P-190 The first cleavage plane in human embryos is dictated by the topology of pronuclei, not by the polar body position

Abstract

Abstract Study question Is the first cleavage plane pre-determined in human embryos? Summary answer Unlike in mice, the human zygotes tend to divide at the right angle to the longitudinal pronuclear axis, irrespective of polar body position. What is known already In lower species, the orientation of the first cleavage plane defines the development of individual blastomeres and the three-dimensional (3D) body plan. The existence of embryonic pre-patterning in mammals remains controversial. Mouse studies suggested that the juxtaposed pronuclei (PN) are aligned with the polar body (PB) and cleavage furrow bisects the zygote along the polar axis thus ensuring symmetrical distribution of maternal resources to arising blastomeres. However, whether the same spatial control applies to humans is unclear. Study design, size, duration Retrospective observational analysis of high-resolution footage from 300 human embryos that gave rise to live births after a single embryo transfer. Participants/materials, setting, methods Embryo monitoring has been performed by the GERI time-lapse incubator. Image acquisition of individual embryo development lasted 6 days, starting within 10 minutes after intracytoplasmic sperm injection (ICSI). Transmitted light images have been automatically captured in 5-minute intervals and 11 focal planes covering the whole embryo volume. Analysis of 3D image data was performed at the time frame corresponding to the onset of the first mitosis. Main results and the role of chance The systematic analysis of the zygotic division pattern showed that the PN axis (defined by apposed pronuclei) was not stereotypically aligned with the animal-vegetal (AV) axis (with animal pole defined by the PB). Instead, PN/AV arrangement categories were equally represented in our cohort (PN/AV angle 0-30° - 34.7% vs. 30-60° - 36.0% and 60-90° - 29.3%; chi-square test p=.91351). Remarkably, we observed that the direction of the first cleavage was preferentially perpendicular to the PN axis (65.3%), while only 2.7% of embryos divided along the longitudinal PN axis. The remaining 32% embryos showed intermediated phenotype with cleavage plane/PN axis angle 30-60o. The results of this human embryo analysis contrast with the previous reports from mice. We hypothesize that this discrepancy might be attributed to the difference in centrosome inheritance and configuration of the zygotic spindle apparatus. Limitations, reasons for caution All analyzed embryos were produced by ICSI. Deposition of spermatozoa might affect the pronuclei positioning. Nevertheless, preliminary data from conventional IVF cycles are in agreement with the ICSI group results. Wider implications of the findings Our observations challenge the textbook view of the first division’s geometry, which has been derived from experiments in mice. The analysis of clinical time-lapse records improves our understanding of symmetry breaking during normal human development and creates scientific ground for evidence-based (de)selection of embryos for infertility treatment. Trial registration number not applicable