Introduction
Asherman's syndrome was first introduced in 1948 by Joseph Asherman as intrauterine adhesions with the symptoms, such as menstrual irregularities (amenorrhea or oligomenorrhea), dysmenorrhea, infertility, recurrent miscarriage, and abnormal placenta [1]. The incidence of this disease varies depending on the extent of the medical intervention and the incidence of infections, such as tuberculosis, schistosoma and endometriosis in different countries [2]. In general, Asherman's syndrome is a rare condition, although it has been reported in 13% of infertile patients and 7% of women with secondary amenorrhea [3]. In this condition, fibrotic tissues are replaced by the endometrial stromal part and the epithelium of inactivated glands is formed a Cubocolumnar structure, in which the basal and functional layers are indistinguishable and do not respond to hormonal stimulation and also fibrotic adhesions occur throughout the cavity. This tissue usually lacks blood vessels and in severe cases, collagen fibers, fibrous bands or muscles similar to the myometrium may form and endometrial cavity may be lost. Uterine adhesions are characterized with better condition, in which the fibrotic tissue is located on sections of the endometrial surface. Symptoms of Asherman's syndrome vary depending on the degree and location of adhesion, including unusual hemorrhages ranging from hypomena to amenorrhea, infertility, and abortion [4].
Diagnosis
Increasing the number and complexity of uterine surgeries along with the increased awareness and stronger diagnostic techniques have led to the increased incidence of Asherman's syndrome [5]. Asherman's syndrome complications in pregnancy include placental abruption, preterm labor, spontaneous rupture of the gallbladder, recurrent miscarriage, and placenta previa. Obstruction of the ostium and endocervix, decreased uterine cavity size, decreased uterine blood flow, decreased endometrial receptivity and myometrium fibrosis have also been reported as causes of infertility [6]. Curettage was previously used to treat Asherman's syndrome, which is still used in centers with no hysteroscopy. Studies show that curettage has poor therapeutic outcomes and is highly likely to cause uterine perforation [1].
Hysteroscopy is the gold standard in the diagnosis of Asherman's syndrome that it also used for its treatment. Other methods, such as saline infusion sonohysterography are also used, which are less invasive and have many advantages than hysteroscopy, but they cannot be used to treat during diagnosis [7]. The primary goals in the treatment of Asherman's syndrome are reforming the size and shape of the uterine cavity and also stimulating endometrial reforming. Secondary goals include the treatment of related symptoms (such as infertility) and the prevention of recurrence of adhesion [8]. Adhesiolysis using hysteroscopic is considered as the gold standard in the treatment, because it is less invasive and can be performed in full vision [9]. Mechanical, electrical and laser techniques are used in hysteroscopy. The mechanical method using scissors is a common and very cost-effective method. It also does not cause heat damages and for the formed holes, the risk of visceral damage is much less likely than thermal methods [10].
Treatment
Adhesiolysis using hysteroscopic reduces resistance to sub-endometrial blood flow, which leads to endometrial repair and increased endometrial thickness, which is essential for the treatment of infertility [11]. It seems that a three-month period is necessary for endometrial repair after hysteroscopy [12]. In addition to hysteroscopic optimization, outpatient hysteroscopy with fewer complications, such as milder anesthesia, less infection, and less cost and time has replaced the conventional procedure that can be performed in the office [13].
In the next step, its management is done by preventing recurrence of adhesion, which is accomplished in several mechanical ways using balloons and intrauterine devices that help to segregate the surfaces opposite the uterine cavity [14]. Another method is hormone therapy (estrogen), which stimulates endometrial regeneration and proliferation by increasing growth factors [8]. Using gels, such as hyaluronic acid prevents adhesion by preventing contact between the uterine walls [15]. Human amniotic membrane grafting in addition to acting as an anatomical barrier, similar to the basement membrane, prevents inflammation and fibrosis [16]. The next step in disease management is endometrial tissue remodeling, which is normally performed by endometrial stem cells. Endometrial stem cells are responsible for periodic endometrial reconstruction in menstrual cycles affected by periodic migration of estrogen and become a group of precursor cells that differentiate into endometrial epithelial, stromal, and vascular cells.
Due to the role of endometrial stem cells in endometrial regeneration in each menstrual cycle, a decrease in their function or number can also cause low thickness and endometrial irregularity that cannot support the fetus [23]. It also seems that in damaged endometrium or endometrium with adhesion, malfunction of stem cells causes malfunctioning endometrium. Especially in Asherman's syndrome, it is assumed that the absence of endometrial stem cells causes the disease, as the complete removal of the uterine cavity due to adhesion causes amenorrhea and infertility [4, 24].
Cell therapy
Stem cells are undifferentiated cells with the ability to self- renew as well as differentiate into all cell types. Almost all tissues and organs of the body include stem cells. Adult stem cells or tissue stem cells play an important role in the regeneration and repair of tissues. These cells proliferate with asymmetric cell division and eventually differentiate into specific cell lines. The endometrium in humans is highly regenerative, with more than 400 cycles of growth, differentiation and destruction throughout the female reproductive life [17]. In the reproductive phase of the menstrual cycle, the endometrium grows between 4-10 mm due to circulating estrogen in each cycle. Endometrial regeneration also occurs in estrogen-treated postmenopausal women. The level of tissue regeneration is similar to the hematopoietic tissue and the epithelium of the gut and skin. It has long been suspected that endometrial stem cells exist due to the dynamic endometrial function of its regeneration in every menstrual cycle, until mature stem cells were first isolated from the endometrium in 2004 [19]. These stem cells have markers and colonization and differentiation properties similar to mesenchymal stem cells that allow the rapid endometrial regeneration required to maintain pregnancy.
Tissue endometrium consists of basal and functional layers, and it seems that stem cells are present in endometrial functional bases [20, 21]. There are three types of stem cells (mesenchymal, epithelial, and endothelial) in the endometrium that play a key role in maintaining tissue homeostasis and appear to be impaired in diseases associated with endometrial defective proliferation. They also migrate under systemic hormonal changes, such as a cyclic increase in serum estradiol levels and transformed into precursor cells that, in a specific microenvironment, are transformed into specific types of differentiated cells such as epithelial, stromal, and vascular cells [22]. For the first time in 2004, Taylor by recording bone marrow stem cells in patients who received allogeneic stem cell transplants sound that bone marrow-derived stem cells are embedded in the endometrium of the receptor and enhanced its regeneration [25, 26] and it was further shown that bone marrow-derived endothelial progenitors were involved in the formation of new blood vessels in the endometrium [26]. In fact, these tissue-specific stem cells need dynamic support for bone marrow-derived stem cells during periodic endometrial regeneration.
Bone marrow-derived hematopoietic stem cells, mesenchymal stem cells, and endothelial progenitor cells circulate in the blood in small numbers and are located in the affected areas. These cells in new tissue cells carry out angiogenesis or differentiation of differentiated lineages [27]. It has suggested that few bone marrow-derived cells enter the endometrium and become endothelial, stromal, and epithelial cells. An experimental study on the rat model examined the feasibility of endometrial remodeling in Asherman's syndrome by bone marrow-derived stem cells [28]. The murine model of Asherman's syndrome was created by damaging the lumen and then bone marrow stem cells and saline were injected into the control group. Histologic analysis after 3 months showed a reduction in fibrosis and 90% pregnancy rate in the treatment group compared to 30% in the control group. Improved fertility after stem cell transplantation in these mice indicates the functional role of cells in uterine regeneration. Intravenous injection of bone marrow mesenchymal stem cells also resulted in an increase in endometrial thickness compared to the control group. There is also an increase in the number of endometrial glands and veins, suggesting that this change appears to be due to migration of MSCs to the injured areas and play an immunomodulatory role [29]. In addition, another study confirmed the role of cell therapy using bone marrow mesenchymal stem cells in increasing endometrial thickness as well as the significant increase in the number of estrogen and progesterone receptors [30]. Injections of human bone marrow-derived stem cells into mice with mechanically induced Asherman's syndrome also showed that these cells were enclosed around small endometrial vessels [31].
The Role of Stem Cells
In 2015, scientists succeeded in transforming embryonic stem cells into endometrial cells and restoring it by placing on collagen scaffolds and transplanting into the damaged uterus; as following 12 weeks in the treatment group, glands and uterine tissue were observed in the affected areas. Restorative uterine function was also evaluated, so that gestational sac and pregnancy rates were significantly higher than negative control or cell-free collagen control groups. The reconstructed endometrium could protect the fetus until birth [41]. Also, transplantation of autologous bone marrow mononuclear cells on collagen scaffold increased uterine regeneration in a patient with asthma [42].
A prospective study examined the role of autologous bone marrow mononuclear stem cell transplantation in 6 patients with estrogen-resistant chronic Asherman's syndrome [43]. Endometrial thickness was measured 3, 6, and 9 months later, which was significantly increased compared to pre-treatment. Also, 5 patients menstruation was initiated. Taken together, the results of these studies indicate the essential role of bone marrow stem cells in the repair of thin and damaged endometrium. This repair may be due to the incorporation of bone marrow stem cells into the endometrium and differentiation into endometrial epithelium and stroma or stem cells have immunomodulatory effect on activation of endometrial progenitor cells by providing growth factors. The CD133 / VEGFR2-expressing cells from bone marrow-derived cells have endothelial properties that are involved in angiogenesis [44] and they have shown favorable outcomes in the treatment of diseases, such as ischemia, post myocardial infarction, refractory angina and atherosclerosis [45, 46].
In 2016, a prospective, unrestricted prospective study was performed on 16 patients with asthma or endometrial atrophy to evaluate the safety and efficacy of bone marrow-derived CD133 + cells. These cells were isolated by moving the cells into venous blood and injected into the spiral arteries (Arteries). Two months later, the endometrial thickness increased significantly (from an average of 3.4 mm to 6.7 mm). After 3 months, and in some patients after 6 months, uterine cavity status was evaluated by hysteroscopy. All patients showed improvement in disease status and stage, although uterine cavity was not normal in all patients, specifically, patients in stage III reached stage I, and two patients in stage II developed a completely normal uterine cavity, and the rest showed stage I. Moreover, the density of adult veins and duration and volume of menstruation increased in the first trimester [44]. The clinical status of the patients showed significant improvement based on the volume and duration of the period, especially in the first trimester. In addition, this effect decreased gradually from 5 days in the first month to 3 days in the third month and menstrual blood volume declined over three months, with a significant improvement in endometrial thickness from 4.3 mm (between 0.5 to 2.7 mm) to 6.7 mm (between 3.1 and 12 mm), which was statistically significant. CD133 + cells are endothelial precursors that can improve angiogenesis. In these patients, after 3 months of cell therapy, a significant increase in the number of blood vessels and a marked improvement in the epithelial and stromal endometrial layers were observed and no adverse events were reported in this study. In 2017, the researchers conducted a study to compare the therapeutic effect of two types of bone marrow-derived stem cells with endometrial stem cells as well as intrauterine or intravenous injection methods on mice. The results of this study after 3 weeks showed that the best method was systemic injection and bone marrow-derived stem cells were much larger than the endometrial cells in the uterus [47].
Subcutaneous fat is an abundant source of access to vascular stromal cells and adipose tissue stem cells that has provided new therapies for patients with tissue damage. The viability and differentiation of these cells after transplantation as well as their anti-apoptotic and anti-inflammatory effects make these cells a suitable alternative for cell therapy and they have been used to treat multiple sclerosis, brain diseases and orthopedic diseases [39]. Stromal cells include mesenchymal stem cells, pericytes, fibroblasts, and progenitor-endothelial cells. A high percentage of these cells express the CD34 marker, which is associated with angiogenesis [33]. This cellular compound is rich in immunomodulatory and regenerative properties and appears to have the potential to regenerate vascular stromal cells due to its heterogeneity, which simultaneously activates several remodeling mechanisms. In addition, vascular stromal cells contain monocyte and macrophage cells with anti-inflammatory properties. Vascular stromal cells have prominent properties, such as anti-inflammatory, immunomodulatory, angiogenic, and healing properties and by mediating secretion they lead to healing effects, such as wound healing, antioxidant and anti-wrinkle effects. These cells are used in the treatment of degenerative diseases, such as orthopedics and nervous system. Research has shown high immunity to these cells after injection into patients in various ways [32]. These cells have the potential to differentiate into bone, fat, muscle, and chondrocytes. They also produce various growth factors associated with wound healing, angiogenesis, and hematopoiesis that help accelerate wound healing, repair of damaged tissue, and local inflammatory mechanisms. Vascular stromal cells from adipose tissue are very similar to mononuclear cells isolated from bone marrow [35] and they even seems to act better in terms of the differentiation [36], angiogenic potential [37] and the immunomodulatory effects [38]. Due to the requirements and potential for infection in vitro for cell culturing and proliferation, the use of these cells in treatment is easier and less risky. Also, adipose tissue preparation can be done by reliable methods, such as liposuction. In 2017, autologous studies of the use of these cells in autologous treatment of several degenerative diseases were performed and several methods of cell transplantation were reviewed. The results of this study on 676 patients at 5 treatment centers in the United States showed that the use of these cells was generally safe and there were few cases of side effects that were not related to cell therapy [32]. These cells have been used autologously in a number of clinical trials, including improvement of traumatic bone defects, breast enlargement, treatment of fistulas in patients with Crohn's disease, and wound healing in chronic wounds [40]. In 2015, stromal vascular cells were used in the treatment of rodent Asherman's syndrome, which reduced Vascular Endothelial Growth Factor (VEGF) expression by 29% and significantly increased uterine blood flow.
Perspective
By February 2019, four clinical trials for the use of cell therapy for the treatment of Asherman's syndrome or endometrial atrophy have been registered at the US Clinical Trials website. Simon's research team from Spain has expanded its remarkable Phase I study results and obtained authorization to study Phase I / II from the European Medical Agency. Twenty-two patients will be enrolled using autologous CD133 + stem cells to optimize the Advanced Therapeutic Medicinal Product (ATMP) protocol for the safety and efficacy of treatment with regard to endometrial regeneration, reception, uterine microbiome, and long-term fertility outcomes. Due to the advances made, there is hope for the treatment of diseases, such as Asherman's syndrome and thin endometrium. Treatment using stem cells derived from bone marrow, endometrium, menstrual blood, and cord blood can aid endometrial remodeling. In 2017, the European Medicines Agency approved the use of hematopoietic stem cells in the treatment of Asherman's syndrome as an orphan biological drug.
Various methods have been discovered to increase the viability of transplanted cells. A variety of biomaterials, such as scaffolds, hydrogels and lipid nanocarriers can increase drug / cell delivery and lead to increased cell therapeutic efficacy, which are currently undergoing clinical trials.
Conclusion
Given the role of the endometrium in pregnancy and women's health, maintaining the physiological structure and regeneration after injury is crucial. The introduction of hysteroscopy as the gold standard of treatment for related patients has had a significant impact on improving treatment outcomes and pregnancy. However, due to the high percentage of postoperative adhesion, various methods to prevent recurrence have been introduced and investigated. Many studies have recently been performed on stem cells that indicate their positive function as a treatment option.