Biology of nidation and ectopic implantation

The incidence of ectopic pregnancy has been steadily growing during the past decade; this fact can only partially be related to known factors (PID, use of IUDs, minipill, inductors of ovulation, sterilization reversal). Ectopic pregnancy is still a frequently misdiagnosed pathology; nevertheless, the recent advances in early diagnosis have allowed a decrease in the mortality rate and an improvement in subsequent fertility. In absence of obvious tubal abnormalities it is difficult to assess the cause of an ectopic implantation. Yet the factors regulating and nidation of the embryo in the human are far from clear. The transport of the embryo in the oviductal fluid depends on ciliary beat (generally regarded as the leading factor) and on muscle contractions mainly in the ampullary-isthmic and in the utero-tubal junction, where a sphincter action is stimulated by estrogens and relaxed by progesterone. Other substances (catecholamines, prostaglandins, oxytocin) are thought to be involved in ovum transport, although their role is unclear. Even less is known about local influence of the embryo on the oviduct. During preimplantation and implantation both the embryo and the mother engage in an elaborate interaction, elsewhere referred to as a 'conversation', involving long-and short-range signals. An essential role is played by the endocrine luminal milieu. A local action of estrogens on a progesterone-primed endometrium may be required for either the release of crucial signals for the blastocyst activation, or to make epithelial cells sensitive to the presence of the embryo, thus inducing the decidualization. In some animals, these estrogens are not ovarian, but synthesized by the blastocyst itself.(ABSTRACT TRUNCATED AT 250 WORDS)