Alteration in Endometrial Remodeling: A Cause for Implantation Failure in Endometriosis?

The process of implantation is an interactive cascade of events between the embryo and the endometrium. It is a dynamic process consisting of three distinct phases. They are (i) apposition of embryo (ii) attachment with the epithelial lining of the endometrium and (iii) invasion into the endometrial stroma gaining access to the maternal circulatory system. Embryo implantation failure may occur due to embryonic defect or unsupportive endometrium. Advances in Assisted Reproductive Technology (ART) have made it possible to obtain good quality embryos; however, successful implantation remains the bottleneck for a successful pregnancy. The endometrium remodels before attaining a state of receptivity. Endometrium remains receptive during a limited period, when it is favourable for blastocyst attachment and implantation. In women, there is clinical evidence of a brief period of optimal uterine receptivity which allows for blastocyst implantation. This period, called the implantation window, is related to changes in the endometrial epithelial morphology. Inappropriate morphological development leads to unreceptive endometrium that causes defective endometrial /embryonic cross talk. This is generally agreed to be one of the main reasons for implantation failure. Endometriosis, characterized by benign growth of endometrial tissue outside the uterus, affects approximately 20%–48% of women during their reproductive years. The occurrence of aberrant hormonal, immunological, genetic and pathophysiological events associated with endometriosis is attributed to the heterogeneous etiology of the disease. The symptoms of endometriosis do not depend on the severity or stage of the disease. Women with even mildest degree of endometriosis can have a 3 – 4 fold reduction in their annual birth rate compared to normal non-endometriotic women.