**3. Recurrent implantation failure**

As one of the most intricate and sensitive processes that takes place in the human body, the relative inefficiency of implantation is ironic given that continuous reproduction is critical to species survival. For those seeking fertility treatment, recurrent implantation failure remains a frustrating and difficult possible underlying cause to an otherwise unexplained diagnosis of infertility or in conjunction with another inhibitory diagnoses. While there is no universal definition for recurrent implantation failure (RIF) despite multiple publications on the topic, broadly speaking, Das et al. defined it as "the repeated transfer of morphologically good embryos to a normal uterus without achieving successful implantation or clinical pregnancy" [29].

Ordinarily, the probability that an embryo will successfully implant is about 30% [30]. This means there is a 70% chance of implantation failure. In these cases, implantation failure may be due to one of two factors: inadequate uterine receptivity and/or problems with the embryo itself. When it comes to selecting good quality embryos, few objective methods of embryo assessment exist. Most rely on embryo morphologic grading, a subjective assessment of embryonic development based on the expansion and quality of the inner cell mass and trophectoderm (see **Figure 4**). Embryo grading has long stood as the gold standard of embryonic assessment for quality and to this day continues to be a reliable indicator of embryonic competence.

Over the last decade, preimplantation genetic testing has made its way onto the market as a means of objective chromosomal evaluation of the embryo. Preimplantation genetic testing for aneuploidy (PGT-A) requires a small biopsy of a few cells from the developing embryo. Currently, this is most often done during the blastocyst stage, in which a small biopsy is taken from the cells of the trophectoderm. Yet, biopsies can also be performed at the blastomere stage, and new research suggests improved efficacy when the biopsy is taken from the inner cell mass of the blastocyst or the spent media culture where the developing embryo has grown in vitro [31, 32]. From there, chromosomal evaluation is done to attest for the number of chromosomes present, with the assumption that euploid embryos (blastocysts with a normal 46 chromosome count) are healthy and deemed optimal for embryo transfer. However, PGT-A is expensive, costing patients thousands of dollars, and imperfect, where many embryos often result as mosaic (some cells have normal chromosomal count, and some do not) or "undetermined."

Yet, for those who opt to undergo PGT-A and continue to suffer the loss of RIF, asynchrony in uterine receptivity is most often the cause. A displaced window of implantation may be caused by a variety of factors, including abnormal cytokine and hormonal signaling, among other things, in which the endometrium is not prepared to accept a blastocyst at the otherwise appropriate time. Thus, evaluation

**101**

**Figure 4.**

*Implantation: Cross Talk of the Developing Embryo and Endometrium*

of implantation markers via an endometrial biopsy taken at the time of supposed implantation may be the key to predicting pregnancy outcome and adequate

*Visual of embryonic development and morphologic grading according to blastocyst expansion and cellular* 

Over the last decade, advancements in technology have made it possible for us to evaluate the window of implantation and diagnose displacements in one's uterine cycle activity. Of the numerous tests that exist, including the window of implantation test, which uses reverse transcriptase PCR analysis of endometrial tissue, and the endometrial function test, a histologic analysis of endometrial sampling, the most well-known example remains the endometrial receptivity array (ERA) by Igenomix. ERA utilizes next-generation sequencing (NGS) to analyze the RNA composition of the endometrial tissue to detect for expression and suppression of known endometrial biomarkers characteristic of the window of implantation. These biomarkers include LIF, MUC16, as well as various integrins and cytokines mentioned earlier in this chapter [33]. Diagnosis of a displaced window of receptivity is based on the RNA expression and transcriptomic signature found within the endometrial cell sampling. Based on this genetic analysis, technology is then able to diagnose whether or not the endometrium is receptive, pre-receptive, postreceptive, or non-receptive. **Figure 5** is a display of the transcriptomic signature of

progesterone administration for optimal uterine.

**4. Endometrial receptivity testing**

*differentiation receptivity at embryo transfer.*

*DOI: http://dx.doi.org/10.5772/intechopen.90748*

*Implantation: Cross Talk of the Developing Embryo and Endometrium DOI: http://dx.doi.org/10.5772/intechopen.90748*

### **Figure 4.**

*Innovations in Assisted Reproduction Technology*

**3. Recurrent implantation failure**

embryonic competence.

If implantation does not occur, either due to asynchrony of the previously described events or the lack of fertilization of the oocyte, the onset of menses is initiated, and a new uterine cycle begins. In order for progesterone secretion during the uterine secretory phase to be maintained, the corpus luteum must receive a positive feedback, and it must be received from an implanted and developing embryo via human chorionic gonadotropin hormone (hCG). Without the presence of hCG, the corpus luteum begins to degenerate after about 10 days, resulting in a decrease

As one of the most intricate and sensitive processes that takes place in the human body, the relative inefficiency of implantation is ironic given that continuous reproduction is critical to species survival. For those seeking fertility treatment, recurrent implantation failure remains a frustrating and difficult possible underlying cause to an otherwise unexplained diagnosis of infertility or in conjunction with another inhibitory diagnoses. While there is no universal definition for recurrent implantation failure (RIF) despite multiple publications on the topic, broadly speaking, Das et al. defined it as "the repeated transfer of morphologically good embryos to a normal uterus without achieving successful implantation or clinical pregnancy" [29]. Ordinarily, the probability that an embryo will successfully implant is about 30% [30]. This means there is a 70% chance of implantation failure. In these cases, implantation failure may be due to one of two factors: inadequate uterine receptivity and/or problems with the embryo itself. When it comes to selecting good quality embryos, few objective methods of embryo assessment exist. Most rely on embryo morphologic grading, a subjective assessment of embryonic development based on the expansion and quality of the inner cell mass and trophectoderm (see **Figure 4**). Embryo grading has long stood as the gold standard of embryonic assessment for quality and to this day continues to be a reliable indicator of

Over the last decade, preimplantation genetic testing has made its way onto the market as a means of objective chromosomal evaluation of the embryo. Preimplantation genetic testing for aneuploidy (PGT-A) requires a small biopsy of a few cells from the developing embryo. Currently, this is most often done during the blastocyst stage, in which a small biopsy is taken from the cells of the trophectoderm. Yet, biopsies can also be performed at the blastomere stage, and new research suggests improved efficacy when the biopsy is taken from the inner cell mass of the blastocyst or the spent media culture where the developing embryo has grown in vitro [31, 32]. From there, chromosomal evaluation is done to attest for the number of chromosomes present, with the assumption that euploid embryos (blastocysts with a normal 46 chromosome count) are healthy and deemed optimal for embryo transfer. However, PGT-A is expensive, costing patients thousands of dollars, and imperfect, where many embryos often result as mosaic (some cells have

Yet, for those who opt to undergo PGT-A and continue to suffer the loss of RIF, asynchrony in uterine receptivity is most often the cause. A displaced window of implantation may be caused by a variety of factors, including abnormal cytokine and hormonal signaling, among other things, in which the endometrium is not prepared to accept a blastocyst at the otherwise appropriate time. Thus, evaluation

normal chromosomal count, and some do not) or "undetermined."

in progesterone production and a breakdown of the uterine lining.

**2.3 Menses**

**100**

*Visual of embryonic development and morphologic grading according to blastocyst expansion and cellular differentiation receptivity at embryo transfer.*

of implantation markers via an endometrial biopsy taken at the time of supposed implantation may be the key to predicting pregnancy outcome and adequate progesterone administration for optimal uterine.

### **4. Endometrial receptivity testing**

Over the last decade, advancements in technology have made it possible for us to evaluate the window of implantation and diagnose displacements in one's uterine cycle activity. Of the numerous tests that exist, including the window of implantation test, which uses reverse transcriptase PCR analysis of endometrial tissue, and the endometrial function test, a histologic analysis of endometrial sampling, the most well-known example remains the endometrial receptivity array (ERA) by Igenomix. ERA utilizes next-generation sequencing (NGS) to analyze the RNA composition of the endometrial tissue to detect for expression and suppression of known endometrial biomarkers characteristic of the window of implantation. These biomarkers include LIF, MUC16, as well as various integrins and cytokines mentioned earlier in this chapter [33]. Diagnosis of a displaced window of receptivity is based on the RNA expression and transcriptomic signature found within the endometrial cell sampling. Based on this genetic analysis, technology is then able to diagnose whether or not the endometrium is receptive, pre-receptive, postreceptive, or non-receptive. **Figure 5** is a display of the transcriptomic signature of

**Figure 5.** *Transcriptomic signature of the human endometrium based on next-generation sequencing.*

**103**

**5. Summary**

implantation.

*Implantation: Cross Talk of the Developing Embryo and Endometrium*

previously unheard of for those facing infertility [34].

our understanding of the window of implantation is still needed.

the endometrium at its various phases throughout the menstrual cycle, including the proliferative phase, pre-receptive secretory phase, and receptive secretory phase. A receptive endometrium indicates that the endometrial lining consists of all the correct biomarkers for proper implantation to take place, indicating the embryo should be transferred the same time as the biopsy took place. A non-receptive endometrial lining, on the other hand, refers to one of two cases: an endometrium that is either prereceptive or post-receptive [34]. A pre-receptive endometrium refers to a window of implantation that takes place 12–48 h after the time of biopsy, whereas a post-receptive endometrium occurs when the window of implantation takes place at an earlier time (12–48 h) than the time of biopsy. In these cases, a corrective course of exogenous progesterone administration is typically advised wherein the embryo transfer takes place earlier or later to when the biopsy took place/the original time implantation was otherwise thought to occur, resulting in more or less overall progesterone exposure. Recently, letrozole, an aromatase inhibitor, and GnRh agonist drug therapies have also been found to correct a displaced window of receptivity, especially in women diagnosed with endometriosis, in which both letrozole and GnRh agonists have been shown to alter integrin expression in the endometrium and induce uterine receptivity [35]. Since it was first brought onto the market, endometrial receptivity testing has offered a novel approach to what had otherwise been a black hole in assisted reproductive technology treatment. For those that experience significant RIF, the ERA has been shown to increase clinical pregnancy rate to upwards of 75%, a figure

Yet, while endometrial receptivity testing has come a long way since its original inception, increasing in accuracy and offering hope to patients who otherwise had no other answers, skepticism still exists as to the clinical utility of this relatively new and evolving technology. Some have posited that the act of biopsying the endometrium has a similar effect to uterine scratching, which is a technique that involves superficial wounding of the endometrial lining and is thought to improve uterine receptivity in subsequent menstrual cycles. Others, however, point to the lack of research and evidence-based medicine to prove these tests accurately diagnose uterine receptivity and truly improve pregnancy rates, arguing more research into

Uterine receptivity and the window of implantation are incredibly intricate and complex processes that are meant to result in pregnancy. Following initial apposition of the embryo to the endometrium by MUC1 and MUC6, cytokines, such as LIF, recruit the blastocyst to the optimal spot for implantation along the endometrial lining. Through cellular adhesion molecules, like integrins and L-selectin, the embryo is able to bind the basal lamina of the endometrium, adhering to the uterine wall before invading the epithelial tissue and completing the process of

While this complex biological system often works accurately for the majority, giving way to a healthy pregnancy, many still experience asynchronies between the endometrium and developing embryo, resulting in infertility. As such, in an effort to optimize assisted reproductive technologies, scientists have sought out new and innovative techniques in order to understand and diagnose irregularities in the most important processes of human reproduction. The invention of endometrial receptivity testing now allows clinicians the ability to predict an individual's personalized window of implantation, offering new understanding to the field and hope for

those who previously faced recurrent implantation failure (**Figure 6**).

*DOI: http://dx.doi.org/10.5772/intechopen.90748*

### *Implantation: Cross Talk of the Developing Embryo and Endometrium DOI: http://dx.doi.org/10.5772/intechopen.90748*

the endometrium at its various phases throughout the menstrual cycle, including the proliferative phase, pre-receptive secretory phase, and receptive secretory phase.

A receptive endometrium indicates that the endometrial lining consists of all the correct biomarkers for proper implantation to take place, indicating the embryo should be transferred the same time as the biopsy took place. A non-receptive endometrial lining, on the other hand, refers to one of two cases: an endometrium that is either prereceptive or post-receptive [34]. A pre-receptive endometrium refers to a window of implantation that takes place 12–48 h after the time of biopsy, whereas a post-receptive endometrium occurs when the window of implantation takes place at an earlier time (12–48 h) than the time of biopsy. In these cases, a corrective course of exogenous progesterone administration is typically advised wherein the embryo transfer takes place earlier or later to when the biopsy took place/the original time implantation was otherwise thought to occur, resulting in more or less overall progesterone exposure. Recently, letrozole, an aromatase inhibitor, and GnRh agonist drug therapies have also been found to correct a displaced window of receptivity, especially in women diagnosed with endometriosis, in which both letrozole and GnRh agonists have been shown to alter integrin expression in the endometrium and induce uterine receptivity [35].

Since it was first brought onto the market, endometrial receptivity testing has offered a novel approach to what had otherwise been a black hole in assisted reproductive technology treatment. For those that experience significant RIF, the ERA has been shown to increase clinical pregnancy rate to upwards of 75%, a figure previously unheard of for those facing infertility [34].

Yet, while endometrial receptivity testing has come a long way since its original inception, increasing in accuracy and offering hope to patients who otherwise had no other answers, skepticism still exists as to the clinical utility of this relatively new and evolving technology. Some have posited that the act of biopsying the endometrium has a similar effect to uterine scratching, which is a technique that involves superficial wounding of the endometrial lining and is thought to improve uterine receptivity in subsequent menstrual cycles. Others, however, point to the lack of research and evidence-based medicine to prove these tests accurately diagnose uterine receptivity and truly improve pregnancy rates, arguing more research into our understanding of the window of implantation is still needed.

### **5. Summary**

*Innovations in Assisted Reproduction Technology*

**102**

**Figure 5.**

*Transcriptomic signature of the human endometrium based on next-generation sequencing.*

Uterine receptivity and the window of implantation are incredibly intricate and complex processes that are meant to result in pregnancy. Following initial apposition of the embryo to the endometrium by MUC1 and MUC6, cytokines, such as LIF, recruit the blastocyst to the optimal spot for implantation along the endometrial lining. Through cellular adhesion molecules, like integrins and L-selectin, the embryo is able to bind the basal lamina of the endometrium, adhering to the uterine wall before invading the epithelial tissue and completing the process of implantation.

While this complex biological system often works accurately for the majority, giving way to a healthy pregnancy, many still experience asynchronies between the endometrium and developing embryo, resulting in infertility. As such, in an effort to optimize assisted reproductive technologies, scientists have sought out new and innovative techniques in order to understand and diagnose irregularities in the most important processes of human reproduction. The invention of endometrial receptivity testing now allows clinicians the ability to predict an individual's personalized window of implantation, offering new understanding to the field and hope for those who previously faced recurrent implantation failure (**Figure 6**).

**Figure 6.** *Ultrasound imaging of an embryo transfer (ET).*

Overall endometrial receptivity testing allows us greater insight into the understanding of reproductive infertility and the timing of the window of implantation. While research remains ongoing as to the clinical utility of these tests, including validation studies and the rate of pregnancy and live birth outcomes, endometrial receptivity testing offers another piece to the puzzle in our attempt to completely understand the underlying etiologies of infertility.
