Morphometric and morphokinetic differences in the sperm- and oocyte-originated pronuclei of male and female human zygotes: a time-lapse study

Purpose To study the morphometric and morphokinetic profiles of pronuclei (PN) between male and female human zygotes. Method(s) This retrospective cohort study included 94 consecutive autologous single day 5 transfer cycles leading to a singleton live birth. All oocytes were placed in the EmbryoScope + incubator post-sperm injection with all annotations performed retrospectively by one embryologist (L-SO). Timing parameters included 2nd polar body extrusion (tPB2), sperm-originated PN (tSPNa) or oocyte-originated PN (tOPNa) appearance, and PN fading (tPNF). Morphometrics were evaluated at 8 (stage 1), 4 (stage 2), and 0 h before PNF (stage 3), measuring PN area (um(2)), PN juxtaposition, and nucleolar precursor bodies (NPB) arrangement. Results Male zygotes had longer time intervals of tPB2_tSPNa than female zygotes (4.8 +/- 0.2 vs 4.2 +/- 0.1 h, OR = 1.442, 95% CI 1.009-2.061, p = 0.044). SPN increased in size from stage 1 through 2 to 3 (435.3 +/- 7.2, 506.7 +/- 8.0, and 556.3 +/- 8.9 um(2), p = 0.000) and OPN did similarly (399.0 +/- 6.1, 464.3 +/- 6.7, and 513.8 +/- 6.5 um(2), p = 0.000), with SPN being significantly larger than OPN at each stage (p < 0.05 respectively). More male than female zygotes reached central PN juxtaposition at stage 1 (76.7% vs 51.0%, p = 0.010), stage 2 (97.7% vs 86.3%, p = 0.048), and stage 3 (97.7% vs 86.3%, p = 0.048). More OPN showed aligned NPBs than in SPN at stage 1 only (44.7% vs 28.7%, p = 0.023). Conclusion(s) Embryos with different sexes display different morphokinetic and morphometric features at the zygotic stage. Embryo selection using such parameters may lead to unbalanced sex ratio in resulting offspring.

Automated summary

This study examined the morphometric and morphokinetic features of male and female pronuclei (PN) in human zygotes. The results showed that male and female zygotes had different time intervals and size differences in PN appearance. Male zygotes also had a higher rate of central PN juxtaposition compared to female zygotes. These findings suggest that embryo selection based on these parameters may result in an unbalanced sex ratio in offspring.