Alternative patterns of partial embryo compaction: prevalence, morphokinetic history and possible implications.

Research Question: The morula stage is a poorly understood developmental stage. In the morula, cell compaction can involve all or only some blastomeres, with largely unknown implications. Here, the prevalence, underlying morphokinetic mechanisms and possible consequences of partial compaction, were investigated.

Design: Preimplantation genetic testing for aneuploidies (PGT-A) cycles of women whose embryos were observed by time-lapse technology were studied. PGT-A data, generated by array comparative genomic hybridization analysis and assessed in three age groups (≤34, 35-39 and ≥40 years), were obtained from trophectoderm biopsies after development to blastocyst stage.

Results: Compaction occurred according to three modalities: (i) full compaction, with all blastomeres included (FCM); partial compaction (partially compacted morula [PCM]), with blastomeres (ii) excluded from the outset (excluded-PCM) or (iii) extruded after compaction (extruded-PCM). Partial compaction occurred more frequently than full compaction. Excluded-PCM displayed the slowest morphokinetics at most stages and were most often associated with abnormal cleavage. After compaction, embryo degeneration was more frequently associated with cell extrusion. In excluded-PCM, loss of ≥2 cells impacted blastocyst rate. In embryos of both younger and middle age groups, no statistical differences were observed in the rate of aneuploidy in relation to the three compaction groups, unlike what observed in ≥40 years women. Implantation rates after transfer of euploid blastocysts were not statistically different between the three groups.

Conclusions: Alternative modalities of incomplete compaction were detected. Such patterns are characterized by different morphokinetic behaviours overarching the entire preimplantation development, and by different developmental abilities.