Study on the relationship between pronuclear proximity in zygote and multinucleation of early cleavage embryos in human using time-lapse system

Abstract

It has recently been reported that two separate bipolar spindles aligned their poles before anaphase keeping the parental genomes apart during the first cleavage in mammalian zygote including mouse and human. In mouse, the failure of spindle alignment by increasing the distance between the two pronuclei, which led to a larger gap between the spindles, gives rise to multinucleated two-cell embryos phenocopying frequently observed errors in IVF clinics. The purpose of our study is to examine the relationship between pronuclear (PN) proximity in zygote and multinucleation (MN) of early cleavage embryos in human using time-lapse system (TLS). Data were retrospectively collected from TLS performed from May 2018 to February 2019 in the Fertility center of CHA Gangnam Medical Center. We analyzed 220 zygotes from 64 patients. We assessed the PN proximity, distance, MN, and development of early cleavage embryos using EmbryoScopeTM (Vitrolife, Gothenburg, Sweden). Drawing tool was used to measure the distance from a PN center to another PN center in the same focal plane before PN fade. Zygotes were divided into two groups according to the proximity (group Gap [G; n=17] and group Juxtaposition [J; n=203]). The number of MN in 2-cell and 4-cell embryo was checked then the embryos were divided into three groups (No MN, MN, and N/A; not available due to abnormal division). Embryo development was checked up to day 3. The quantitative variables were expressed as mean ± SD and statistically analyzed with Student t test. p <0.05 was considered to be statistically significant. The incidence of gap between PN was considerably low (G; 7.7% vs. J; 92.3%). The average of PN distance was significantly different between two groups (G; 31.2 ± 5.1 μm vs. J; 21 ± 4.9 μm, * p < 0.05). The rate of No MN (23.5%[2C] and 52.9%[4C]) was highly decreased but the rate of MN (41.2%[2C] and 5.9%[4C]) and N/A (35.3%[2C] and 41.2%[4C]) was increased in the Gap group compared with Juxtaposition group (49.8%/72.9%[2C/4C], 38.4%/5.4%[2C/4C], and 11.8%/21.7%[2C/4C]). Also the rate of 3D good quality embryo is slightly decreased in Gap group (G; 82.4% vs. J; 89.7%). This study suggests that the PN proximity is also one of the applicable explanations for the MN in early cleavage embryos of human due to the failure of zygotic spindle alignment as other mammalian embryos such as mouse. As the presence of MN in human embryos, especially during the first and second mitotic divisions, is generally considered to be abnormal, our study on the relationship between PN proximity and MN combined with TLS could be used as a noninvasive technique to enhance selection of competent embryos likely to have the greatest potential of development. This may be of particular benefit to patients desiring elective single embryo transfer without PGS screening.