Transfer of Day 11 ruptured embryos can prevent luteolysis in recipient mares

Abstract

Previous studies transferring Day 11 embryos to Day 11 or 12 recipient mares and then manually crushing the transferred embryo 24 hours later resulted in 83% (5/6) and 100% (6/6) mares, respectively, entering a period of luteostasis. Undertaking manual reduction 12 hours post transfer resulted in 100% (5/5) of Day 11 recipient mares becoming luteostatic. This work suggested that the mare could recognise the presence of an embryo within the uterus after only a short period of time. The present experiment aimed to shorten the window of the presence of an intact embryo still further by evaluating the ability of ruptured or damaged Day 11 embryos transferred to recipient mares to initiate a period of luteostasis. A total of sixteen Day 11 embryos were flushed from the uteri of donor mares. Embryos were moved to a petri dish containing 5mL of Vigro Hold Plus Solution where they were measured and graded. The intact embryos were ruptured using two 100µm diameter acupuncture needles, ensuring the blastocoele fluid was released into the medium. The ruptured embryo and the medium were then transferred to recipient mares at Day 11 (n = 8) or 12 (n = 8) post ovulation using a standard embryo transfer technique. Control Day 11 (n = 8) and 12 (n = 8) recipient mares received 5mL of Vigro Hold Plus. All recipient mares were monitored on alternate days post transfer using a combination of ultrasonography,including assessment of the corpus luteum using colour Doppler, and serial blood serum samples assayed for progesterone in peripheral blood to determine if, and when, they returned to oestrus. Mares with a functional corpus luteum with blood flow and progesterone levels >1ng/mL up to 25 post ovulation were deemed to have entered a period of luteostasis and at this time point luteolysis was achieved by administration of an i.m injection of prostaglandin. Transfer of the ruptured embryo and medium resulted in significantly more Day 12 recipients entering a period of luteostasis compared to Day 11 recipients (75% vs. 12.5%; 6/8 vs. 1/8; P= 0.04, respectively). Day 11 and 12 control recipient mares that received only medium had an incidence of luteostasis of 12.5% (1/8) in each group. Hence the presence of medium containing blastocoele fluid, a ruptured capsule and trophoblast within the uterus can prevent luteolysis around the time of maternal recognition of pregnancy (MRP) in the mare. If all or only some of the components of the ruptured Day 11 embryo are required to initiate luteostasis needs further study. If the mechanisms underlying luteostasis with this model mirror those of the MRP signal in pregnant mares this study also raises questions about the duration of the MRP signal and how long embryo migration within the uterus is needed to prevent luteolysis.