Twin embryos in mares. II: Post fixation embryo reduction.

The research studied the natural progression of twin embryos in mares from fixation (day 17) to day 40, focusing on the phenomenon of post-fixation embryo reduction. The factors contributing to this occurrence such as fixation location and vesicle size were examined. The results indicated that embryo reduction was more likely in cases where the vesicles were unequally sized and fixed unilaterally. The deprivation hypothesis was proposed to shed light on the conditions under which an embryo reduces.

Key Research Findings

• The incidence of embryo reduction increased significantly when the vesicles became fixed unilaterally, rather than bilaterally, and when the vesicles were unequal in diameter.
• Out of 68 mares studied with twins on the day of fixation, post fixation embryo reduction occurred in 41 (60%).
• The incidence of reduction was significantly higher following unilateral fixation. In mares with vesicles of different sizes (4mm or more difference), reduction occurred in all cases.
• Embryo reductions were found to be completed by Day 20 in the majority of the cases (59% reductions). The remaining reductions were completed either between Days 21 and 30 (27%) or between Days 31 and 38 (14%).
• A remarkable 80% of the early reductions (by Day 20) resulted in the disappeared vesicle within one day. Reductions that happened after Day 20 were typically preceded by a gradual decrease in size.
• As the days after Day 17 advanced, the frequency of reduction decreased while the time required to complete reduction increased.
• When the twins’ vesicles differed in diameter by 4mm or more, they were more likely to undergo reduction by Day 20. If the twins exhibit a significant difference in diameter, it enhanced the possibility of unilateral fixation, increased reduction frequency, and hastened the reduction timeframe.

Deprivation Hypothesis

• The deprivation hypothesis posits that embryo reduction happens when a significant portion of the yolk sac’s three-walled area or the vascularized wall of the yolk sac or allantoic sac aligns with the wall of the neighboring vesicle rather than the endometrium.
• As a result of this positioning, the vesicle might not get enough embryonal-maternal exchange, leading to regression.

This research provides insightful information about equine reproductive aids and processes, especially in understanding the conditions fostering embryo reduction in mares. Some of these observations and patterns could further be studied in other animal species.