Polarized light microscopy reveals distinct spindle phenotypes and outcomes in non-pronuclear zygotes

Can polarized light microscopy (PLM) spindle phenotypes predict the developmental potential of non-pronuclear (0PN) zygotes?

Retrospective observational study including 1266 0PN zygotes and 69,383 two-pronuclei (2PN) zygotes from patients undergoing IVF and embryo transfer treatments in a reproductive centre affiliated to a university hospital from June 2021 to November 2024. Phenotypic observations were obtained through PLM; all 0PN zygotes were categorized into four distinct groups: adjacent dual spindles (AD-SP); separate dual spindles (SD-SP); mono-spindle (M-SP) and no spindle (N-SP). Logistic regression assessed the effect of these PLM phenotypes on embryological outcomes, considering parental and treatment factors, and evaluated the effects of rescue artificial oocyte activation or rescue ICSI on these outcomes.

Cleavage rates were significantly lower in AD-SP (adjusted odds ratio [aOR] 0.22; P < 0.001), SD-SP (aOR 0.03; P < 0.001), M-SP (aOR 0.02; P < 0.001), and N-SP (aOR 0.02; P < 0.001) categories compared with 2PN zygotes. The rates of four or more cells at day 3 (aOR 1.97; P = 0.041), six or more cells on day 3 (aOR 1.60; P = 0.012) and good-quality embryos at day 3 (aOR 1.58; P = 0.008) were significantly higher in AD-SP categories compared with 2PN zygotes. Blastocyst formation rates of day-3 poor-quality embryos (aOR 1.31; P = 0.351) was comparable between 2PN zygotes and 0PN zygotes with AD-SP.

Four prominent phenotypes of 0PN zygotes were identified based on PLM, along with their corresponding embryonic outcomes. Notably, 0PN zygotes with AD-SP had embryonic outcomes similar to those of 2PN zygotes, except for cleavage rate, and demonstrated superior developmental potential compared with other phenotypes.