Is the endometrium or oocyte/embryo affected in endometriosis?

One of the most puzzling problems of endometriosis is determining which mechanisms link this spectrum of conditions to infertility. There is conflicting evidence about the effect of endometriosis on the endometrium and on oocyte/embryo quality. Clinical studies reveal that implantation rates seem to be lower in women with endometriosis, while spontaneous abortion rates show variable results which are difficult to interpret due to the design of the studies. Biochemical markers (integrins and other cell adhesion molecules), morphological markers (pinopodes), apoptosis and ultrasound studies confirm that not only does the endometrium from women with endometriosis behave differently from the endometrium of women without endometriosis, but ectopic endometrium also behaves differently from eutopic endometrium. Data from oocyte donation programmes suggest that oocyte quality may be hampered in women with endometriosis. Recent reports have focused on the molecular mechanisms that may be altered, such as ovarian steroid production, or inadequate luteal function. In this review, we analyse the most recent literature dealing with the different mechanisms which affect the endometrium and oocyte/embryo quality and which thereby might cause infertility.