Abnormal cleavage patterns tied to early loss in equine IVP embryos

CURRENT FULL VERSION: Abnormal early cleavage behavior may give equine reproduction specialists a clearer way to identify which in vitro-produced embryos are most likely to hold a pregnancy. In a retrospective study published online July 31, 2025, in Equine Veterinary Journal, UC Davis-led researchers reported that abnormal cleavage patterns during the first mitotic division were associated with lower pregnancy success and substantially higher early pregnancy loss after embryo transfer. Embryos with abnormal cleavage had a 53.3% total early pregnancy loss rate, compared with 22.6% for embryos with normal cleavage. (pubmed.ncbi.nlm.nih.gov)

That matters because equine IVP has advanced quickly, but embryo selection still lags behind. The study authors noted that the field lacks standardized embryo classification criteria, even as IVP embryos are known to have higher early loss than in vivo-derived blastocysts. UC Davis has been building clinical and laboratory infrastructure around equine assisted reproduction, including live time-lapse embryo monitoring and commercial ICSI services, reflecting how central embryo selection has become for high-value breeding programs. (pubmed.ncbi.nlm.nih.gov)

In the new study, investigators reviewed time-lapse images and clinical outcomes from 70 transferred embryos with known pregnancy results, alongside 114 embryos that arrested during development. They evaluated morphokinetic characteristics, cleavage abnormalities during the first cell division, and time to vitrification or arrest. Their regression analysis found two signals that stood out: embryos that reached vitrification earlier had better pregnancy odds at day 14 and lower loss through day 25, and embryos with abnormal cleavage patterns were both less likely to establish pregnancy and more likely to be lost early. (pubmed.ncbi.nlm.nih.gov)

The findings build on a growing body of equine IVP evidence pointing to developmental speed as a meaningful quality marker. A 2023 study of 316 equine IVP blastocysts found that day of development significantly affected foaling rate, and concluded that bovine grading systems don't translate cleanly to equine embryos. That broader clinical literature also suggests embryo success is influenced by more than lab kinetics alone: in a separate retrospective analysis of 2,292 IVP blastocyst transfers, slow embryo development, poorer post-thaw grade, advanced donor mare age, and embryo transfer later in the breeding season all reduced pregnancy rates, while the chance of getting at least one pregnancy from an IVEP session rose from 78% with one blastocyst to 91% with two and 98% with three or more. The same study found that overall variation between most donor mares and stallions was modest, but for some underperforming mares, pregnancy likelihood differed by as much as 50% depending on which stallion was used for ICSI. (pubmed.ncbi.nlm.nih.gov)

Direct outside commentary on this specific paper was limited, but the broader industry and academic conversation has been moving in the same direction: toward more precise, equine-specific embryo assessment. UC Davis says its VetART laboratory is among the few centers able to monitor equine embryos continuously with non-invasive time-lapse imaging, capturing cleavage timing and other mitotic events in detail. On the pregnancy-loss side, recent work from Cornell, the Royal Veterinary College, Texas A&M, and UC Davis has also sharpened attention on embryo-level causes of early loss, including chromosomal abnormalities. Taken together, that suggests embryo selection in mares is becoming less about static appearance alone and more about measurable developmental behavior and underlying biology—while still needing to account for donor age, transfer timing, and occasional mare-stallion interaction effects seen in large clinical datasets. (vetart.vetmed.ucdavis.edu)

Why it matters: For veterinarians and theriogenologists, the practical takeaway is that first-division cleavage patterns may offer a more actionable way to triage embryos before transfer, counsel pet parents and breeders about risk, and potentially improve use of recipient mares. In programs where each oocyte retrieval, ICSI cycle, and transfer carries meaningful cost and limited opportunity, better prediction of which embryos are likely to survive beyond the first month could reduce avoidable loss and improve case selection. The study doesn't prove that cleavage-based selection alone will solve early loss, but it strengthens the case for adding morphokinetic data to equine-specific grading frameworks that already need to account for donor mare age, embryo quality after thawing, and seasonal timing. (pubmed.ncbi.nlm.nih.gov)

What to watch: The next step is likely prospective validation, ideally across multiple centers, to test whether using abnormal cleavage patterns as a formal exclusion or ranking criterion improves retained pregnancies and foaling outcomes in routine practice. It will also be worth watching whether these time-lapse markers are paired with embryo biopsy, genetic testing, or other lab measures to create more standardized equine IVP selection protocols—and whether clinics begin integrating them with session-level predictors such as blastocyst yield and stallion choice for mares with historically weaker results. (research-portal.uu.nl)