The mule (Equus mulus) as a recipient of horse (Equus caballus) embryos: Comparative aspects of early pregnancy with mares

Abstract The aim of this study was to compare the embryonic and early fetal development of horse embryos between recipient mules and mares from day 10–60 of pregnancy, in addition to hormonal (eCG and progesterone), ovarian, and uterine characteristics for approximately 4 months. Embryo donor mares (n = 5) and two groups of recipients (acyclic mules, n = 7; cyclic mares, n = 7) were used. Donor mares were monitored daily by transrectal ultrasonography and inseminated using fresh semen. Cyclic recipient mares were synchronized with the donor’s ovulation using PGF2α and deslorelin acetate. Mules were prepared for the embryo transfers with estrogen and progestagen. Embryo collection and transfer were performed 8 days after ovulation of the donor mares. Pregnancy diagnosis with ultrasonography began 1 day after embryo transfer. After pregnancy confirmation, the recipient mules received long-acting progesterone once weekly for at least 120 days. The first day of detection (day 10) of an embryonic vesicle (EV) was similar between mules and mares. A period of extensive intrauterine mobility of the embryonic vesicle was observed similarly in mules and mares from days 10–17. The day of fixation of the EV in mules tended to be 1-day earlier than in mares; however, the diameter and growth rate of the EV did not differ between the two species. The embryo proper was first detected at day 20, and the crown-rump, width, and diameter were similar between the two recipient types. The heartbeat and allantoic sac tended to be detected 1 day later in mules than in mares, while the umbilical cord was first observed around day 40 in both species. Besides the expected differences found in ovarian aspects and eCG production, similar endometrial diameter, uterine tone and echotexture, and progesterone levels were seen between the two types of recipients. In conclusion, striking ultrasound similarities in equine embryo and fetal development, and uterine characteristics were seen between mules and mares used as recipients of horse embryos.