Lodgement of the equine blastocyst in the uterus from fixation through endometrial cup formation.

The equine blastocyst becomes fixed in position in the uterus on approximately Day 16 of gestation, but allantochorionic villi are not formed until about Day 50. The purpose of this study was to examine evidence that the blastocyst is orientated during this time period, and to determine what morphological features might assist retention of the position of the blastocyst within the uterus. Implantation sites were collected on Days 10-42 of gestation, and the reproductive tracts perfused with fixative for light and electron microscopic examination. The conceptus is found at the bend of a uterine horn near the junction with the body of the uterus. As the conceptus expands, the ventral antimesometrial aspect of the horn is dilated, forming a distinct chamber. Despite the prescence of the capsule around the conceptus through Day 20, the conceptus is orientated with the embryo situated anti-mesometrially. The endometrial folds at the ends of the chamber are closely apposed and exhibit cross ridges that may aid in stabilizing the chamber. After displacement of the capsule, the trophoblast is closely apposed to the endometrium over most of the surface of the conceptus. As the expanding allantois pushes the embryo and yolk sac toward the mesometrial side, the sinus terminalis region of the yolk sac becomes progressively modified. A ridge or annulus of trophoblast indents the endometrium prior to the stage of girdle cell migration. The structure of the sinus terminalis complex is consistent with a role in prevention of rotation of the conceptus within the chamber. Close association of vascularized trophoblast and uterine epithelium, underlain by dilated subepithelial capillaries, is present as early as Day 25 in the allantochorion. Orientation of the blastocyst is important for apposition of vascularized trophoblast to the flattened antimesometrial surface and for proper sitting of the endometrial cups in the mesometrial area.