Recurrent Implantation Failure (RIF) is considered to be an implantation failure for those women who have undergone three or more than three IVF cycles and have not been able to achieve a viable pregnancy. However , considering cause for failed embryo transfer may include factors like embryo quality , genetic factors , hormonal imbalance and Endometrial receptivity. Synchrony between an embryo and endometrium is one of the most considered factors for implantation leading to viable pregnancy and live births. Endometrium receptivity for implantation to occur requires monitoring of molecular , histological as well as hormonal mechanism. Endometrial receptivity is the ability of the endometrium to attach to the blastocyst , nourish it to a successful pregnancy. Endometrial receptivity is monitored throughout the ART Cycle , which suggests that the clinicians look for appropriate thickness of the endometrium for embryo transfer. Apparently some studies suggest that the endometrium thickness should between 8-12mm , but no results have claimed it to be true 100%. Monitoring the changes in endometrium throughout the cycle of ART and how to regulate it will help to improve chances of implantation failure.
Molecular Mechanism for Implantation.
A successful implantation occurs when a Blastocyst attaches to the Endometrium , which further nourishes it grow into a pregnancy leading to live birth. The crosstalk between receptive uterus and a competent blastocyst will only occur during a limited time period also known as “Window of Implantation”. This window is restricted to 16-22 day of a normal menstrual cycle. The processes involved in human implantation are primarily : Orientation , Hatching , Apposition , Adhesion , Invasion. As the blastocyst reaches the uterus , the inner cell mass attaches towards the endometrial epithelial lining .The Zona Pellucida is dissolved by the secretion of proteases by trophectoderm cells. Soon after the zona pellucida dissolves , the blastocyst is now in close contact with the endometrium . However, no direct contact is yet made between the endometrial walls and the embryo. Once the embryo has enters through the uterus and the zona pellucida is digested , now the mechanism to maintain synchronisation between embryo and endometrial walls starts to build. During apposition , in this phase of implantation the dialogue relies between mediators such as cytokines , chemokine. These mediator molecules regulate expression and functional activity of adhesion molecules(L-selectin and integrins). Carbohydrate ligands binds with L-selectin present on the luminal epithelium. A multitude of small profusion referred as pinopodes develop on apical surface of luminal uterine epithelium , which further interdigitate with blastocyst through surface microvilli on apical cytotrophoblast.The synchronised molecular dialogue is mediated by a variety of factors : Endometrium integrins , extracellular matrix molecules , adhesion molecules are responsible for successful implantation to occur. This diagram illustrates the mechanism of how the embryo reaches the endometrial surface and the events happening at the molecular level. The embryo starts to evolve through apposition and final contact between the embryo and endometrium is formed by the completion of invasion.
The connection between embryo and uterus is primarily of two attachment phases (1 and 2). The epical surface of uterine epithelial layer is coated with glycocalyx , a biological barrier which prevents microbial and proteolytic attack. The anti cellular adhesion molecule MUCIN1_(MUC1) , a glycoprotein expressed on the epical surface of secretary epithelial cells. Thus , embryo comes in contact with glycocalyx , MUC1 clearance allows to access the apical surface receptors for implantation. Endometrial integrins such as α5β1 , αVβ3 +αVβ6 are integrins expressed by trophectoderm during early apposition and adhesion. However , α1β1 , αβ1 , α7β1 tends to dominate during embryonic invasion. Osteopontins are secrets glycoproteins that interacts with integrins to carry out maturation in tissues , integrity and angiogenesis etc. interactions between OPN and integrins (α1β1 , α4β1 , αVβ3) activates integrin receptors and cytoskeleton proteins like CD44. Subsequently promoting focal adhesions of trophectoderm of embryo. Trophoblast is further differentiated into syncytiotrophoblast and cytotrophoblast only when the adhesion is completed. The inner cell mass of an embryo penetrate syncytiotrophoblast via uterine epithelium and underlying base membrane. This eventually leads to the invasion of inner cell mass invade endometrial stroma to interact with maternal vascular system. Although , to achieve successful invasion trophoblast must induce a pronounced expression of gene involved in the degradation of extracellular matrix. metalloproteinase-9 (MMP-9) is closely associated with the invasive phenotype of trophoblast. Under the influence of ovarian hormones and feedback mechanism preceding implantation acquire a receptive state.
Alteration in molecular mechanism leading to repeated implantation failure (RIF).
While the root cause for RIF cannot be directly identified , there are alterations at the molecular levels which can be monitored and altered for improved fertility results. Th1/Th2 ratio and TNFα levels: Tumour Necrosis Factor are immune cells regulators which induce apoptotic cell death that triggers production of several immune system molecules , which further leads to symptoms of many autoimmune diseases. Reportedly, it was observed that women with recurrent implantation failure had comparatively lower th1/th2 ratios expressed by interleukin10. Cytokines produces TNFα which suppress trophoblastic growth promoting inflammatory response in maternal uterine vessels , thus adversely affecting implantation. Auto-Antibodies: Several autoimmune antibodies such as anti-nuclear , anti-cardiolipin antibodies (ACA’s) are involved in biochemical pregnancy loss. The interactions between anti-β2 glycoprotein1 and anti-nuclear abs which leads to implantation failure. Cellular Adhesion Molecules: Integrins that particularly function in cell to cell interactions play an important role in implantation , specifically αVβ3 , α4β1 , α1β1. However , low levels of these integrins leads to recurrent implantation failure. Apparently lack of αVβ3 expression is often seen causing delayed histological development of phase endometrium. Leukaemia Inhibitory Factors (LIF): IL-6 expressions are part of cytokines , LIF and LIF mRNA are expressed throughout the menstrual cycle with a striking increase in the mid secretory phase, coinciding with a supposed window of implantation. Correlation in the expression of LIF and some other markers of implantation has been reported. LIF acts on cells by binding to the LIF receptor (LIFR) and gp130. Human blastocysts express mRNAs for LIFR and gp130, participating actively in establishing contact with the endometrium. However , low levels of these LIF have be associated with high risk of implantation failure. Infections: While every alteration in endometrial receptivity or embryo quality will lead to implantation failure , apparently any microbial infection or inflammation of the endometrium will lead to abnormal lymphocyte count and is a result of altering endometrial receptivity.
Conclusion
While more than 186 million people suffering with infertility worldwide (according to scales of WHO) , specialist across the globe are working to improve the results of ART. Over the last decade results have significantly improved , studying the molecular mechanism involved in Recurrent implantation failure is essential to treat those patients where alterations of endometrium are on the molecular level. No significant evidence has been found for the relation between RIF and age. The binding of endometrial surface and embryo is crucial and precise , therefore imbalance of any gene expression or immune cells will directly affect the implantation. In summary, optimising endometrial receptivity in fertility treatment will improve success rates. Evaluation of the implantation markers may help to predict pregnancy outcome and detect mystical implantation deficiency. Manipulating the expression of the endometrial genes with medical, psychological, surgical or future cells and gene-based therapies might improve implantation rate.
However currently , therapy relies on treating the underlying gynaecological and medical disorders.{[3]}
