P-118 Behavior of the second polar body and its relationship to subsequent embryonic development, embryo ploidy status and viability in human embryos
Authors: Shimura T , Yumoto K , Mio Y 
Published: July 2024

Abstract

Study question
Can the behavior of the second polar body (PBII) be an indicator of embryo viability?

Summary answer
The behavior of PBII was shown to possibly reflect the viability of human embryos depending on the type of its behavior.

What is known already
Lately, time-lapse incubators have become widely available. Using time-lapse imaging, second polar bodies (PBII) can be observed from the time of extrusion to the first cleavage, exhibiting a variety of behaviors such as morphologically static, moving like an amoeba, shrinking, fragmenting and ruffling (Shimura et al, Hum Reprod:38,Suppl_1,2023). Although the ploidy and morphology of the first polar body have been variously reported to reflect the quality of embryos after fertilization, there are few studies on PBII, which is thought to directly reflect the state of oocyte after sperm entry, and its relationship to the subsequent embryonic development and embryo viability.

Study design, size, duration
Retrospective study involving 282 ICSI zygotes derived from 69 ART treatment cycles from January to August 2019. Zygotes were classified into two groups according to the type of PBII behaviors (Group1: static, Group2: others), and developmental outcome was compared (Study1). The relationship between the type of PBII behaviors and ploidy in 64 biopsied blastocysts from 11 cycles in which one or more euploid embryos were obtained between August 2021 and October 2023 was examined (Study2).

Participants/materials, setting, methods
Time-lapse imaging was performed in EmbryoScope® or Geri®. Study1: Zygotes with morphologically static PBII (n = 70) and PBII showing various behaviors (n = 212) were classified into Group1 and Group2, respectively. The rates of irregular division, good quality embryos, and the time from the PBII extrusion and pronuclear breakdown to the first cleavage were compared between groups. Study2: Biopsied blastocysts were classified according to the type of PBII behavior, and the euploid rates were examined.

Main results and the role of chance
Study1: The incidence of irregular divisions at the first cleavage was 21.4% (n = 15/70) in Group1 and 28.3% (n = 60/212) in Group2. Good quality embryos at the early cleavage stage were cryopreserved/transferred on Day-2/3 according to our clinic policy, and only those that were not cryopreserved or freshly transferred on Day-2/3 were extendedly cultured up to Day-7. Of those, the rate of good quality blastocysts was 32.4% (n = 12/37) in Group1 and 22.5% (n = 32/142) in Group2. Meanwhile, the rate of good quality embryos at the early cleavage stage was significantly higher in Group1 than in Group2 [P &lt; 0.001; 58.6% (n = 41/70) in Group1, 35.4% (n = 75/212) in Group2]. The utilization rate for cryopreservation and embryo transfer was also significantly higher in Group1 than in Group2 [P &lt; 0.05; 64.3% (n = 45/70) in Group1, 48.1% (n = 102/212) in Group2]. The time elapsed from the pronuclear breakdown to the first cleavage was significantly longer for Group2 than for Group1 [P &lt; 0.05; 2.57 ± 0.42h in Group1, 3.07 ± 1.90h in Group2]. Study2: There were 20 euploid embryos in embryos with PBII showing static, amoebic and shrinking behavior (37.0%; n = 20/54), whereas there were no euploid embryos in embryos with ruffling and fragmenting PBII (0%, n = 0/10) (P &lt; 0.05).

Limitations, reasons for caution
Categorizing PBII behavior is difficult and requires some proficiency because it varies widely including time-based changes. For the conventional IVF-derived embryos examined in Study2, time-lapse imaging was mostly started after PBII extrusion to allow sufficient insemination time.

Wider implications of the findings
This study showed that good quality embryos tended to have morphologically static PBII, and that euploid embryos were absent in embryos with PBII showing certain types of behavior. The behavior of PBII may be a new indicator for embryo assessment, but further clarification is needed for clinical use.

Trial registration number
not applicable