P-124 Behavior of the second polar body and its relationship to subsequent human embryonic development
Authors: Shimura T , Yumoto K , Mio Y 
Published: June 2023

Abstract

Study question
Is the behavior of the second polar body (PBII) associated with subsequent human embryonic development?

Summary answer
Human zygotes with morphologically static PBII showed significantly higher rates of good quality embryos and utilization than zygotes with PBII showing various behaviors.

What is known already
In recent years, the time-lapse incubators have become widely available, enabling continuous observation of human embryos. With the time-lapse imaging, zygotes with second polar bodies that exhibit a variety of behaviors can be observed from the time of PBII extrusion to the first cleavage. 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 the second polar body, which is thought to directly reflect the state of oocyte after sperm entry, and its relationship to the embryonic development.

Study design, size, duration
This is a retrospective study involving the time-lapse imaging of 285 normally fertilized ICSI zygotes between January and August 2019. Of those, 262 showing suitable images of PBII from the extrusion to the first cleavage were examined. Zygotes with morphologically static PBII during the observation were classified into Group 1 (n = 68), and zygotes with moving-like-amoeba, shrinking, fragmenting, and/or ruffling PBII were classified into Group 2 (n = 194).

Participants/materials, setting, methods
This study included 64 cycles of ART treatment. Gonadotrophin-releasing hormone analogues (GnRHa) and gonadotrophins were used for ovarian stimulation prior to the oocyte retrieval. Time-lapse imaging was performed in EmbryoScope®. The rates of irregular divisions at the first cleavage, good quality embryos at the early cleavage stage based on the modified Veeck’s criteria, good quality blastocysts (≥4BB) based on the Gardner grading system and utilization for cryopreservation and embryo transfer were compared between groups.

Main results and the role of chance
The proportion of Group 1 (morphologically static PBII) and Group 2 (PBII with various behaviors) was 26.0% (n = 68/262) and 74.0% (n = 194/262). The mean maternal age (years) was 34.8 ± 5.4 in Group 1 and 35.0 ± 4.8 in Group 2. The incidence of irregular (direct/reverse) divisions at the first cleavage was 22.1% in Group 1 and 28.9% in Group 2. 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 30.6% (n = 11/36) in Group 1 and 23.3% (n = 30/129) in Group 2. Thus, there were no significant differences between Group 1 and 2 in the mean maternal age and the rates of irregular divisions and good quality blastocysts. Meanwhile, the rate of good quality embryos at the early cleavage stage was significantly higher in Group 1 than in Group 2 [P&lt;0.01; 57.4% (n = 39/68) in Group 1, 37.1% (n = 72/194) in Group 2]. The utilization rate for cryopreservation and embryo transfer was also significantly higher in Group 1 than in Group 2 [P&lt;0.05; 64.7% (n = 44/68) in Group 1, 49.5% (n = 96/194) in Group 2].

Limitations, reasons for caution
This is an observational study using only ICSI zygotes. Since good quality embryos were cryopreserved or freshly transferred on Day-2/3 according to our clinic policy, the number of embryos cultured up to the blastocyst stage was limited to those which were fair or poor quality at the early cleavage stage.

Wider implications of the findings
This study revealed that the rates of good quality embryos and the utilization for cryopreservation or embryo transfer were significantly higher in zygotes with morphologically static PBII than in those with PBII showing various behaviors. This suggests that the behavior of PBII may be a new indicator for embryo evaluation.

Trial registration number
not applicable