Failure of endometrial cup development in the donkey-in-horse model of equine abortion

The mature preinvasive chorionic girdles of horse, mule, donkey and extraspecies donkey-in-horse conceptuses, and the very young endometrial cups on d 37 of gestation in mares carrying horse, mule and transferred donkey-in-horse conceptuses, were compared histologically and ultrastructurally to determine possible mechanisms underlying failure of endometrial cup development in the donkey-in-horse model of equine abortion. The progenitor chorionic girdle from the failing donkey-in-house pregnancy was similar in size to the normal donkey chorionic girdle but the trophoblast cells within the former were smaller, less organised and showed definite signs of degeneration and pyknosis. In the 37 d endometrial cups both in the horse and mule pregnancies, the recently invaded, differentiated and enlarging endometrial cup cells had penetrated deeply into the endometrial stroma and were becoming tightly packed between the persisting endometrial glands. In the donkey-in-horse pregnancy, on the other hand, relatively few donkey chorionic girdle cells had begun the invasion process and the majority of these, having penetrated and dislodged the horse luminal epithelium, did not penetrate the basement membrane beneath. Very few cells had reached the endometrial stroma and these had already attracted considerable numbers of lymphocytes to the area. It is concluded that unknown factors in the horse uterus affect adversely all phases of the development, attachment and invasion of donkey chorionic girdle cells, thereby leading to very little or no endometrial cup development and equine chorionic gonadotropin secretion in the extraspecific donkey-in-horse pregnancy created by embryo transfer.