Eutopic or ectopic pregnancy: a competition between signals derived from the endometrium and the fallopian tube for blastocyst implantation.

Embryo retention in the fallopian tube is thought to lead to ectopic pregnancy, which is a significant cause of morbidity. This pathological condition does not occur in laboratory rodents. Moreover, incidences of tubal pregnancy after assisted reproduction (ART) are continuously increasing. What are the factors that need to be considered responsible for this condition? Ectopic pregnancies occur because of conflicting signals to the blastocyst from the two epithelia (uterine and fallopian). The signals consist of cytokines, chemokines and adhesion molecules that mediate both blastocyst adhesion to the uterine (and fallopian) epithelium and leukocyte adhesion to the vascular endothelium and, presumably, the fallopian epithelium. Chronic inflammation in the fallopian tube caused by infections or misplacements of the blastocyst (in the case of ART) can alter expression (upregulate) of the signals emanating from the fallopian tube and thereby can compete with the uterine (normal) site of implantation. That is, in ectopic pregnancy, a blastocyst may receive stronger signals from the tubal epithelia, migrate to the fallopian tube, and be implanted at that site.