the embryo attaches to the epithelial surface of endometrium, invades the epithelial surface, and proceeds decidualization. In general, successful clinical pregnancy depends on synchronization of blastocyst embryos and endometrial receptivity with optimal decidualization. 22 Decidualization is the process in which the stromal cells of endometrium change their morphology in order to preparing the suitable conditions for embryo implantation. 22 In human, however, decidua is formed routinely in relation to menstruation and shed off if the embryo implantation does not occur. 23 In contrast, in case of fertilization followed by implanting of embryo, the decidua stroma remains cotact. 24 It is generally accepted that decidualization plays a key role in embryo implantation and maintenance of pregnancy.
During the process of decidualization, there are cellular and molecular changes occur in the endometrial layer include, increasing of oxidative stress resistance, modulating a local inflammatory response resulting in allowing of trophoblast invasion. In contrast, diminishing of decidualization causes some reproductive problems, such as failure of implantation, miscarriage, and dysfunction of uterus. 25 Interestingly, low implantation rate after transferring of high-quality embryos in infertile patients emphasizes the importance of a diminished decidualization. Therefore, impairment of decidualization is one of a major causes of pregnancy failure and infertility. 26

| Infertility
Infertility often creates problems for the couples, and the women are frequently blamed for the cause of problem. 27 In a larger scale, however, infertility involves in decreasing of birthrate in many country and influences the national policy, especially in the country that is facing aging society. Infertility is clinically defined as a disease of the reproductive system, failure to achieve a clinical pregnancy after 12 months of having regular unprotected sexual intercourse. 28 According to World Health Organization (WHO), infertility can be classified into primary and secondary infertility. Primary infertility is referred to a woman who has never been pregnant and secondary infertility is referred to a woman who has had at least one successful pregnancy but later experiences the incapability to pregnant. 29 Mascarenhas and colleagues (2012) 30 reported that globally, 19.2 million out of 48.5 million of infertile couple suffered from primary infertility and 29.3 million infertile couple were suffered from secondary infertility. Interestingly, primary infertility was a common cause for couple in the developed countries, whereas in developing countries, secondary infertility was found to be a major cause. 30 Despite of the country factor, approximately 25% of infertility has been attributed to male, whereas 58% to female, and 17% to unexplained cause. 31 Focusing on female factor, female infertility may cause by disturbances of reproductive organs or central nervous system that secrets hormones controlling the reproductive system.
The abnormalities including anovulation, fallopian tube disease, pelvic adhesions, endometriosis, and unexplained factors are reported as the cause of female infertility. 32 However, it is generally accepted that main factor affecting female fertility is an age. Theoretically, woman is born with limited number of oocytes. Female fertility dramatically declines after the certain age, due to the degeneration rate of follicles in the ovaries, increasing of abnormal mitochondria and chromosomal abnormalities in oocytes as well as increasing the incidence of miscarriage. 33 Treatment of female infertility is still complicated; therefore, novel treatment is needed to be developed and evaluated for safety and efficiency.

| Female infertility caused by endometrium diseases or abnormalities
As mentioned in the previous section, several factors cause female infertility including anovulation, fallopian tube disease, pelvic adhesions, endometriosis, and unexplained infertility. 32 However, this review highlights only infertility that caused by endometrium dysfunction.
Fibroids or scaring in the uterine tissue can deform the uterine cavity resulting in difficulty for implantation. The major causes of scarring or adhesion include previous uterine surgery or infection of the endometrium. Intrauterine adhesions (IUAs), also known as Asherman's syndrome, caused fibrous strings connecting inside the uterine wall. Damage of the endometrial layer can lead to scar formation resulting in partially or completely obstructs the uterine cavity. 34 Several abnormalities of the reproductive system including hypomenorrhea, amenorrhea, infertility, and recurrent miscarriages can be found in patients with IUAs. 34 To date, numerous treatments such as hormonal treatment, insertion of an intrauterine device (IUD), or barrier gels can be applied for treatment of IUAs. 34,35 Interestingly, existing of stem cells in the basal layer of endometrium is probably the key factors for treatment of IUAs by stem cell-based therapy. 36 Endometriosis is a disease with unknown cause, characterized by ectopic endometrial implants throughout the pelvis, and negatively impacts fertility. 37 Some patients that develop minimal to mild endometriosis have an almost normal probability of pregnancy but some experience infertility for unknown reasons. In cases of moderate and severe endometriosis, more scar adhesions occur and interrupt the movement of oocyte to the fallopian tube, causing the reduction of natural conception. As a result, endometriosis may lead to an abnormal implantation resulting in subfertility. 38 Congenital mesonephric anomalies and Mullerian malformation results in uterine abnormalities. Anatomical abnormalities of female reproductive organs including duplication of uterus, cervix and vagina, failure of forming the uterus, and abnormalities of uterine structures cause serious clinical symptoms, affects the quality of life, and creates the fertility problems. 39,40 Endometrial thickness can be used to predict the endometrial receptivity. 41 Endometrial thickness is clinically defined as the distance between the myometrium and endometrium which can be measured by ultrasonography. 42 Normally, the endometrial thickness shows different ranges according to the uterine cycles. In menstrual phase, the thickness ranges from 1 to 4 mm, mid-proliferative phase ranges from 4 to 8 mm, late follicular phase ranges from 8 to 14 mm, and secretory phase ranges from 7 to 14 mm. 43,44 Slow proliferation of uterine epithelium and stroma cells can lead to thin endometrium. Several pathological conditions such as Asherman's syndrome, previous intrauterine surgery, pelvic radiation have been reported to be the causes of thin endometrium. 45,46 Theoretically, thin endometrium can be improved by regeneration of endometrial stromal and epithelial cells resulting in improvement of implantation. 45 Some reports suggested that the endometrium which represents the thickness more than 17 mm is related with improving pregnancy rate. 47,48 However, it has to be considered that thickened endometrium is associated with fibroid or polyps which have adversely affect the implantation of embryo. 49 In order to solve the infertility caused by thin endometrium, several methods for endometrial preparation have been described. It is still in a premature stage to conclude for the best procedure for improving implantation of thin endometrial patient.

| Current application of mesenchymal stem cells for infertility caused by endometrial dysfunction
Stem cells exhibit unique characteristics of self-renewal and differentiation potential. Recently, cell therapy using stem cells becomes a novel treatment for several diseases including infertility. However, only the uses of MSCs for treatment of endometrial infertility are highlighted in the present review.

| Bone marrow-derived stem cells
Bone marrow-derived stem cells (BMSCs) can be differentiated into many cell types such as skeletal myoblasts, hepatic epithelium, neuroectodermal cells, and endometrial cells. 50,51 BMSCs display immunoregulatory properties. 52 In addition, it has been demonstrated that BMSCs can secrete several growth factors such as hepatocyte growth factor, platelet-derived growth factor, and transforming growth factor-β. 53  Strikingly, some patients established pregnancy without any medical intervention after treatment.

| Adipose-derived stem cells
Adipose-derived stem cells (ADSCs) can be isolated from adipose tissue. ADSCs have been intensively examined for the therapeutic ef-

| Umbilical cord-derived stem cells
Human umbilical cord-derived stem cells (hUC-MSCs) can be iso- these results indicate a promising future for IUA patients, but also prove that tissue engineering can be applied with hUC-MSCs in order to increasing their therapeutic efficiency. 72

| Amniotic tissue-derived stem cells
Human amniotic tissue is a postnatal membrane, normally discarded after delivery. Human MSCs can be isolated from amniotic tissue called human amniotic-derived stem cells (hAMSCs). Interestingly, human amniotic epithelial cells (hAECs) which are the epithelial cells derived from amniotic tissue exhibit multipotential differentiation, immune-regulating potential, and low tumorigenicity similar to hAM-SCs and other type of MSCs. 73 Therefore, hAECs have a potential in cell-based therapy.
In an IUA mouse models, Li et al 73 showed that endometrial morphology was improved after transplantation of hAECs. hAECs

| CON CLUS I ON S AND FUTURE PER S PEC TIVE
MSCs have exhibited a great potential for treatment of infertility

ACK N OWLED G EM ENT
This study is supported by the grants from Faculty of Medicine, Srinakharinwirot University (MED-RES-200).

CO N FLI C T O F I NTE R E S T
There are no conflicts of interest to declare.

H U M A N/A N I M A L S R I G HTS
This article was performed without having any human or animal subjects.