Stem Cell Therapy for Endometrial Insufficiency
The hallmarks of success in achieving a sustained pregnancy are having both a reproductively competent embryo and endometrium. The endometrium is a dynamic tissue that consists of two distinct cellular components, the surface epithelium, and the stromal cells. It is a very dynamic tissue that undergoes characteristic changes throughout the menstrual cycle, orchestrating a complex symphony of morphological, biochemical and molecular changes. These changes are under the direction of cyclic changes in estrogen and progesterone associated with folliculogenesis and ovulation. In addition, the endometrium goes through repeated rapid growth as it regenerates following its complete sloughing with the end of each monthly cycle.
Unfortunately, our desire to avoid interfering with embryo implantation and early pregnancy restricts our ability to evaluate the endometrium during a treatment cycle. Despite the incredible functional complexity of the endometrium, our assessment of the endometrium is limited the ultrasound measurements of thickness and echogenicity with the former being the most critical factor. While pregnancy rates gradually decline with a thinning endometrium, there is no universally accepted definition of a thin or insufficient endometrium. A common cutoff for a sufficient endometrial thickness is ≥ 7mm to 8mm at time of LH surge or trigger for ovulation. Estimates of the number of patients that have endometrial insufficiency range from 0.6% to 0.8%.
A thin endometrium may be idiopathic or the consequence of uterine trauma following a delivery or surgical procedure, often a dilatation and curettage (D&C). It is often a very difficult problem as there is no single effective treatment. As a last resort, patients with severe, refractory endometrial insufficiency will be advised to use a gestational carrier.
Treatments for endometrial insufficiency have traditionally been directed at supplying additional estrogen or use of medications or supplements to foster improved uterine and or endometrial vascularization. A more recent development in the treatment of this difficulty condition is the use of stem cell therapy.
All of the cells in our body have the same genetic blueprints or DNA. As we develop from early embryos, molecular switches turn on or turn off certain segments of our cellular DNA. These changes result in the cells differentiating into different structures and organs. A stem cell is defined as an undifferentiated cell that is capable of reproducing itself and also of differentiating into different cell types that can produce at least one highly differentiated descendant. The most undifferentiated stem cell is called a totipotent. These stem cells exist in early embryos that are less than four days old. They have the potential to differentiate into any cell type. Pluripotent stem cells can give rise to the cells that derive from any of the three germ layers (endoderm, mesoderm, and ectoderm). These cells exist in the inner cell mass of the early embryo and are embryonic stem cells. Multipotent stem cells are capable of producing a limited number of different cell lines and exist in adults, e.g. bone marrow stem cells (BMSC). Unipotent stem cells can only differentiate into a single lineage, e.g. sperm or epidermal stem cells.
Given the dynamic nature of the endometrium, as early as 1978 it was proposed that endometrial stem cells reside in the basalis layer and serve as a source of the cells that differentiate to regenerate the endometrium. The first direct evidence to confirm this hypothesis came from two separate groups in 2004. Chan et al. published a report that demonstrated the presence of a small population of epithelial (0.22%) and stromal (1.25%) stem cells that are responsible for endometrial regeneration.
Also in 2004, Taylor et al. published a study that revealed that the bone marrow (BM) is an exogenous source of endometrial stem cells. The authors studied four female allogeneic bone marrow transplant (BMT) recipients who received marrow from a single-HLA antigen mismatched related donor, allowing the ability to differentiate donor cells from the patient cells. Endometrial epithelial cells and stromal cells of donor origin were found in the uterus of women who were bone marrow transplant recipients. The percentage of cells of BM origin ranged from 0.3% to 50% with a preponderance found in the stroma. The finding of local clusters originating from donor cells suggests the clonal expansion and differentiation of single BM-derived progenitor cells. Several years later, a similar study by Ikoma et al examined the endometrium from three women who had received bone marrow transplants from a male. They also demonstrated chimeric endometrial glands with 0.6–8.4% of the epithelial cells and 9% of stromal cells containing a Y chromosome. Around the same time several other groups confirmed that the normal human endometrium does, in fact, contain a low number of native mesenchymal stem cells that serve as endometrial stromal stem/progenitor cells.
In 2007, the population of BM-derived stem cells in the endometrium following BMT from a male donor was reported in the mouse model. Interestingly, this report also demonstrated bone marrow stem cells (BMSC) also gave rise to the stem cells responsible for the endometriotic implants of endometriosis. Also in the mouse model, Bratincsak et al. published a report showing that CD45 cells give rise to endometrial epithelial cells. Hematopoietic stem cells produce CD45 cells and their progeny. This activity suggests that circulating CD45 cells may provide a renewable pool of endometrial epithelial precursor cells in the uterus. In addition, the following year, Mints et al. reported that in both humans and mice, BM-derived endothelial progenitors contribute to the formation of new blood vessels in the endometrium.
The obvious clinical implication of these findings is that BMSCs may represent a source of endometrial stem cells that could positively impact patients with endometrial insufficiency. An enhanced migration and grafting of endometrium by BMSCs is also likely to occur after endometrial injury or an inﬂammatory insult. This concept was successfully demonstrated in both the mouse and rat model of uterine injury.
In 2012 Du et al., used a mouse model to infuse BMSCs intravenously after uterine injury. Study results showed that stem cells of BM origin are recruited to the endometrium in response to injury. They also demonstrated that BMSC migration is unlikely to play a role in cyclic endometrial regeneration during the menstrual cycle. Bone marrow stem cells develop into hematopoietic stem cells (HSCs) and mesenchymal stem cells (MSCs). The natural lineage of HSCs are erythrocytes, thrombocytes, and leukocytes. The MSCs, on the other hand, give rise to osteoblasts, chondrocytes, myoblasts, and adipocytes. In this study, the MSCs were the class of BMSCs that were found to respond to uterine injury and migrated to replace and regenerate damaged endometrium. However, it remains unclear if HSCs may also participate in this process.
This month, in the Journal of Reproductive Science, Zhao et al. reported the successful repair of a thin endometrium with BMSCs treatment in the rat model. They created a damaged endometrium by uterine infusion of anhydrous ethanol. They then exposed the damaged uterus to BMSC either through IV infusion or direct installation into the uterus. They found that with either route of infusion, BMSCs are preferentially recruited to the injured endometrium. The BMSCs exhibit a therapeutic effect on the thin, damaged endometrium by exerting protective actions against cell damage and promoting the regeneration of the endometrium.
The first report of use of BMSC to treat a damaged, thin endometrium was by Nagori et al. in 2011. They describe a 33-year-old woman had suffered endometrial damage following a D&C with multiple intrauterine adhesions. Following resection of these adhesions, despite aggressive hormone treatments, her endometrium never reached a thickness greater than 3.2 mm. A bone marrow aspiration was performed from the patient’s iliac crest. An endometrial curettage was performed immediately followed by an ultrasound guided, slow, trans-cervical uterine installation of 0.7ml of BM stem cell suspension. The patient was given oestradiol valerate 6 mg daily, starting on the same day for 25 days. In addition, aspirin 75 mg was started from the same day. The endometrium showed an immediate response to this therapy with an ultrasound documented endometrial thickness of 5.0 and 5.2 mm. Soon afterward, a subsequent embryo transfer treatment cycle with Estrace, achieved an endometrial thickness over 7mm and resulted in a successful ongoing pregnancy.
More recently in 2014, Singh et al. reported a series of six patients with secondary amenorrhea resulting from damaged, thin endometrium, that was refractory to standard treatment options. Using a transvaginal ultrasound guided needle, the BMSCs was implanted in the sub-endometrial zone using the transmyometrial route. A volume of 3 ml of BMSCs was delivered at 2-3 sites (fundus, anterior and posterior part) of the myometrium. Exogenous oral estrogen therapy followed, and researchers assessed endometrial thickness (ET) at three, six, and nine months. Five out of six women started menstruating at three months post-transplant with regular cyclical bleeding. The mean endometrial thickness increased from an average of 1.4mm to 4.3mm in 3 months. At nine months, the maximum endometrial thickness was 6.7mm with an average of 5.5mm.
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