Dynamic analysis of human parthenogenetic zygotes induced by artificial oocyte activation

Abstract

The origin of the pronucleus (PN) in a single PN zygote (1PN), and whether its genome is normal still remains controversial. We recently established a novel method of discriminating between maternally- and paternally-derived PN using immunofluorescence staining and demonstrated the possibility that both the male and female genome could be packed in 1PN in some cases. However, currently analyzing karyotypes is an invasive technique, limiting its clinical application. Therefore, we tried to distinguish between normally-fertilized zygotes and 1PN zygotes by their morphology or developmental behavior. In this study, we used a microscope with time-lapse system to analyze the developmental time course and morphology of human 1PN zygotes, especially parthenogenetic zygotes induced by artificial oocyte activation. Research study. This study used 32 MII oocytes donated between October 2014 and August 2015 by patients who gave informed consent for this study. Fresh or freeze-thawed MII oocytes were activated electronically and the oocytes were observed by EmbryoScope®. We compared the developmental time course and morphology between parthenogenetic zygotes and normal 2PN zygotes fertilized by assisted reproductive technology. There was no difference in the diameter of the PN in parthenogenetic zygotes compared to the female PN in normal fertilized zygotes (28.9 ± 2.2 vs 26.4 ± 2.0 μ m, respectively). There were significant differences between normal 2PN and parthenogenetic zygotes for the time from intracytoplasmic sperm injection or electronic activation to the 2nd polar body (PB) extrusion (3.0 ± 1.7 vs 2.3 ± 0.5 h, respectively), from 2nd PB extrusion to syngamy (20.8 ± 4.1 vs 18.7 ± 2.9 h, respectively), from syngamy to 1st cleavage (3.0 ± 2.3 vs 3.9 ± 1.1 h, respectively), and from 1st cleavage to 2nd cleavage (9.8 ± 4.8 vs 13.6 ± 5.2 h, respectively). In addition, some parthenogenetic zygotes (6 of 21) developed to blastocysts. The time required from electronic activation to 2nd PB extrusion and from 2nd PB extrusion to syngamy in parthenogenetic zygotes was significantly shorter than in normal zygotes. This may be because oocyte activation or decondensation of the sperm nucleus is not required in parthenogenetic zygotes. In addition, the time required from syngamy to 1st cleavage, and from 1st cleavage to 2nd cleavage in parthenogenetic zygotes was significantly longer than in normal embryos. Thus, differences in the time course of embryonic development in activated zygotes could be used to identify the characteristics of parthenogenetic zygotes, even though further studies are needed. Furthermore, although some parthenogenetic zygotes develop to blastocysts, the clinical use of zygotes with 1PN should be questioned.