Equine embryo mobility. A game changer

The equine embryo or embryonic vesicle on Days 11-15 postovulation travels with profound physiologic purpose throughout the lumen of the two uterine horns and uterine body making 12 to 22 trips between the two uterine horns per day. This phenomenon is termed embryo mobility and is unique in equids among domestic species. Apparently, the embryo first reaches the uterine body on Days 8 or 9. Mobility increases to maximum by Days 11 or 12 and continues until an abrupt cessation of mobility (fixation) on Days 15 (ponies) or 16 (horses and donkeys). The embryo is propelled by uterine contractions in response to the production of apparently both PGF2α and PGE2 by both the embryo and uterus. An increase in endometrial vascular perfusion accompanies the mobile embryo as it moves from horn to horn. Restricting the embryo to one uterine horn by a ligature has indicated that specific roles of the traveling embryo include the stimulation of uterine contractions, tone, vascularity, and edema and to curtail the production of the luteolysin (PGF2α) by the uterus. The increase in uterine tone, decrease in diameter of the uterine horns, and a flexure in the caudal portion of each horn collaborate in the selection of a horn of fixation. Embryo mobility is a game changer that has solved several long-time enigmas in mare reproduction and has provided a needed and effective finger/thumb compression method for eliminating one member of a twin set.