O-288 Association of initial zygote cleavage, compaction pattern and euploidy rates: possible implications for embryo correction mechanisms
Authors: De Martin H , Gomes A , Fujii M , Bonetti T , Monteleone P 
Published: June 2022

Abstract

Study question
Is the morula compaction pattern associated with early zygote cleavage anomalies and euploidy rates? Can it represent a possible mechanism of embryonic correction?

Summary answer
Abnormalities in the first divisions give rise to dysfunctional blastomeres, which were excluded from compaction process. This process represent a possible embryonic correction mechanism.

What is known already
IVF embryos have historically been analyzed based on morphological characteristics focusing on days 2/3 (cleavage stage) and 5/6 (blastocyst). The morula stage, usually reached on day 4, represent a transitional phase resulting from the embryonic genome activation and has received little attention in clinical embryology. However, during this crucial stage, cell differentiation begins, metabolism reaches higher levels, changes in cell shape and cell-destination mechanisms are enhanced, providing opportunities for “quality control”. At morula stage, embryos with severe genetic abnormalities are sorted out by natural selection or possibly self-correction events may occurs, reducing the load of aneuploid cells in mosaic embryos.

Study design, size, duration
We evaluated laboratory and clinical outcomes of 836 blastocysts (195 ICSI cycles) cultured in a time-lapse system and biopsied for preimplantation genetic testing for aneuploidy (PGT-A), from April-2018 to June-2020. Time-lapse videos were retrospectively analyzed to assess morphokinetic parameters of initial zygote cleavages and morula compaction. These parameters were evaluated and associated to genetic and embryonic outcomes. Blastocyst outcomes were also assessed according to maternal age &lt;35 (n = 140), 35-37 (n = 175), 38-40 (n = 314) and &gt;40 (n = 207).

Participants/materials, setting, methods
The zygote initial cleavages were classified as normal (1-2-4 cleavage) or anomalous: 1-3 cleavage (zygote splits directly into three blastomeres); 1-2-3 cleavage (2-3 in &lt; 3h); and 1-2-5 cleavage (after the first normal cleavage, one blastomere splits directly into three blastomeres). Morula were classified as: Fully Compacted Morulae (FCM); Partially Compacted Morulae (PCM). The PCM could have cells excluded from the compaction process (Exc-PCM), extruded from an already compacted morula (Ext-PCM), or both situations (Exc/Ext-PCM).

Main results and the role of chance
Most blastocysts showed normal zygotic cleavage (83.3%). Anomalous cleavage such as 1-3 (0.8%), 1-2-3 (7.2%) and 2-5 (8.7%) were less prevalent and no differences were observed according to female age (p = 0.268). Overall, FCM (61.0%) was a more frequent pattern than PCM (39.0%). According to compaction pattern of PCM, Exc-PCM (25.1%) was more prevalent than Ext-PCM (11.5%) and Exc/Ext-PCM (2.4%). There was also no difference in the prevalence of the morulae compaction pattern according to female age (p = 0.124). FCM occurred in 511 embryos (73.4%) from zygotes with normal cleavage (n = 696), but no FCM was observed among anomalous cleaved zygotes (p &lt; 0.001). Genetic and embryonic outcomes were displayed as follows: normal cleavage and FCM (NC-FCM, n = 511), normal cleavage and PCM (NC-PCM, n = 185), and anomalous cleavage and PCM (AC-PCM, n = 140). NC-FCM and AC-PCM groups showed statistically similar euploidy rates (39.3% and 38.6%, p = 0.870), that were both significantly higher than the NC-PCM group euploidy rate (23.8%, p &lt; 0.001). However, morphokinetic scores (KidScore) and percentage of top-quality blastocysts at morphological evaluation were discordant: NC-FCM (6.4±1.7; 81.4%), NC-PCM (5.0±1.8; 47.6%) and AC-PCM (4.3±2.0; 37.9%). Euploidy rate decreased with female age for all groups: NC-FCM (&lt;35:54.7%, 35-37:56.0%, 38-40:33.3%, &gt;40:24.1%); NC-PCM (&lt;35:37.5%, 35-37:19.0%, 38-40:25.0%, &gt;40:19.0%); AC-PCM (&lt;35:66.7%, 35-37:48.5%, 38-40:39.0%, &gt;40:3.7%).

Limitations, reasons for caution
The retrospective design of the study represents a general limitation. In addition, this study evaluated only embryos that reached the blastocyst stage and showed enough quality to be biopsied. Therefore, the association between anomalies in the first and second mitotic cleavages and the embryonic development could not be evaluated.

Wider implications of the findings
Similar euploidy rates among NC-FCM and AC-PCM support the hypothesis that cell exclusion/extrusion in compaction stage of anomalous-cleaved zygotes can represent a potential correction mechanism, even though lower morphological/morphokinetic scores are seen on AC-PCM. That process is supposed to rescue embryos from the mosaic aneuploidy which occurs after abnormal cleavage.

Trial registration number
not applicable