An oocyte score for use in assisted reproduction

Abstract

Purpose: In this work, we describe a system for the morphological scoring of human oocytes prior to fertilisation and use this system to test whether oocyte morphology is an indicator of fertilisation, embryo development and implantation potential.

Methods: The study is a prospective trial of the use of oocyte morphological scores in 822 patients undergoing their first cycle of ICSI. Analyses of oocytes were performed prior to ICSI procedures and the scores compared with fertilisation rates, embryo quality and clinical results.

Results: 'Top quality' oocytes had a significantly higher level of fertilisation (96%) as compared to low scoring oocytes (25.6%). Where top quality oocytes formed top quality embryos, we noted a clinical success rate of 63.4%.

Conclusions: Clinical success rates were increased in cases where top quality oocytes formed top quality embryos after ICSI. The analysis of oocyte morphology may represent a positive selection feature during ICSI.

Fig. 1

Morphological features selected for analysis. The figure is a representative figure of morphological parameters selected for analysis. a granularity describes the observation of granules in the cytoplasm. When these are polarised to one, or both sides of the oocytes, a score of 3 points is given. Granularity localised to the centre of the oocyte is scored as 2 points, and oocytes with a complete absence of granularity awarded 1 point. b The presence of vacuoles and inclusions (consisting of any deformity of the oocyte cytoplasm) was scored, 3 points awarded to oocytes not containing any features, 2 points if the oocyte cytoplasm contained either vacuoles or inclusions, and 1 point in cases where the oocyte contains both features. c Injection properties of the oocyte were scored during the ICSI procedure. In cases where a small amount of pressure was required to rupture the oocyte plasma membrane (the membrane did not stretch further than the natural limit of the plasma membrane - see arrow) 3 points were awarded. In cases where the membrane ruptured without aspiration pressure, 2 points were awarded and where an excessive pressure was required for the rupture of the plasma membrane (see arrow), 1 point was awarded.

Fig. 2

Biological results after morphological analysis of human oocytes during ICSI cycles. Results show percentages of normal and abnormal fertilisation rates and percentage of oocyte lysis after intracytoplasmic sperm injection in oocytes in which morphological analysis preceded the ICSI procedure. Black bars are percentages of normal fertilisation, light grey bars percentages of oocyte degeneration and dark grey bars percentages of abnormal fertilisation (i.e. one pronucleus). The subtitles are the single morphological parameters observed. I Inclusions, V vacuoles. Asterisk denotes data significantly different from first parameter (p < 0.05, two-tailed t test). Two asterisks represent data highly significantly different from first parameter (p < 0.001, two-tailed t test). The data is derived from a total of 2,017 oocytes in which fertilisation was verified after the analysis of morphology

Fig. 3

Correlation between normal fertilisation rates and combinations of oocyte morphological scores. Oocyte morphological scores can vary between three and nine. Fertilisation rates at each level of morphological analysis are shown. Long dashed line is the correlation coefficient and dotted lines are the 95% confidence intervals. The data is derived from a total of 2,017 oocytes in which fertilisation was verified after the analysis of morphology

Fig. 4

Correlations between oocyte morphological scores and morphological scores applied during the first two days of development. Top Correlation between oocyte and zygote morphology. Centre Correlation between oocyte morphology and embryo morphology. Embryo morphology was assessed on day 2 (40–41 h after fertilisation). Bottom Correlation between oocyte morphology and cumulative embryo score. Zygote, embryo and cumulative scores were assessed according to previous work [51]. Continuous line shows correlation coefficient and dotted lines are the 95% confidence intervals. The data is derived from a total of 2,017 oocytes in which fertilisation was verified after the analysis of morphology