Morphokinetic Behavior of the Second Polar Body in Human Zygotes as a Predictor for Embryonic Developmental Potential: An Exploratory Study Based on Time-Lapse Observation

Time-lapse imaging has made possible the detailed observation of all stages of embryonic development, including also from the extrusion of the second polar body up to the first cleavage. By extensive observation, we achieved detection of a variety of behaviors of PBIIs such as (a) morphologically static behavior, (b) amoeboid movement, (c) shrinking, (d) fragmenting, and (e) ruffling. Retrospective analysis was performed on 282 ICSI zygotes derived from 69 ART treatment cycles from January to August 2019. Zygotes with morphologically static PBIIs (a) and PBIIs showing various behaviors (b)~(e) were classified into Group 1 (n = 70) and Group 2 (n = 212), respectively. Based on the rates of irregular division, good quality embryos, and the time from the PBII extrusion, pronuclear breakdown to the first cleavage was compared between groups (Study 1). Furthermore, the relationship between the type of PBII behaviors and ploidy in 94 biopsied blastocysts from 15 cycles was examined, in which one or more euploid embryos were obtained between August 2021 and July 2024 (Study 2). The results showed that good quality embryos tended to have morphologically static PBIIs, and that euploid embryos were absent in embryos with fragmenting and ruffling PBIIs. The behavior of PBIIs may be a new predictor of embryonic developmental potential, and, in the future, morphokinetic behaviors of PBIIs may be a useful parameter for AI-assisted embryo evaluation systems.