158 Predicting embryo development success with physical parameters of oocytes

There is a continual search for reliable, noninvasive methods of selecting viable oocytes and embryos. Previous studies have indicated that the physical size of oocytes may reflect their developmental potential. The objective of this study was to observe the correlation between an oocyte’s diameter [including zona pellucida (ZP), cell area, and ZP thickness] and its ability to develop. Bovine cumulus-oocyte complexes were collected from abattoir-derived ovaries and incubated for 24 h in TCM-199-based maturation medium. The cumulus-oocyte complexes were denuded by vortexing with hyaluronidase, and mature oocytes were selected based on presence of a visible polar body. Selected oocytes were artificially activated by incubating in 5 μM ionomycin for 5 min followed by incubation in 2 mM 6-DMAP for 3 h (n = 723). After activation, oocytes were placed in individual 5-μL culture droplets under oil and photographed using an inverted scope with digital camera. Oocytes were then group cultured in 50-μL droplets in a polyester micromesh for identification of individual oocytes. Development to the blastocyst stage was noted on Days 7 and 8 of culture. ImageJ was used to measure diameter, area, and ZP thickness from images. A logistic regression using the lme4 package in R was run with Day 7 or 8 blastocyst development as the response; diameter, area, and ZP thickness as predictors; and replicate and culture droplet as random effects. The residual estimates of area and ZP thickness from diameter were used to account for correlation between predictors. Only significant interactions were kept in the model. The ZP thickness ranged from 6.1 to 17.7 μm with a mean of 12 μm. There was a significant correlation between diameter and ZP thickness (P < 0.01, R2 = 0.12) with a correlation coefficient of 0.35. Oocyte diameter had a significant effect on subsequent blastocyst development on Day 7 (P < 0.01) and Day 8 (P < 0.01), with larger oocytes more likely to develop on both days. Oocytes were also grouped into quantiles by diameter. Larger groups were more likely to develop to the blastocyst stage on Day 7 (P < 0.001) and Day 8 (P < 0.001). Blastocyst rate on Day 8 for oocytes with diameters <149.5 μm was 24.2%, whereas blastocyst rate on Day 8 for those with diameters = 159 μm was 41.2%. In addition, ZP thickness also had an effect: oocytes with thinner ZP were more likely to develop to the blastocyst stage on Day 7 (P < 0.001) and Day 8 (P < 0.001). Blastocyst rate for oocytes with a ZP thickness <11 μm was 37%, whereas the rate for those with a ZP thickness of 12.9 μm was 27.6%. Area did not have an effect on blastocyst formation on Day 7 or 8 (P = 0.21). Here we have demonstrated that the oocyte diameter, including the ZP and ZP thickness, affects its probability of development. Larger oocytes and those with thinner ZP are more likely to develop to the blastocyst stage. Differences in the size of the perivitelline space could explain why diameter had a significant effect on development and area did not. Further studies will focus on determining the relationship between these physical parameters of oocytes and embryo quality.