Association of equine oocyte and cleavage stage embryo morphology with maternal age and pregnancy after intracytoplasmic sperm injection.

The research study revolves around understanding the connection between the morphology of equine oocytes and cleavage stage embryos with maternal age, and the resulting pregnancies after Intracytoplasmic Sperm Injection (ICSI) in mares. Essentially the study demonstrates that, while a mare’s age doesn’t impact the cleavage rates, it does result in declining pregnancy rates and rising rates of pregnancy loss.

Methodology and Sample

• The study was retrospective and used images of horse oocytes and embryos at the stage of cleavage. They sourced the oocytes from preovulatory follicles.
• A total of 401 images were used for measurements of ooplasm, the perivitelline space, and the zona pellucida.
• For the cleavage stage embryo images, a total of 178 samples were used.
• The research categorized the ages of the oocyte donors into four groups: 3-13, 14-19, 20-23 and 24-27 years.
• The researchers also compared the mares aged between 3-13 years with and without the history of recent performance or injury stress.

Findings

• The study found that the cleavage rates in mares do not undergo any significant change with the increase in their ages. However, the pregnancy rates did decline with age, while the rates of pregnancy loss between 11-50 days gestation went up with age.
• The study also found that oocyte donor mares that had undergone performance or injury stress yielded lower pregnancy rates compared to mares of the same age bracket maintained under typical broodmare management.
• There was no consistent connection discovered between oocyte morphology and either mare age or pregnancy outcome.
• Even the morphology of the cleavage stage embryo did not show a consistent association with pregnancy outcomes. However, it was observed that embryos that resulted in early pregnancies (within or equal to 17 days) developed at a slightly faster rate.
• Towards the end, the researchers discovered that embryonic vesicles that had an expedited increment in diameter were more likely to sustain a pregnancy up to a period of 50 days’ gestation.

Implications

• While the reasons behind these findings need further exploration, these early observations can be important for equine breeders or veterinarians who are using or considering ICSI procedures in mares.
• The understanding that certain morphological characteristics of the oocyte or cleavage stage embryo do not consistently impact pregnancy outcomes can guide further research into other factors that may have more significance.
• The associations with maternal age and stressors such as performance or injury could influence management practices for broodmares, particularly those intended for ICSI procedures.