Morphological evaluation of Day 8 embryos developed during induced aluteal cycles in the mare

A novel in vivo model utilizing serial administrations of PGF_2α to induce aluteal cycles in the mare was used to evaluate the effects of progesterone-deprivation on the morphology of in vivo preimplantation embryos. We hypothesized that equine embryos produced during induced aluteal cycles (AL) would be developmentally affected, characterized by earlier embryo stage at collection, smaller embryo diameter, and lower quality grade, compared with those collected on the same day post-ovulation from control cycles during diestrus (high progesterone; > 4 ng/mL). Seven cyclic mares with a median age of 6.5 years (range 3-16) were utilized in a crossover design. Mares in estrus were artificially inseminated to a fertile stallion and randomly assigned to control or AL groups. Mares received either saline solution (control mares) or PGF_2α (AL mares), twice daily on days 0, 1, and 2 and once daily on days 3 and 4. Serial blood samples were collected daily during estrus and until the day of embryo collection 8 days after ovulation. Mares were monitored until they returned to estrus, and artificially inseminated. Mares were switched to the opposite treatment group only after a successful embryo collection occurred during the previous cycle. Only cycles that produced embryos were used for analyses. No significant rise in progesterone was observed in the AL group with mean concentrations of plasma progesterone remaining <1.0 ng/mL from ovulation until embryo collection on Day 8. This is in sharp contrast to the control (luteal) cycle where a post-ovulatory rise in plasma progesterone was observed. The mean daily concentrations of plasma progesterone were significantly higher in control vs. AL group beginning at Day 3 and remained so until Day 8. The mean (±SEM) embryo diameter of AL embryos was 171 ± 5 μm compared to 756 ± 99 μm for control embryos. The majority of the Day 8 AL embryos were classified as morulas (3/9) or early blastocysts (5/9) with only 2 embryos of quality grade 1 compared to the Day 8 control embryos that were mostly expanded blastocysts (6/7) with 5 of 6 being of quality grade 1. This study shows that serial administrations of PGF_2α were able to prevent significant rises in plasma progesterone, thus inducing aluteal cycles characterized by a progesterone-deprived environment for developing embryos. Embryos collected from induced aluteal cycles were adversely affected as demonstrated by a lower quality grade, smaller diameter and earlier embryo stage at collection when compared to control embryos.