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

The research investigates the comparison of embryonic and early fetal development of horse embryos between recipient mules and mares. It also looks at the hormonal, ovarian, and uterine aspects throughout the first four months of the process.

Study Design and Methodology

• This study involved donor mares which were monitored daily via transrectal ultrasonography and inseminated using fresh semen. The recipients were divided into two groups — acyclic mules and cyclic mares — for comparison.
• The mares’ ovulation was synchronized with the donor’s schedule using PGF2α and deslorelin acetate. The mules, however, were prepared for embryo transfers with estrogen and progestagen.
• Embryo collection and transfer were performed 8 days post-ovulation of the donor mares. Upon confirmation of pregnancy, the recipient mules were treated with long-acting progesterone once a week for at least 120 days.

Results and Observations

• Both mules and mares displayed similar patterns for the first detection of an embryonic vesicle (EV) on day 10. The mobility of the EV was also similar in both species.
• The day of fixation of the EV in mules was observed to be a day earlier than in the mares, but the diameter and growth rate of the EV did not significantly vary between the two groups.
• The first detection of the embryo proper was noted on day 20. Here again, parameters like crown-rump length, width, and diameter were alike in both recipient types.
• Mules were noted to have the heartbeat and the formation of the allantoic sac slightly delayed as compared to mares. Despite this, the umbilical cord was noticed to appear approximately around day 40 in both species.
• Apart from the expected differences in ovarian functions and eCG production, other aspects like the endometrial diameter, uterine tone and echotexture, and progesterone levels displayed considerable similarity between mules and mares.

Conclusion

• The study concluded that there were noteworthy similarities in the ultrasound diagnosis of equine embryo and fetal development, and uterine characteristics between mules and mares when used as recipients of horse embryos.