EMBRYO MIGRATION

As plasma levels of progesterone increase with luteal development, oviductal contractions are suppressed and the opening through the tubo-uterine junction is expanded; this allows for release of the embryo into the uterus. The embryo resides within the oviduct three to four days.

In polytocous species, multiple embryos must maneuver for limited developmental space within the uterine cavity. Uniform positioning within the uterus is dictated by myometrial contractions directed outward from each embryo; this suggests a localized control mechanism involving a secretory product of the conceptus. Under conditions of overcrowding, some embryos will fail to establish viable implantation sites - an important contributing factor to early embryonic mortality in pigs. Up to 40% of porcine embryos are lost during early pregnancy (conception rate is > 90%). The maximal uterine quota of piglets is around 16.