Twin embryos in mares. I: From ovulation to fixation.

This research article discusses the development of twin horse embryos from the time of ovulation to the point of fixation, specifically exploring their origins, growth rates, and movement in the uterine lumen. They also studied the natural process of embryo reduction during this period.

Origination of Twins

• The researchers started by investigating the origin of twins in mares.
• Based on the data they reviewed, they established that the majority of twins come from multiple ovulations. This implies that two or more ovarian follicles mature and release eggs within the same estrus cycle.

Impact of Double Ovulation

• The authors used ultrasound studies to determine if there was a relationship between the number of days between double ovulations and the conception rate or embryo survival in the first 16 days post-ovulation.
• They discovered no correlation, implying that the timing of double ovulation doesn’t affect the probability of both ova being fertilized or the embryos’ early survival rate.

Embryo Reduction

• The researchers then turned their attention to the natural process called embryo reduction – the elimination of excess embryos to make sure only one embryo reaches the fetal stage.
• Two studies highlighted indicated that embryo reduction before or on the day of fixation is not a significant factor in correcting twin situations naturally.

Development and Growth of Embryos

• Comparison of diameters and growth rates between Days 11 to 16 for single and twin embryos revealed no significant differences.
• The presence of two body vesicles (surrounding the embryos) didn’t directly affect their diameter, except as dictated by their age.

Movement of Embryonic Vesicles

• The authors observed the embryonic vesicles’ movement in the uterus from the first day they could detect them (Days 9 to 11), up to the day of fixation (average: Day 16).
• In over 50% of the time during Days 9 to 12, the vesicles remained in the uterine body, subsequently they were most often found in the uterine horns.
• For twins of different sizes, preference for the uterine body or horns seemed to be a function determined independently by each vesicle, possibly based on its age or size.
• After Day 12, the vesicles started a maximum mobility phase, transitioning frequently from the uterine body to the horns. This phase of increased mobility continued until the day of fixation.