Location of equine oviductal embryos on Day 5 post ovulation and oviductal transport time of Day 5 embryos autotransferred to the contralateral oviduct

Abstract

The location of equine embryos and their rate of transport through the mare's oviduct prior to uterine entry on Day 6 post ovulation are not well defined. We tested the hypothesis that equine embryos are located at the ampullary-isthmic junction until just prior to uterine entry by attempting to recover Day 5 equine embryos from 4 separately flushed oviductal segments (ampulla, ampullary-isthmic junction, proximal isthmus and distal isthmus) of 16 bred mares. At least 1 embryo was located in the oviductal segment that included the ampullary-isthmic junction in each of 12 mares, and 1 embryo was located in the uterus of each of the 4 remaining mares. The recovery of most Day 5 embryos near the ampullary-isthmic junction less than 24 h before the expected time of uterine entry supports the hypothesis that equine embryos are retained for most of the oviductal period near the ampullary-isthmic junction and suggests that embryo transport through the oviductal isthmus is rapid. We estimated the rate of embryo transport through the entire oviduct by transferring Day 5 embryos from the ipsilateral to the contralateral oviducts of 9 mares and subsequently attempting to recover the embryos from the uterus at 36, 48 or 72 h post autotransfer. Fewer (P < 0.02) embryos were recovered from the uterus at 36 h (0 in 3 attempts) or 48 h (0 in 3 attempts) postautotransfer than at 72 h postautotransfer (3 expanded, hatched blastocysts in 3 attempts). The extended oviductal transport time of transferred embryos indicates that an aspect of the embryo transfer procedure may have interfered with oviductal transport.