P-267 The morula stage. Is it time to repair?
Authors: Cuevas-Saiz I , Rodríguez Trujillo K  Y , Olmedo-Illueca C , Torres-Serrano M , Royo-Bolea S , Morales-Vicente A , Barea-Gómez M 
Published: July 2024

Abstract

Study question
Are there statistically significant differences in embryo’s fate when exclusion or extrusion of blastomeres is detected along development?

Summary answer
According to our results, exclusion or extrusion of blastomeres affects the embryo’s subsequent viability, suggesting that different biological processes are involved in its appearance.

What is known already
The morula stage is a clear example of embryo plasticity, being a critical step towards blastocyst development. One morphokinetic parameter observed at this stage is the exclusion (before compaction) and extrusion (during or after compaction) of blastomeres. Exclusion and extrusion seem to be related to embryo self-correction phenomena and they are considered negative criteria due to potential associations with tight junction failures, gene expression, aneuploidies, or a high degree of mosaicism. However, the moment in which those phenomena are detected, can have different origins and though different consequences in embryo development.

Study design, size, duration
A total of 668 fertilized embryos and cultured to the blastocyst stage were included in the study. Embryo cohorts correspond to 158 consecutive patients that underwent an IVF treatment with fresh single blastocyst transfer and all their cohorts, between June 2017 and September 2022. Data recovered during culture in time-lapse incubator regarding the extrusion or exclusion of cells and the final fate of the transferred and the cohort embryos were retrospectively analysed.

Participants/materials, setting, methods
Embryos were cultured in single culture medium (CSCC®, Irvine Scientific) in a time-lapse incubator (MIRI-TL®, ESCO Medical). Embryos were evaluated for the time of exclusion/extrusion, number and volume of excluded/extruded cells and their final fate (transferred and the result, vitrified, discarded). For those transferred, Beta-hCG was performed 14 days after transfer and if positive clinical pregnancies were confirmed 3 weeks later. Data were analysed using IBM SPSS Statistics v25.0, with p &lt; 0.05 indicating statistical significance.

Main results and the role of chance
Of the 668 analysed embryos, 340 were deemed “useful”: 225 were transferred (158 fresh, 67 cryotransferred after fresh negative pregnancy test), and 115 remain still vitrified. The rest, 328, were considered “non-useful”: 84 discarded and 244 blocked. 37% of the embryos presented total compaction and 63% partial, being 70.1% exclusion, 9.7% extrusion and 20.2% both phenomena. Exclusion/extrusion was significantly higher (p &lt; 0.05) in non-useful embryos compared to the useful ones. In the distribution with regards to the final fate, significant differences were found (p &lt; 0.05) being 78% exclusion, 8.8% extrusion and 13.3% both for useful, and 53.8% exclusion, 5.8% extrusion and 40.4% both for non-useful. There were statistically significant differences (p &lt; 0.05) in the volume and number of cells excluded/extruded and the fate, being higher in those that resulted non-useful. ROC analysis showed that the volume occupied by excluded cells influences the embryo’s fate (AUC=0.738). Regarding transferred embryos, those that presented the phenomenon and resulted in a positive beta-hCGs, 95.8% was exclusion, none of them extruded cells and 4.2% both. For the negatives, 71% of exclusion, 11.4% of extrusion and 17.1% both, resulting in non-significant differences (p = 0.055). No significative differences were found either for the ones that resulted in a clinical pregnancy.

Limitations, reasons for caution
Although no statistical differences were found according to the moment of exclusion of blastomeres, before or after compaction, and the pregnancies, there is a clear trend in the sooner it happens the more embryo viability. And increase in the sample size might be helpful to confirm it.

Wider implications of the findings
The establishment of new parameters can improve embryo selection to help embryologists discriminating between embryos that appear to be of similar quality.

Trial registration number
not applicable