P-207 Analysis of oocyte morphology: differences between rescued and non-rescued metaphase II oocytes after conventional ovarian hyperstimulation
Authors: Carrión T , Murria L , Gómez-Pastor R , Conversa L , Bori L , Meseguer M 
Published: July 2024

Abstract

Study question
Are there morphological differences between rescued and non-rescued metaphase II oocytes after conventional ovarian hyperstimulation (COH)?

Summary answer
Rescued metaphase II oocytes have a smaller diameter and a higher zona pellucida/perivitelline space ratio than non-rescued metaphase II oocytes.

What is known already
The rescue of germinal vesicles and metaphase I in assisted reproduction clinics has increased in recent years, but it is still not widely used because these oocytes have a lower developmental potential compared to oocytes matured in the follicular environment. Recent studies focus on the different strategies for the oocyte maturation process in the laboratory, one of them showed that rescued metaphase II oocytes have a smaller diameter than oocytes matured within the follicular environment. To our knowledge, this is the first study to analyze in detail the morphological differences between in vivo and in vitro matured oocytes.

Study design, size, duration
This retrospective study includes the analysis of 144 non-rescued metaphase II oocytes images and 181 rescued metaphase II oocytes images obtained after decumulation, or decumulation and in vitro maturation. Images were acquired over four months using Embryoscope TimeLapse systems. Morphological parameters were manually annotated by one single operator, to reduce the discrepancy of measurements, using the drawing tools of the EmbryoViewer software: oocyte area (μm2), polar body area (PBA)(μm2), zona pellucida (μm), perivitelline space (PVS)(μm).

Participants/materials, setting, methods
First, the different variables measured were analyzed to see if they were related to each other. Then, measurements obtained from images of non-rescued metaphase II oocytes were compared with those of the rescued. In addition, relationships between the different morphological parameters (oocyte area/PBA ratio, oocyte area/zona pellucida ratio, oocyte area/PVS ratio, zona pellucida/PVS ratio, PBA/PVS ratio and PBA/zona pellucida ratio) were calculated and their values were compared between oocytes from different maturation origins.

Main results and the role of chance
In both rescued and non-rescued metaphase II oocytes, a significantly positive correlation was detected between oocyte area and PBA*, and zona pellucida thickness*. It was also found a significant negative correlation between PVS size and PBA*. For rescued oocytes, a significantly negative correlation was also found between oocyte area and PVS*. Comparison of the means of age, BMI, AMH and all the morphological parameters analyzed between the two groups showed significant differences in age*, oocyte área*, PVS*, oocyte area-zona pellucida ratio*, oocyte area -PVS ratio*, zona pellucida -PVS ratio*, PBA -PVS ratio *. The average of these variables in the rescued oocytes group were: 28.77 ± 6.25, 9734.70 ± 681.28, 8.79 ± 4.43, 628.10 ± 116.29, 1391.86 ± 689.13, 2.24 ± 1.11, 50.84 ± 40.81; while their mean in the non-rescued oocytes group were: 24.41 ± 4.15, 10152.63 ± 711.3, 11.73 ± 5.62, 675,08 ± 111.48, 1063.32 ± 554.35, 1.60 ± 0.82, 37.65 ± 24.55. In addition, multivariable logistic regression showed that the differences in oocyte area size and zona pellucida/PVS ratio remained statistically different between the two groups despite the difference in patient age*. (p&lt;.05*)

Limitations, reasons for caution
This study is limited by its descriptive and single-center nature. A multicenter study that also evaluates the developmental potential of oocytes would be needed.

Wider implications of the findings
Our study shows morphological differences between rescued and non-rescued metaphase II oocytes. It could be studied whether these differences are associated with the lower developmental competence of rescued metaphase II oocytes that has been observed in previous studies.

Trial registration number
Not applicable